Question

A 1000kg car moving at 97.2 kph is stopped in 2 seconds, how much force was required to this?

Answer 1

The initial momentum of the car is:

$$p_i = 1000kg\cdot 97.2kph = 1000kg\cdot 27m/s = 27000kg\cdot m/s$$

The final momentum of the car is $p_f = 0$. Thus, the force reqired to stop can be found from the second Newton's law:

$$F = \dfrac{p_i-p_f}{\Delta t}$$

where $\Delta t = 2s$. Thus, obtain:

$$F = \dfrac{27000kg\cdot m/s-0}{2s} = 13500N$$

Answer. 13500 N.