Question

4. A car of mass 1200 kg travelling at 20 m/s brakes and comes to rest in 4

seconds. What is the average force exerted by the brakes on the car?*

Answer 1

Step-by-step explanation:

The car has an average acceleration of

Then, by Newton's second law, the average force exerted by the brakes on the car is

which is to say, the average force has a magnitude of 6000 N and is directed opposite the car's motion.

Answer 2

The car has an average acceleration of

`a_(\rm ave) = (\Delta v)/(\Delta t) = (0(\rm m)/(\rm s) - 20(\rm m)/(\rm s))/(4\,\mathrm s) = -5(\rm  m)/(\mathrm s^2)`

Then, by Newton's second law, the average force exerted by the brakes on the car is

`F_(\rm ave) = ma_(\rm ave) = (1200\,\mathrm{kg})\left(-5(\rm m)/(\mathrm s^2)\right) = \boxed{-6000 N}`

which is to say, the average force has a magnitude of 6000 N and is directed opposite the car's motion.