Question

A father and son pushed their car to bring it to the side of a road as it had stalled in the middle of the road. They experienced that they had to push with all their might initially to move the car. But the push required to keep the car rolling was smaller. Explain.

Answer 1

Laws of friction at work here!!!...when we push something against friction then we have to apply force so as to just move the thing..when we apply force static friction comes into play ...it does not allow the object to move..when we increase force value of static friction also inc....but at one point when we keep increasing the force the static friction cannot increase any more...at this moment the object is about to move...when we apply more force the object stars moving and then friction decreases a little bit as the motion starts and then finally it becomes constant and then we can apply a constant force to move the object (i.e we don't need to keep increasing our force) ....this to overcome static friction we need to apply a larger force to move an object

Answer 2

Because initially they had to apply force to set the car in motion but once the car started rolling, they had to apply force only to balance rolling friction of the car, the value of which is very less.