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Introduction and guidance

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

Welcome to this free course, Microgravity: living on the International Space Station. The course lasts eight weeks, with approximately three hours of study each week. You can work through the course at your own pace, so if you have more time one week there is no problem with pushing on to complete another week’s study.

You will be able to test your understanding of the course through the weekly interactive quizzes, of which Weeks 4 and 8 will provide you with an opportunity to earn a badge to demonstrate your new skills. You can read more on how to study the course and about badges in the next sections.

After completing this course, you will be able to:

understand what microgravity is and the difference between mass and weight
explain the types of scientific research that can benefit from microgravity environments, focusing on ageing
explain the types of scientific research that can benefit from microgravity environments, focusing on bacterial resistance and planet formation
understand some key concepts and principles that underpin scientific knowledge and why it is important to continue to ask moral questions about scientific research.

Moving around the course

In the ‘Summary’ at the end of each week, you can find a link to the next week. If at any time you want to return to the start of the course, click on ‘Course content’. From here you can navigate to any part of the course. Alternatively, use the week links at the top of every page of the course.

It’s also good practice, if you access a link from within a course page (including links to the quizzes), to open it in a new window or tab. That way you can easily return to where you’ve come from without having to use the back button on your browser.

The Open University would really appreciate a few minutes of your time to tell us about yourself and your expectations for the course before you begin, in our optional start-of-course survey. Participation will be completely confidential and we will not pass on your details to others.

What is a badged course?

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

While studying Microgravity: living on the International Space Station you have the option to work towards gaining a digital badge.

Badged courses are a key part of The Open University’s mission to promote the educational well-being of the community. The courses also provide another way of helping you to progress from informal to formal learning.

To complete a course you need to be able to find about 24 hours of study time, over a period of about 8 weeks. However, it is possible to study them at any time, and at a pace to suit you.

Badged courses are all available on The Open University’s OpenLearn website and do not cost anything to study. They differ from Open University courses because you do not receive support from a tutor. But you do get useful feedback from the interactive quizzes.

What is a badge?

Digital badges are a new way of demonstrating online that you have gained a skill. Schools, colleges and universities are working with employers and other organisations to develop open badges that help learners gain recognition for their skills, and support employers to identify the right candidate for a job.

Badges demonstrate your work and achievement on the course. You can share your achievement with friends, family and employers, and on social media. Badges are a great motivation, helping you to reach the end of the course. Gaining a badge often boosts confidence in the skills and abilities that underpin successful study. So, completing this course should encourage you to think about taking other courses.

How to get a badge

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

Getting a badge is straightforward! Here’s what you have to do:

read each week of the course
score 50% or more in the two badge quizzes in Week 4 and Week 8.

For all the quizzes, you can have three attempts at most of the questions (for true or false type questions you usually only get one attempt). If you get the answer right first time you will get more marks than for a correct answer the second or third time. Therefore, please be aware that for the two badge quizzes it is possible to get all the questions right but not score 50% and be eligible for the badge on that attempt. If one of your answers is incorrect you will often receive helpful feedback and suggestions about how to work out the correct answer.

For the badge quizzes, if you’re not successful in getting 50% the first time, after 24 hours you can attempt the whole quiz, and come back as many times as you like.

We hope that as many people as possible will gain an Open University badge – so you should see getting a badge as an opportunity to reflect on what you have learned rather than as a test.

If you need more guidance on getting a badge and what you can do with it, take a look at the OpenLearn FAQs. When you gain your badge you will receive an email to notify you and you will be able to view and manage all your badges in My OpenLearn within 24 hours of completing the criteria to gain a badge.

Get started with Week 1.

Introduction

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

In this first week you will look at weightlessness, zero gravity and microgravity in the context of a parabolic flight. You will study the physics of the International Space Station (ISS), its altitude and speed. You will consider the various rockets used to send Astronauts into space over the decades and you will use a software simulation to launch your own rocket. You will consider units of measurement for forces, mass and acceleration. Finally, you will carry out a practical experiment using a cup of rice to model circular motion similar to the ISS orbiting the Earth.

By the end of this week, you should be able to:

understand what microgravity is and the difference between mass and weight
understand equations of motion, forces and fields
substitute numbers into an equation and understand what the answer means in context
compare numerical values and understand their scientific significance
interpret an online interactive diagram and extract information from it.

1 Microgravity and the ‘vomit comet’

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

There are several ways of describing the apparent absence of gravity. You may have heard of ‘weightlessness’ where there seems to be no weight, or a ‘zero g’ environment where gravity (g) doesn’t seem to be acting. What about ‘microgravity’ though? Do these terms mean the same thing?

Watch Video 1 which introduces ‘microgravity’ environments. Then complete Activity 1.

Download this video clip.Video player: Video 1 >

Video _unit2.2.1 Video 1 What is microgravity?

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Transcript: Video 1 What is microgravity?

NARRATOR:

60 Second Adventures in Microgravity. Number 1, What Is Microgravity? Gravity is pretty useful. It keeps our feet on the ground, ensures the Earth orbits the Sun, and shapes the whole universe. But sometimes it would be really handy if it wasn't there, because when gravity acts on something, there is always an effect, usually from the other forces counteracting it-- like the ground pushing back on us, which we perceive as weight.

So to understand many physical and biological processes, it would be better to take gravity out of the equation. But that's impossible. So instead, we create an environment on Earth in which, as far as possible, all these other forces are balanced out so the thing we're studying appears to be weightless. We call this environment microgravity.

There are many ways to achieve microgravity, but one of the simplest is by using something called a drop tower, which is, well, a tower you drop things off. So relative to each other, objects become weightless and are in an environment of microgravity, but only for a few seconds, until they're back down to Earth with a bang.

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Video _unit2.2.1 Video 1 What is microgravity?

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Activity _unit2.2.1 Activity 1 What is microgravity?

Allow approximately 15 minutes

Complete the following statements, based on what you learned in Video 1.

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Now watch Video 2, which discusses parabolic flights and how they can create a microgravity environment. This video also discusses how it is thought planets are formed. Then complete Activity 2.

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Video _unit2.2.2 Video 2 The ‘vomit comet’

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Transcript: Video 2 The ‘vomit comet’

NARRATOR:

60 Second Adventures in Microgravity. Number Two, The Vomit Comet. When you're on a roller coaster and you feel your stomach being left behind, for that tiny moment, you're experiencing microgravity. And we can use microgravity to study how planets like the Earth are formed when tiny ice and dust particles collide and somehow stick together in space.

To test how this happens, the particles have to be thrown together very gently or else they break up. But at that speed on Earth, gravity acts on them and they don't even reach each other, like a really pathetic snowball fight. So scientists head off on something called a parabolic flight, where a plane travels in a series of huge parabolas, or curves. As it goes over the top of the curve, the pilots adjust the plane's speed to counteract the effect of gravity inside. And for about 22 seconds, we can create a microgravity environment to test these planet-building collisions, just like a four-hour roller-coaster ride, which is why they call it the ‘vomit comet.’

[RETCHING]

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Video _unit2.2.2 Video 2 The ‘vomit comet’

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Activity _unit2.2.2 Activity 2 Microgravity environments

Allow approximately 15 minutes

Study Figure 1, which shows the parabolic flight pattern in more detail. Then select the answer to the questions below, based on this figure and Video 2.

View larger image

Figure _unit2.2.1 Figure 1 Parabolic flight of an aircraft.

Long description

1. What height does the aircraft need to achieve to start the experiment? (Hint: look at the top of the curve in Figure 1 and draw a line from this to the vertical axis.)

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2. Roughly how long does the microgravity environment last?

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3. Which part of the parabolic flight curve can create a microgravity environment?

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4. How are planets formed? (Hint: this was discussed in Video 2.)

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Moving on from parabolic motion and planetary formation, next you will consider the physics behind the orbit of the International Space Station (ISS).

2 The International Space Station

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

How high is the International Space Station (ISS) and how fast is it travelling?

Currently, the ISS is in orbit around the Earth at an altitude of about 400 kilometres (km). It travels at a speed of about 28 000 kilometres per hour (km/h). The ISS’s orbit around the Earth is very similar to the Moon’s orbit of the Earth. Both the ISS and the Moon are technically falling towards the Earth. However, they are falling at exactly the right rate to remain in orbit. Unlike the parabolic flight described in Section 1, there is no need to keep boosting the ISS back up to create another 20-second period of microgravity.

Figure _unit2.3.1 Figure 2 The ISS, photographed on 5 March 2008 from the Atlanta Space Shuttle.

Long description

To find out where the ISS currently is, you can ‘spot the station’ (Figure 3) here: https://spotthestation.nasa.gov/. This website gives you lots of opportunities to track the ISS from your computer.

Figure _unit2.3.2 Figure 3 Spot the Station website.

Long description

You can also track the ISS’s position ‘in real time’ (Figure 4) using another website called www.isstracker.com/. Go to this website and find out some facts about the ISS’s flight path.

Figure _unit2.3.3 Figure 4 A screenshot from the ISS tracker website.

Long description

Did you discover that the three yellow lines show three orbital paths of the ISS directly over the Earth’s surface? The longitude of the ISS is shown on the top horizontal axis and its latitude is shown on the left vertical axis. You can see that these trajectories cover roughly between +50 degrees latitude in the Northern Hemisphere to –50 degrees latitude in the Southern Hemisphere. This means that the ISS covers roughly 80% of the Earth.

The top box on the right of the screen – ‘VEL’ for ‘velocity’ – shows values of KMH. This is the speed of the ISS in km/h. The middle box on the right – ‘ALT’ for ‘altitude’ – shows values of KM. This is the altitude of the ISS in km. The bottom box on the right – ‘UNITS’ – gives the two options of units, in either imperial or metric. There are additional functions on the bottom bar. ‘Map’ gives the options of ‘Satellite’, ‘Terrain’ or ‘Hybrid’. ‘Show Orbital Path’ provides the three orbital paths in yellow (Figure 4).

You can also click on and move the ISS. Pressing ‘ISS centred’ returns the ISS image to the centre of the screen. ‘Show horizon’ then provides the circle around the ISS, indicating the horizon view of the Earth’s surface from the ISS. Finally, using the ‘zoom’ buttons on the bottom left changes the scale of the image from 10 km to 1000 km.

Activity _unit2.3.1 Activity 3 Exploring the ISS

Allow approximately 5 minutes

Now select the correct option to complete the following statements.

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There are also numerous apps about the ISS which can be downloaded to your mobile device. For example, there is a free app called ISS HD Live (Figure 5). You can even use this app to record video footage of the view of the Earth from the ISS!

Figure _unit2.3.4 Figure 5 ISS HD Live app for Android devices.

Long description

You’ve now seen how to observe the Earth from the ISS. But how do Astronauts travel from the Earth to the ISS?

3 How astronauts get up there

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

Astronauts first need to get into space – but how? Obviously, a powerful rocket is needed. In the 1960s, the rocket that sent men to the Moon – Saturn V (Five) – was the most powerful machine ever built (Figure 6).

Figure _unit2.4.1 Figure 6 The Saturn V rocket and Apollo 11, Kennedy Space Center, Florida, USA, in 1969.

Long description

Between 1981 and 2011, NASA then sent astronauts to space using the Space Shuttle Program (Space Transportation System) (Figure 7).

Figure _unit2.4.2 Figure 7 The last flight of the Space Shuttle Atlantis, 8 July 2011.

Long description

The Space Shuttle was invaluable in building the ISS and the Hubble Space Telescope. It flew for 135 missions and was launched from the Kennedy Space Center in Florida, USA. Space Shuttles docked with the Russian Space Station, Mir, nine times and visited the ISS 37 times. A total of 355 people representing 16 countries flew on the Shuttle. Unfortunately, Challenger and Columbia had catastrophic accidents, leading to the deaths of 14 astronauts. Take a look at this full list of Space Shuttle missions.

Figure 8 shows the Space Shuttle launch profile as it lifts off and the external fuel tanks separate, returning to Earth. You will consider how recent developments have changed this ‘launch profile’ later in Week 8.

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Figure _unit2.4.3 Figure 8 Space Shuttle launch profile.

Long description

Since the Space Shuttle was retired in 2011, the only way to get to the ISS now is on the Soyuz rocket from the Russian Mission Control Centre in Kazakhstan (Figure 9).

Figure _unit2.4.4 Figure 9 Expedition 33 Soyuz launch, 23 October 2012.

Long description

How do all these rockets compare by size? You can see from Figure 10 that the Saturn V rocket is still the tallest.

Figure _unit2.4.5 Figure 10 A comparison of the size of rockets to date.

Long description

3.1 How to launch a rocket

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

How can you launch a rocket into space? Obviously, you need something to push it upwards, so providing a force. This force is often called a ‘thrust’, which is measured in newtons (N). Burning rocket fuel to provide this force is one obvious solution. Opposing this motion, in addition to gravity, is air resistance. This is often called ‘drag’ and is also measured in N.

You then need to consider the mass of the rocket – measured in kilograms (kg) – and how long the fuel burns – measured in seconds (s). As the fuel is gradually exhausted, there is a reduction in the overall mass of the whole rocket. This is because the mass of fuel is reduced as time increases. This change of mass in time is given in kg/s.

Now complete Activity 4.

Activity _unit2.4.1 Activity 4 Launching your own rocket

Allow approximately 25 minutes

Using the link below to simulate a rocket launch, try to overcome the gravitational ‘pull’ of the Earth – the Earth’s gravitational field. In the following simulation you can change the mass, thrust, thrust time, drag and mass change. See how far you can launch a rocket into space. Now record what happens when you change the parameters as follows.

Task 1

Mass change = ‘off’
Drag forces = ‘on’
Time = 5 s
Thrust = 400 N
Mass = 1 kg

Answer

Your rocket exploded because the thrust was too high for the size of rocket you selected. Try again by selecting a smaller thrust or a larger mass for your rocket.

Now try Task 2 where the mass has been increased but all the other variables have been kept constant.

Task 2

Mass = 20 kg
Thrust = 400 N
Time = 5 s
Drag forces = ‘on’
Mass change = ‘off’

Answer

You should have made a successful launch. Did your rocket also reach a maximum height of 267 m?

The ‘extra challenge’ is to switch off drag and mass change to see how the flight of the rocket would be different. Try this now in Task 3. Again, we have only changed one variable, keeping the others constant.

Task 3

Mass = 20 kg
Thrust = 400 N
Time = 5 s
Drag forces = ‘off’
Mass change = ‘off’

Answer

Again, you should have made another successful launch. This time the rocket reached a maximum of 270 m, only 3 m more than the last one.

Task 4

Now see if you can beat our best result of a maximum height of 14 769 m (14.769 km) and a maximum speed of 517 metres per second (m/s).

Discussion

To do this, you need to use the following parameters.

Mass = 5 kg
Thrust = 400 N
Thrust time = 5 s
Drag forces = ‘off’
Mass change = ‘on’

There are many other online rocket simulations. Here are a few to explore if you have time.

So now, when people say ‘It’s not rocket science’, you can reply by saying that it is!

3.2 Location of launch sites

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

You should now appreciate how difficult it is to send a rocket into space. But what about the location of the launch site on Earth? Take a look at Figure 11 and then complete Activity 5.

Figure _unit2.4.6 Figure 11 Worldwide rocket launch sites.

Long description

Activity _unit2.4.2 Activity 5 Launching a rocket

Allow approximately 15 minutes

Using the information in Figure 11, answer the following questions.

1. Where do you think the best places on Earth to launch a rocket from are? Choose the one correct option below.

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Discussion

At the Equator, Earth is rotating at nearly 1700 kilometres per hour (km/h). If the rocket is launched half-way between the Equator and the North or South Pole, the speed is reduced by nearly 500 km/h. This makes it harder for the rocket to escape Earth’s gravity.

Now choose the correct options to complete the following statements.

2. The largest number of launch sites are in

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Answer

The largest number of launch sites are in Russia and the USA.

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How can you calculate the acceleration of a rocket from the launch pad to the edge of the Earth’s atmosphere? To do this, you need to use one of Newton’s Laws. Here, you need the Second Law which relates force (F), mass (m) and acceleration (a) as shown in Equation 1.

Equation label: Equation 1

You need to change the mass of the rocket – in this case, say 40 tonnes – into kg. So, the mass m = 40 × 1000 kg = 40 000 kg = 4 × 10⁴kg.

Question 4 in Activity 5 provided an approximate value of the thrust of such a rocket, that is, F = 800 000 N = 8 × 10⁵N.

Now, if you rearrange Equation 1 and substitute in these values, you get Equation 2.

Equation label: Equation 2

Here you can see that the derived units of newtons (N) have been broken down into the fundamental units of kilograms (kg), m and s. The numerical value in Equation 2 is just over twice the value of the acceleration due to gravity (g) near the surface of the Earth, which is 9.81 m/s² (or m s^–2). It means that, for every second you fall under Earth’s gravity, you travel another 9.81 m/s faster.

Now you’ve calculated how to get to the ISS. But how does it stay up in Earth’s orbit?

4 How the ISS stays up there

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

The ISS is just a satellite in orbit. It wasn’t launched in one go. It is a combination of many smaller satellites launched over several years and joined together. It is obviously a massive satellite when compared with the others. It does occupy a very low orbit though, mainly to avoid all of the other satellites and especially space debris. This was shown graphically in the film Gravity (2013)!

However, because of this relatively low orbit on the edge of the Earth’s atmosphere, the ISS must be reboosted occasionally. The Earth’s atmosphere slows it down, resulting in the ISS falling slowly back to Earth.

You can now test your knowledge in the next activity.

Activity _unit2.5.1 Activity 6 A test on the ISS

Allow approximately 15 minutes

1. How high is the ISS from Earth? Choose the correct answer from the options below.

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2. Using the correct answer from Question 1, what is the approximate percentage of the distance from the ISS to the Earth’s surface compared with the radius of the Earth? (Hint: divide the distance in Question 1 by the radius of the Earth (about 6000 km) and then multiply by 100. This is your answer in %.)

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Answer

This is about 7%.

3. The ISS’s speed is approximately:

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Answer

The ISS’s speed is approximately 28 000 km/h.

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But what keeps the ISS in orbit around the Earth? Your first (and correct) answer is probably ‘gravity’. This ‘force’ of gravity’ gives the feeling of weight. But people often mix up scientific terms in everyday speech. For example, how often do you hear people saying that they need to lose weight? Mass and weight are not the same. Mass is a measure of the amount of ‘stuff’ you have and is measured in kg, whereas weight is a force, due to gravity, which is measured in N.

This force of gravity helps the ISS to orbit the Earth in a similar way to the Moon orbiting the Earth. The speed of the ISS and the Moon around the Earth are just enough to keep them orbiting.

You know that the Moon is a natural satellite of the Earth (that is, not manufactured), but what types of satellite are launched by humans into orbit? Two particularly distinct orbit trajectories are:

geostationary – staying above the same part of the Earth (for example, Sky TV)
polar orbiting (for example, monitoring weather systems).

These are shown in Figure 12.

Figure _unit2.5.1 Figure 12 Geostationary and polar orbiting satellites.

Long description

Various global positioning and communication satellites are placed in orbits that are somewhere between geostationary and polar.

But how else other than gravity does the ISS stay in motion around the Earth? The answer to this is circular motion. Watch Video 3 which demonstrates circular motion with water in a bucket. Pay attention to what happens to the water when the swinging motion is stopped.

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Video _unit2.5.1 Video 3 Circular motion of water.

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Transcript: Video 3 Circular motion of water.

TOM:

OK, the first thing we do is take the bucket. We have the water in the bucket, and we start to emulate circular motion by gradually building up speed, and then confidently swinging it around in a motion where the water stays in the bucket. If you slow it down very carefully, the water doesn't fall out of the bucket.

The trick now is to show you circular motion as we're going around, what happens when the force stops. The water falls out of the bucket. At the wrong time, the water will fall out and continue towards the centre. As proof, there's very little water left in the bucket.

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In the next video, circular motion is demonstrated using a bicycle. Watch this video before moving onto the first practical experiment in this course.

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Video _unit2.5.2 Video 4 Circular motion of a bicycle.

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Transcript: Video 4 Circular motion of a bicycle.

[SIGHING]

HELEN:

So I may not be the best bicyclist in the whole world, but ever since I learned to ride, and probably when you learned to ride as well, we learnt that when we go around the corner on our bicycles, we have to lean into the corner so that we don't fall over. And that's all about balancing forces, something we do very naturally when riding our bikes. And this next practical is all about motion in a circle and learning about how we balance those forces and how those forces help us to maintain that motion in a circle.

So Tom and I are now back inside in the warm in the lab. And we've gathered together all the things we need to do the experiment about motion in a circle. First of all, we've got a jar of lentils, pasta, rice. Anything like that will do. Second, we've got some plastic cups. This one is a clear one. This one is a paper one. Either one is just as good. And what we need to do is we need to take the cup, and we need to make a small hole on either side of the cup.

Now, I happen to be using the point of a compass, but you could use scissors or actually a pencil. Either one will do. And what you need to do is thread some string through each side of the cup to make something like this, which Tom made earlier-- a piece of string with two knots on it that can hang equally from either side of the cup. And now what I'm going to do is I'm going to ask Tom to fill the cup as bravely as he feels like with some lentils from the jar. Can you do that, Tom?

TOM:

Thank you, Helen. Well, I'm going to take the jar of lentils. I'm going to place some of these into the cup. I think that should be enough.

HELEN:

OK, Tom. Why don't you go and give that cup a whirl around in a circle?

TOM:

I shall certainly try, Helen. Thank you.

HELEN:

Now, the important thing for Tom to do is not to spill any of the lentils. Let's see if it's possible.

[LAUGHTER]

Well done, Tom.

TOM:

Thank you, Helen.

[CLAPPING]

There we go, and not a single lentil lost.

HELEN:

Wow, Tom. That was amazing. But I'm really confused. At some points, the cup was clearly upside down. How did the lentils not all just fall out?

TOM:

And it's amazing when you look at that. The fact that the lentils don't fall out, the reason behind that is the forces are balanced. So as it's going round in circular motion, the forces are balanced so that the lentils stay in as it's going round in a circle, very similar to what happens in the International Space Station. Now, you need to look at the text now as we move again to talking about circular motion and the International Space Station, how it stays in orbit around the Earth.

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4.1 Practical experiment 1

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

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Video _unit2.5.3 Video 5 Demonstration of Practical Experiment.

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HELEN:

TOM:

Thank you, Helen. Well, I'm going to take the jar of lentils. I'm going to place some of these into the cup. I think that should be enough.

HELEN:

OK, Tom. Why don't you go and give that cup a whirl around in a circle?

TOM:

I shall certainly try, Helen. Thank you.

HELEN:

Now, the important thing for Tom to do is not to spill any of the lentils. Let's see if it's possible.

[LAUGHTER]

Well done, Tom.

TOM:

Thank you, Helen.

[CLAPPING]

There we go, and not a single lentil lost.

HELEN:

Wow, Tom. That was amazing. But I'm really confused. At some points, the cup was clearly upside down. How did the lentils not all just fall out?

TOM:

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Practical experiment 1: Modelling how the ISS stays in motion around the Earth with the cup-of-rice experiment

To do this experiment, you need to be in a place with lots of space, ideally in a garden or a field. You will need a plastic cup, a piece of string and some rice. Cut two holes in the plastic cup opposite each other, thread the string through and tie it off. Half-fill the cup with rice. Taking care to avoid hitting anyone, swing this briskly around in a vertical circle.

To keep the cup moving, you need to exert a force (in the string, called tension). To increase the force on the cup, you need to increase the speed. If you put more rice into the cup, the mass has increased, so it will move slower. These are all factors which will change the motion of the cup. These variables are included in Equation 3 where F = force (N), m = mass (kg), v = velocity (speed) and r = radius (m).

Equation label: Equation 3

If the mass m increases, the force F increases (when r and v are constant).

If v increases, F increases (when r and m are constant).

However, if r increases, Fdecreases (when v and m are constant).

If m = 5 kg, and you swing it on a string where r = 1 m and v = 4 m/s, then, using Equation 3, you have a resultant force with a numerical value, as shown in Equation 4.

Equation label: Equation 4

Note that, while the forces are balanced in the rice cup experiment, the rice itself is in freefall while in motion, so that it doesn’t fall out of the cup. You know from Equation 3 that the units for force in Equation 4 should be N. This is the same as kg m/s².

In terms of the orbit of the ISS, obviously there is no string connecting it to Earth! There must be some other kind of force acting on it. The gravitational pull of Earth is the force F acting here. However, if you then add the effects of friction from the atmosphere, the ISS slows down. It is no longer able to maintain the same gravitational orbit and you would have to either increase v or push it out to a higher orbit.

5 Is there any gravity on the ISS?

The Open University — Wed, 17 Oct 2018 15:05:47 GMT

‘Weightlessness’ is where you don’t experience the force of contact, for example when on a fairground ride or skydiving. It occurs when the forces are balanced. This feeling doesn’t mean that there are no forces, however. For example, parachutists reach what is known as terminal velocity when the force of gravity is equal to the air resistance. These forces are balanced, although the speed of the parachutist is high and too dangerous to land! When a parachute is opened, it increases the drag forces to the point where a lower terminal velocity is achieved, and the forces are balanced again (see Week 6).

Most people are familiar with the term ‘g-force’ which tells you how many times heavier you feel compared with the everyday experience of 1 g. Astronauts frequently train in ‘high g-force centrifuge’ environments to prepare them for space travel. But what is ‘zero-g’? The gravitational force has not disappeared but there is a feeling of weightlessness.

The correct term is microgravity. It happens whenever an object is in freefall. You now know that the ISS orbits the Earth at a distance of 400 km and travels at a speed of 28 000 km/h. Remember that the astronauts are travelling at the same speed as the ISS. Both the astronauts and the ISS are in orbit around (or about) the Earth, which also means they are in a continual state of freefall towards the Earth.

But how does gravity itself relate to masses and weights? Equation 5 shows how the weight W of an object can be calculated when you know its mass m and the acceleration due to gravity g.

Equation label: Equation 5

If you have an object with a mass of 50 kg then, using Equation 5, you can calculate its weight (Equation 6).

Equation label: Equation 6

Clearly, the units of weight are N, so weight is a force.

Now try the next activity, where you can calculate the weight of the same object on different planets.

Activity _unit2.6.1 Activity 7 Calculating weights on different planets

Allow approximately 15 minutes

Use Equation 6 to calculate the answers to the following questions. The numerical value of acceleration due to gravity g on each planet is given.