Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Multiflash photographs of free fall


This provides an interesting and graphic way of obtaining a value for the acceleration of a falling object. 
Click here for a video showing free fall from which you can make measurements (please note this runs only in Internet Explorer 4+).

Apparatus and materials

Bail or other object to be photographed

Camera and stroboscope or multiflash system

Scale, 10 cm, with strong contrast

Lamp, bright (500 W) or slide projector

Box lined with soft material

Health & Safety and Technical notes

Do not allow anyone to climb on stools or benches to release the ball. Use a pair of steps (stepladder) or a kick-stool.

A steel ball (15 mm - 25 mm), golf ball, or lamp with cell are all suitable objects to drop.  

Read the Multiflash Photography guidance page for detail of specific methods and for general hints. 



The setup

a Set up the arrangement as shown in the diagram. 

set up project lamp, table stools and falling ball

b Darken the room and shine light on the falling object but not the background. See guidance page Classroom mangement in semi-darkness
c Set the stroboscope rotating. 

Making an image 
d Prepare to drop the ball from near the top of the camera's field of view. 

ruler and ball in motione Open the camera shutter using 3-2-1-0 countdown and drop the ball. 
f Close the shutter when the ball lands or leaves the field of view. 
g Record the image frequency in hertz, which is the same as the number of images per second. Use this to work out the time between each image, using time = 1/frequency. Frequency is measured in hertz (or images per second). 
Analysing the image 
h Choose the first image of the ball after its release. You can define 'time zero' to be the time at which this image was produced. Use the grid to measure the distance travelled by the object for each later image. 
i Use the time and distance data to plot a distance-time graph, manually or with a computer. Draw a smooth curve to fit the points. It will not pass through the origin if t does not = 0 when s=0. 
j The gradient of the distance-time graph, at any time, is equal in value to the velocity. Measure the gradient of the curve at several times, to obtain a set of values of time and velocity. Use this information to produce a velocity-time graph. 
k The value of the gradient of the velocity-time graph is equal to the acceleration at any time. If the ball fell under gravity and if air resistance was not significant, then the velocity-time graph should be a straight line. The ball had a constant acceleration. This is the acceleration due to gravity. 

Teaching notes

1 The accepted value of the acceleration due to gravity is 9.81 m/s2. Any experimental value in the region of 10 m/s2 is a reasonable one. 

2 If air resistance is significant compared with the weight of the falling object, then the gradient of the speed-time graph will decrease. This indicates a decreasing acceleration caused by the increasing effect of air resistance matching the increasing speed of the object. 
As an extension activity, it is worth comparing the fall of a golf ball with that of a polystyrene ball. In the latter case, air resistance is more significant relative to weight, and acceleration quickly decreases. 
3 Extension: investigate the effect of object mass on its acceleration. (Provided that air resistance is not significant, object mass should make no difference.) 
A multiflash photograph of a free falling polystyrene
This experiment was safety-checked in April 2006


Related guidance

Multiflash photography

Classroom mangement in semi-darkness