Question

A car of mass 1000 kg is moving at speed of 20 m/s on horizontal surface if the car moves and come to rest after traveling 50m distance then what force of friction tends to stop it ?

Answer 1

We can find the force of friction from the work-kinetic energy theorem:

$$\Delta KE=W,$$ $$KE_f-KE_i=-F_{fr}d,$$ $$-\dfrac{1}{2}mv_i^2=-F_{fr}d,$$ $$F_{fr}=\dfrac{mv_i^2}{2d},$$ $$F_{fr}=\dfrac{1000\ kg\cdot(20\ \dfrac{m}{s})^2}{2\cdot50\ m}=4000\ N.$$