Ramp Height and Toy Car Speed
Students investigate how the release height of a ramp affects a toy car's speed. They use the same 0.25 kg car and the same 1.2 m ramp. The car is released from rest at heights of 0.20 m, 0.40 m, and 0.60 m above the table. A 2.0 m flat section follows the ramp. Light gates record the time for the car to travel that 2.0 m section. The average times for three trials at each height are 1.80 s at 0.20 m, 1.40 s at 0.40 m, and 1.10 s at 0.60 m. On one 0.60 m trial the car rubbed against a side rail and took 1.45 s; the group excluded that trial. A student says, 'The higher ramp makes the car heavier, so it moves faster.' Another student says the car's mass never changed. The group wants to know whether their data support a claim about height, not mass.
1. Calculate the average speed of the car on the 2.0 m section for each release height. Describe the relationship between release height and speed, and explain whether excluding the rubbing trial is reasonable.
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Use speed = distance / time.
- 0.20 m: 2.0 m / 1.80 s = 1.11 m/s
- 0.40 m: 2.0 m / 1.40 s = 1.43 m/s
- 0.60 m: 2.0 m / 1.10 s = 1.82 m/s
The speed increases as release height increases, but it does not double when height doubles. Going from 0.20 m to 0.40 m increases speed by only about 29%, not 100%. Excluding the rubbing trial is reasonable because the car hit the side rail, adding friction that was not part of the intended test. If that trial were included, the 0.60 m result would be lower than the valid trials, but the mechanical problem makes it a poor measure of the effect of height.
2. Evaluate the student's claim that 'the higher ramp makes the car heavier, so it moves faster.' Use the setup and the data.
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The claim is incorrect because the car's mass stays 0.25 kg at every height. Raising the release height increases the car's gravitational potential energy, because potential energy depends on height. As the car rolls down, that stored energy is converted mostly into kinetic energy, so the car speeds up. Mass is not created by a taller ramp. In fact, if the car were made heavier while the height stayed the same, the gravitational potential energy would increase, but the kinetic energy for a given speed would also increase; the simple data here do not show that extra mass by itself causes higher speed. The pattern is best explained by greater height producing greater energy conversion, not by a change in mass.
3. Describe one controlled variable and one change the group could make to test whether mass affects speed without changing the height.
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One controlled variable is the release height; it should stay at the same value, such as 0.40 m, for every mass trial. Other controlled variables could include the same ramp, same surface, same car shape, and same starting position. To test mass, the group could add identical small masses to the car, for example 0.10 kg, 0.20 kg, and 0.30 kg, while keeping the release height and ramp unchanged. They would measure the time over the same 2.0 m section and calculate speed. They should repeat each mass several times and average the results so that random timing errors do not decide the conclusion.