Chapter 8

NCERT
Class 9
Science
Solutions
11. According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside. It will probably not move.

Question:

According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside. It will probably not move, A student justifies this by answering that two opposite and equal forces cancel each other comment on this logic and explain why the truck does not move.

Answer:

The mass of the truck in question is large so that its inertia is also high. Due to greater inertia, the truck is at rest. This is from Newton's first law of motion. The statement given by the student is not correct. Two equal and opposite forces cancel each other, when they act on the same object. According to Newton's third law of motion, two equal and opposite forces act on two different objects, so they do not cancel each other. Due to large mass of truck, the acceleration produced in it is negligible so truck can not move.

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