A father and son pushed their car to bring it to the side of road as it had stalled in the middle of the road. They experienced that although they had to push with all their might initially to move the car, the push required to keep the car rolling was smaller, once the car started rolling. Explain.
We need to apply larger force to move the car (heavier object) from its rest position. As the mass increases, the force of friction increases.
⇒ Once the car starts moving, the friction becomes rolling friction. Rolling reduces friction. Thus, the force applied now is less than the previous one.
⇒ So, to keep the car in motion and balanced, we only need to exert the lesser force.
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