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. First, watch Video 2.

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 (t ²).
• 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 2 shows the results taken from Video 2.

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.