# Physics QuestionWatch

#1
A car is travelling at a speed of 20 m/s when the driver applies the brakes. The car decelerates at a constant rate and stops,

How much work is done by the braking force to stop the car and driver?

This is what I know:

Work done is equal to force times distance. But you don't get given a value for force nor do you get one for distance.

I know that force is equal to mass times acceleration, but if the car is decelerating, then you'd get negative acceleration...

wouldn't it mean that the whole value for the work done is negative X joules?
0
4 years ago
#2
The reason you would get a negative work is because you're calculating the work done on the car, when actually the car is doing work itself (and so work done on the car is negative).

There's an easier way to look at it, however. Work and energy are the same (we measure them using the same units after all), and so you just need to calculate the change in energy. Which energy is changing?
0
#3
(Original post by tory88)
The reason you would get a negative work is because you're calculating the work done on the car, when actually the car is doing work itself (and so work done on the car is negative).

There's an easier way to look at it, however. Work and energy are the same (we measure them using the same units after all), and so you just need to calculate the change in energy. Which energy is changing?
Well, if the car starts off moving and stops, I assume it's the kinetic energy which is changing. Am I right?
0
4 years ago
#4
(Original post by flibber)
Well, if the car starts off moving and stops, I assume it's the kinetic energy which is changing. Am I right?
Correct. If you want a numerical answer, you need the speed and mass of the combined driver and car. If you can use a symbolic answer then just calculate the change in kinetic energy.
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