# What average force is required to stop a 1600 kg car in 9.0 s if the car is traveling at 80 km/h?

##### 1 Answer

#### Answer:

First find the acceleration, then apply Newton's second law to show that the force is -3950 N.

#### Explanation:

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.