Microgravity: living on the International Space Station

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# 3 Practical experiment 3

Practical experiment 3: Investigating gravity

Here you will time objects that fall under gravity to calculate the acceleration of gravity on Earth. This is often called ‘little g’ or g. This terminology is used to distinguish it from Newton’s gravitational constant G, which, unsurprisingly, is called ‘big G’. You met these terms in Section 5 of Week 1 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . First, watch Video 2.

Skip transcript: Video 2 Calculating gravity.

#### Transcript: Video 2 Calculating gravity.

HELEN:
So in this experiment, we're going to measure little g, or the acceleration due to gravity, and you're going to recreate, if you like, your own little drop tower at home.
So we've done this by imagining our wall is this piece of cardboard, and we've got a tape measure. Now, ours is a haberdasher's tape measure, but a DIY one works just as well. And at 50 centimetres, we've marked very carefully with a ruler a simple line. This is going to be our drop point for our experiment.
We've tacked everything on with blue tack, and we've got all ready a handy smartphone with a stopwatch. And what are we going to drop? Well, we're going to drop some toy balls. So, let's have a go at looking at all these different balls, and as we do drop these balls, I want you to think about their size, their mass, and what's going to happen to them.
From what you've learnt so far, are some of them going to drop faster or slower? And afterwards, what you'll be able to do is watch the slow motion of each ball dropping, and then in the text, we're going to explain to you how to do the calculations and get your own value of little g.
Tom, are we ready to drop the balls?
TOM:
Here we go.
HELEN:
OK, so now you've seen us drop our balls, and we've obviously got slow motion and the advantage of finding the exact time they've taken to drop. We want you to go and find your selection of toys, and drop them and time them, and use your information with the same method as ours, following the information in the text to try and calculate your own value of little g. Let's compare the two answers.
End transcript: Video 2 Calculating gravity.
Video 2 Calculating gravity.
Interactive feature not available in single page view (see it in standard view).

Now complete Table 1 using the following guidelines.

• Collect as many objects as you can with different sizes and masses (for example, balls)
• Measure the vertical distance s (in metres, m) from where you are going to drop your object to the ground
• Time the drop t (in seconds, s). Do this for all of your objects.
• Calculate the vertical distance doubled (2 × s).
• Square the time (t2).
• Finally, record your value of the doubled distance (2 × s).

Take care. In the mathematics of motion it is quite common to use the letter s to denote distance. Watch out for also using the same letter as an abbreviation for seconds. When it is used for the unit of time, then it should be an upright symbol, s, if it means distance in an equation it will look like s. That means you have to be especially careful in handwriting!

## Table 1 Results of Practical experiment 3

ObjectDistance s / m2 sTime t / st2g = 2s / t2
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Table 2 shows the results taken from Video 2.

## Table 2 The course team’s results for Practical experiment 3

BallDistance s / m2 sTime t / st2g = 2s / t2
10.501.00.320.109.8
20.501.00.320.109.8

The calculations from the results in Table 2 provide a final answer of 9.8 m/s2 for ‘little g’. You met the acceleration due to gravity in Week 1 where the value given was 9.81 m/s2. So this experimental result is quite accurate!

Note that the two values taken from Video 2 are the same (to 2 significant figures). But what if the size of the balls was different? What about their masses? Do these factors affect the final calculations? What if you dropped a feather instead?

To help answer these questions, watch Video 3 which shows a hammer and a feather being dropped at the same time on the Moon’s surface, then complete Activity 3.

Skip transcript: Video 3 Apollo 15 mission experiment on the Moon.

#### Transcript: Video 3 Apollo 15 mission experiment on the Moon.

[BEEP]

JIM:
Can we copy to both solar wind and a [INAUDIBLE] [INAUDIBLE] in the ETB.
[BEEP]
DAVE:
Not quite, Jim. I haven't put the solar wind in yet, but I will shortly. I want to watch this. A good picture
JIM:
Beautiful picture, Dave.

[BEEP]

DAVE:
In my left hand, I have a feather. In my right hand a hammer. I guess one of the reasons we got here today was because of a gentleman named Galileo a long time ago, who made a rather significant discovery about falling objects in gravity fields. And we thought, where would be a better place to confirm his findings than on the moon?
And so we thought we'd try it here for you. And the feather happens to be appropriately a falcon feather for our Falcon. And I'll drop the two of them here. And hopefully, they'll hit the ground at the same time.

[BEEPING]

That proves that Mr. Galileo was correct in his findings.

[BEEPING]

JIM:
Superb.
End transcript: Video 3 Apollo 15 mission experiment on the Moon.
Video 3 Apollo 15 mission experiment on the Moon.
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## Activity 3 Effect of air resistance

Timing: Allow approximately 15 minutes

Choose the correct answers to the following questions.

1. If you drop a hammer and a feather together at the same time on Earth, what would you expect to happen?

a.

The hammer and feather arrive on the ground at the same time.

b.

The hammer arrives first.

c.

The feather arrives first.

d.

The hammer’s speed is reduced more than the speed of the feather.

e.

The feather’s speed is greater than the hammer’s speed.

2. What happens when a hammer and a feather are dropped together at the same time on the Moon?

a.

The hammer and feather arrive on the ground at the same time.

b.

The feather arrives first.

c.

The hammer’s speed is reduced more than the feather’s speed.

d.

The feather’s speed is greater than the hammer’s speed.

3. How do the conditions on the Moon differ from those on Earth?

a.

They are the same.

b.

There is no atmosphere on Earth.

c.

The gravity on the Moon is stronger than the Earth’s gravity.

d.

The Earth’s gravity is weaker than the Moon’s gravity.

e.

There is no atmosphere on the Moon.

You will now look at how planets are formed.

MG_1

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