What average force is required to stop a 1600 kg car in 9.0 s if the car is traveling at 80 km/h?
First, we must calculate the acceleration of the car, but before we begin, we should get the initial speed into SI base units:
(You may find it easier to just remember that to change km/h to m/s you divide by 3.6)
Now, the acceleration
With this, we are able to apply Newton's second law:
The negative sign means that the force acts in a direction opposite the that which the car is travelling.