We worked with a group of A-level students (16-18) at Wyedean School and Sixth Form Centre (@WyedeanScience) on different ways to measure the gravitational field strength g. What we were really interested in was the students’ ability to use a range of methods and devices to calculate g from a series of measurements, agree on a justified calculation of their uncertainties and compare their results with the accepted value of g = 9.81 ms^-2.
One method they used was using the G-ball, which allowed the students to drop the ball from different heights and measure the time of fall, then from the equations of motion they could calculate g.
But a method that seemed pretty interesting, especially when the calculation of their uncertainty was done, was to use Vernier Video Physics to video a tennis ball dropping from 1 m. If you are familiar with Vernier Video Physics, you know that the app lets you track the motion of the ball on the screen and develops a displacement – time and its respective velocity – time graph. The graph the students obtained is shown below.
Having multiple bounces on the graph, allowed the students to analyse the acceleration from different episodes of the ball falling and bouncing by calculating the gradient of each bounce and fall.
And they used the gradients calculated by the app by selecting different sections to calculate a mean g.
Their mean value of g was g(mean) = (10.99 + 10.93 + 10.19 + 12.58) / 4 = 11.17 ms^-2
They also calculated the value of their uncertainty by taking the range divided by 2 as:
Uncertainty = (12.58 – 10.19) / 2 = 1.20 ms^-2
The value of g = 9.81 ms^-2 does not lie within the degree of uncertainty of this group of students, but their method was quite good and the calculations intuitive to make. We think the students learnt a lot from this process and they could appreciate the importance of calculating the uncertainties in their practical work. There are a range of factors that could have affected their experimental results, for example the position at which the ball was bouncing which might not have been aligned very well with the position chosen by the students as their origin of the axes on Video Physics, or they might have misplaced the target to plot the position of the ball on some of the frames, as the ball is moving quite fast at some points and becomes quite blurred for a 30 fps camera.