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Investigate the correlation between the height at which a ball is dropped and the height to which it bounces.

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Introduction

GCSE Physics Coursework

PLANNING

Aim

To investigate the correlation between the height at which a ball is dropped and the height to which it bounces.

Theoretical background

What causes a dropped ball to bounce?

When a ball is lifted off the floor, energy is transferred to it. This energy is stored in the gravitational force between the ball and the earth, and is called gravitational potential energy (GPE). When the ball is released, its weight makes it accelerate downward and its GPE gradually becomes kinetic energy (KE). When the ball hits the floor, both the ball’s bottom surface and the floor’s upper surface begin to distort and the ball’s KE becomes elastic potential energy in these two distorted surfaces. The ball accelerates upward during this process and eventually comes to a complete stop. When it does most of the energy that was initially GPE and later KE has become elastic potential energy in the surfaces. However, some of the original energy has been converted into thermal energy by frictional forces caused by the ball hitting the floor. The distorted ball and floor then push apart and the ball rebounds into the air. Some or most of the elastic potential energy becomes KE in the ball, and the rising ball then converts this KE into GPE.

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Middle

54

53

53

90

60

59.5

59.5

59.7

100

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110

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66.7

120

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71.3

The range most suitable for this experiment would start from 20 cm. If the ball is bounced from lower e.g. 10cm, its bounce is so quick that reading off its bounce height will be hard to do, and therefore results will be less accurate. As many heights as time allows must be done, but at least 10 readings are needed to give suitable accuracy.

Method

Safety

Don’t throw the ball down as it can bounce out of control and hit someone. Drop the ball at arm’s length so that it doesn’t bounce back up into body or face.

Accuracy and control of variables

  • Make sure that the ball has no splits, which will affect its bounce and therefore the accuracy of the measurements.
  • Keep the surface that the ball is bouncing on the same. This is important because surfaces such as carpet absorb more of the ball’s elastic energy when the ball hits it. The carpet will also produce more friction when the ball hits it so more of the original energy will be converted into waste energy. Therefore if the surfaces change, the results will be altered, so for this experiment the classroom floor (vinyl) will be the surface used.
  • Drop the ball, do not throw it. As the force for KE = ½mv², if the force is altered then the speed at which the ball travels will be altered and so the amount of energy produced will be different. Just dropping the ball will ensure that the same force is used each time, ensure a fait test and accurate measurements.
  • Ensure that there is nothing that the ball can hit against that will distort the readings. Be very careful how you drop it, so that it doesn’t hit against the wall during its bounce, and change the measurements.
  • Keep the temperature of the ball the same. If the ball is heated up, this means there will be more elastic energy when the ball bounces, and s it will travel further, and affect the speed at which the ball travels distorting the results.
  • Use the same ball each time. The weight of the ball may vary from one to another so it is important to keep this variable constant. As the formula for KE = ½mv², if the mass is changed then the energy will change.
  • Practice before doing the experiment. When first trying to read off the height that the ball has bounced to, it will prove difficult, but with practice this will become easier and so results will be more accurate.
  • Throw with the same hand each time, to ensure that the same force is used.
  • In both the preliminary and the actual experiments, check constantly to see that the ruler is at 90° to the surface. This will improve the accuracy of the measurements recorded.
  • The only variable that will change is the height the ball is dropped from. The tennis ball will be dropped from heights of 20cm, 30cm, 40cm, 50cm, 60 cm, 70 cm, 80 cm, 90 cm, 100 cm and 110 cm. Each height will be repeated 3 times and an average will be taken to ensure accurate results.
  • Using the preliminary experiment’s results, for every height put your eye level to where approximately the ball will bounce – this will improve accuracy of readings.
  • Read off the bounce height with one eye closed (the same eye). This will result in clearer vision and so the bounce height will be easier to measure.
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Conclusion

Another improvement on the experiment we did would be to calculate the energy lost when the ball bounced. We will measure the ball first and then work out the energy lost by doing this simple equation:

Gravitational Potential Energy= Mass × Gravity × Height.

Just before the ball hits the floor, the Gravitational Potential Energy is converted into Kinetic Energy.

Calculating power and work as an additional experiment could be another extension.

I could extend this experiment by looking at temperature as the variable as this would mean I could look at kinetic theory and see if I can prove it using the results form my experiment as there was no scientific model or theory that I could use in the experiment that I did.

NB all measurements are to one decimal place where appropriate                

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