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

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

Force = mass x 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 find the force exerted by the brakes:

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

Therefore, the force exerted by the brakes is 1500 N.

Step 1: Calculate the initial kinetic energy of the car.

```
Ek = (1/2)mv^2
Ek = (1/2)(300 kg)(20 m/s)^2
Ek = 60,000 J
```

Step 2: Calculate the change in kinetic energy of the car.

```
ΔEk = Ek_final - Ek_initial
ΔEk = 0 J - 60,000 J
ΔEk = -60,000 J
```

Step 3: Calculate the force exerted by the brakes.

```
Force = ΔEk / time
Force = -60,000 J / 4 s
Force = -15,000 N
```

Therefore, the force exerted by the brakes is 15,000 N.

Note that the negative sign indicates that the force is acting in the opposite direction to the motion of the car, which is what is needed to slow it down.