A car of mass 300kg is brought to rest in 4s from the speed of 20m/s. If there is no resistance to motion, you are required to find the force exerted by the brakes, assuming it to be constant?

To find the force exerted by the brakes, we can use the equation:

Force = mass * acceleration

First, we need to find the acceleration of the car. We can use the equation:

acceleration = (final velocity - initial velocity) / time

acceleration = (0 - 20) / 4
acceleration = -5 m/s^2

Now, we can calculate the force exerted by the brakes:

Force = 300kg * (-5m/s^2)
Force = -1500 N

Therefore, the force exerted by the brakes to bring the car to rest in 4 seconds is 1500 N.

Given:
Mass of the car, m = 300 kg
Initial velocity, u = 20 m/s
Final velocity, v = 0 m/s
Time taken, t = 4 s
Force exerted by the brakes, F = ?

Using the first equation of motion, we have:
v = u + at
0 = 20 + a(-4)
a = -5 m/s²

Now, using Newton's second law of motion, we have:
F = ma
F = 300 kg × (-5 m/s²)
F = -1500 N

Therefore, the force exerted by the brakes is 1500 N. The negative sign indicates that the force is acting in the opposite direction to the motion of the car.