4.3

Stopping Distance of a Car

Student Notes

This simulation shows the distance covered by a car when stopping. By adjusting the initial values of the car's speed, the driver's reaction time, and the coefficient of friction, you should be able to see what effect each of these parameters has on the stopping distance. In this situation, during the reaction time interval the car travels at constant speed. The brakes are then applied, and the car decelerates at a constant rate (equal to what?). Here are some things to consider:

1. | Before running the simulation, can you predict how far the car will travel? How far will it travel during the reaction period? How far will it travel after the reaction period, when the brakes are applied?
2. | Does changing the coefficient of friction affect the total distance covered? What about the distance covered during the reaction time? Why?
3. | How does the initial speed of the car impact the distance traveled after the reaction period? If the speed is changed by a factor of 2, by what factor does this distance change? Why?
4. | Investigate the effect of the driver's reaction time by running the simulation with zero for the reaction time, and then re-running it with the maximum possible reaction time. In particular, notice how the graphs of position and velocity change. Is it easy to see on the graphs exactly when the brakes are applied?

Self-test

1. | Before applying the brakes, the car has plenty of kinetic energy. After the car has stopped, where has this energy gone?
2. | Driving at a speed of 100 km/h and assuming a coefficient of friction of 0.9 (a reasonable assumption under good driving conditions), how long does it take you to stop your car, and how much distance do you cover, if your reaction time is 0.2 s?
3. | The mass of the car is not given in this simulation. Does the mass matter?

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