Bouncing ball experiment

A squash ball is a hollow ball made of rubber, with air inside. Before starting a game of squash, most players will "warm up" the ball by knocking it around the court. This raises the temperature of the ball and increases the "bounciness". In this experiment you can investigate the effect of temperature on the height to which a ball bounces. An alternative experiment is to investigate how the height of each successive bounce changes.

Planning your experiment

The height to which the ball bounces will depend on the initial height and the temperature, so you must decide which of the variables you are going to investigate and how you can ensure that your experiment is a "fair test". You must then decide the range over which you are going to make measurements and how many measurements you should make. e.g. if you were varying the temperature of the ball, what would be suitable value for the maximum temperature? Should you make measurements every degree, every 5 degrees, every 10 degrees, etc.

How could you change the temperature of the ball and measure the temperature? Should the heights be measured from the top, bottom or middle of the ball? In a real experiment, it is obviously not possible to stop the ball at the top of its bounce - so how could you measure the height of the bounce? In an actual experiment, what precautions could

you take to ensure that the results were as accurate as possible?

Making a prediction

Try to predict what your results will show. E.g. if you were planning to change the temperature of the ball, would you expect the bounce height to increase or decrease as the temperature is increased? Can you think of a scientific reason for your prediction?

Fair tests

In any experiment there are usually several factors or variables that you could change. For example, suppose you were asked to investigate what affects the rate at which a ...

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you take to ensure that the results were as accurate as possible?

Making a prediction

Fair tests

In any experiment there are usually several factors or variables that you could change. For example, suppose you were asked to investigate what affects the rate at which a container of hot water cools down. Some possible variables might be

The volume of water.
The starting temperature of the water.
The size and shape of the container.
Whether the container was insulated.

You might then decide to measure the temperature of the water at say 1-minute intervals as it cools down. To make the experiment a fair test, you would need to change just one of these variables and keep the rest

constant. I.e. if you did one experiment with 100 cm3 of water starting at 80 oC in an insulated container and then a second experiment with 50 cm3 of water starting at 60 oC in an uninsulated container, you

Couldn’t compare the two because you wouldn’t know if any differences were caused by the different masses of water, the different starting temperatures or the different containers.

Processing your data

Usually, the best way to display your results is in the form of a graph. (Remember that all graphs should have titles and the axes should have labels and units).

The points you should cover are:

Do your results support your prediction?
Are there any trends or patterns in your results?
Would you expect your graphs to go through the origin?

Energy changes in a bouncing ball experiment

The screen on the left shows the energy changes that take place in the form of a bar chart. You can move through the sequence by using the scrollbar. Alternatively you can automate the sequence by clicking the drop button to start the motion. You can stop the motion at any point and reset to the start.

Initially, before the ball is released energy is stored a gravitational potential energy. (mgh)As the ball falls its speed increases and the potential energy is converted to kinetic energy (½mv2 ) of the moving ball until at the moment of impact with the ground it has no potential energy. Notice that if there are no energy losses the total energy (i.e. PE + KE) stays constant.

When the ball hits the ground it begins to slow down and as it does so it deforms. What's happening here is that the kinetic energy of the ball is being used to do work deforming the ball. Some of this energy is stored as potential energy in the deformed ball (sometimes called elastic potential energy) and some is converted to heat and sound.

When the ball reaches zero speed and maximum deformation, it has no kinetic energy (its not moving) and because some energy has been converted to heat and sound, its elastic potential energy is less than its

initial gravitational potential energy. Again notice that the total energy (elastic PE + heat +sound) is constant.

As the ball returns to its original shape, some of the energy that was stored as elastic PE is converted back to kinetic energy, heat and sound. I.e. the ball starts to bounce.

Once the ball leaves the ground it will start to slow down as it rises and its kinetic energy is converted back to gravitational potential energy. Because some of its initial energy has been converted to heat and sound it will finish up with less gravitational energy than it started with. I.e. the rebound height is less than the starting height.

Deformation

For a soft ball like a squash ball the deformation of the ball is quite large. (During a squash game the ball is completely flattened when it hits the wall of the court.) For more rigid balls, the deformation is less. Steel ball bearings and glass marbles will bounce quite effectively when dropped onto a hard surface and in these cases the deformation is very small indeed , but because it takes a very large force to produce the small deformation, the energy stored can still be large.