Physics' Concept -4

Classical Mechanics Level 2

A boy sits in a car and tries to push the car from inside of it.

Will the car move?

Yes, if the friction of the boy's feet and car exceeds the friction of car's tyres and ground. No , By Newton's third law , the car also applies force on the boy and the forces get cancelled. No , Because the force that the boy applies fails to overcome the friction of the tyres of the car. No , Because no external force is applied on the car. Sometimes , if the boy is too much strong and has the ability to apply tremendous force.

Nihar Mahajan by Nihar Mahajan , 20, India

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3 solutions

Nihar Mahajan
May 18, 2015

If we consider the boy and the car , whole as one system , we can easily conclude that the boy applies "internal " force and no external external force is appled on the car. Since there is no external force is applied , By Newton's first law , the car remains at rest.

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If you're going to study the interactions between the car and the boy, you can not consider them as one body, even if they are part of a system. Have you considered the principle of momentum conservation? When the boy pushes the car, the friction with the carpet will push him back in the other direction. So, if the boy moves back, the car would have to move forward, if he is resting on a surface where friction is null, in order to maintain the total momentum from before, when everyone was still, equal to the total momentum after they start moving. When you add the frinction between the car and ground, there might not be movement or there will be movemrnt, depending on the boy's strengh, but the total momentum will not be conserved.

Carlos Dias - 6 years ago

Oh, you got me! Nice question :D

Sravanth C. - 6 years ago

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Hehe.. Hard luck. I want to troll you again. So,try this.

Nihar Mahajan - 6 years ago
Aditya Kumar
May 19, 2015

We can also use the concept of center of mass in this. The cm of the car+boy doesn't change as there is no external force applied. Now, since the boy is sitting in the and not making any movement, the car won't move in order to keep the cm in place. Hence, the car won't move.

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Panshul Rastogi
May 30, 2015

Yes, because in the Free Body Diagram of an object internal forces are not included. So, the net force is zero.

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