Question

A car of mass 900 kg moves at 10 m/s. What is the braking force required to stop the car in a time of 5 seconds?

Answer 1

The average veloicity during braking is 5m/s, so in five seconds, it goes 25m.

brakingforce*5m=1/2 900 10^2
solve for braking force.

Answer 2

well if F=ma and a=v-u/t (final velocity-initial velocity/time) so combining these two equations we get F=m(v-u/t) which gives us F=mv-mu/t

if the car is to stop then obviously the final velocity is zero (now this will give us a negative answer but that is OK because it is a retarding force)

so our equation is F = (0-9000)/5
which = -1800
so therefor that is 1800 Newtons of force required to stop the car from moving.

If you have any questions ask away because i know this can get a little confusing.