Physics · Laws Of Motion · NEET
For two forces to add to zero (cancel), they must act on the SAME object. Newton's third law forces are a pair on TWO different objects: force on A by B, and force on B by A. Since they are on different bodies, you can never add them to get the motion of one body. The force on A by B is what moves A; the force on B by A is what moves B. NCERT states it plainly: 'Action and reaction act on different bodies and so they cannot be cancelled out.'
When you walk, your foot pushes the ground backward (action), and the ground pushes you forward (reaction). The backward push is ON the ground; the forward push is ON you. Only the force ON YOU decides your motion, so you move forward. In a rocket, hot gas is pushed down (action on gas) and the gas pushes the rocket up (reaction on rocket). Different bodies, so nothing cancels.
When both act on the same body. Example: a book resting on a table. Gravity pulls the book down (mg) and the table's normal reaction pushes the book up (N). Both act on the BOOK, they are equal and opposite here, so they cancel and the book stays still. But note: N and mg are NOT an action-reaction pair. The reaction to mg (Earth pulling book) is the book pulling Earth; the reaction to N (table on book) is the book on table.
The horse pulls the cart forward (force ON cart) and the cart pulls the horse backward (force ON horse). These are on different bodies, so they do not cancel. The horse moves because the ground pushes its hooves forward with a force larger than the cart's backward pull on it. Net force on the horse is forward, so the whole system accelerates.
No. This is the most common mistake. Weight (mg) acts on the body due to Earth; its reaction is the body pulling Earth. Normal force (N) acts on the body due to the surface; its reaction is the body pushing the surface. Weight and normal force both act on the SAME body, so they are a balancing pair, not an action-reaction pair.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. NCERT is clear: there is no cause-effect relation. The force on A by B and the force on B by A happen at the same instant. Either one may be called action and the other reaction.
Yes. A truck hitting a small car pushes the car with exactly the same force that the car pushes the truck. The masses differ, so the accelerations differ (small mass gets large acceleration), but the paired forces are always equal and opposite.
No. An action-reaction pair is always the same type of interaction between the same two bodies. If the action is a gravitational pull of A on B, the reaction is the gravitational pull of B on A. They cannot be a gravity force paired with a normal force.
NEET regularly tests whether you can identify true action-reaction pairs and separate them from balancing forces (like weight vs normal force). Getting this right is the base for momentum conservation, recoil, rocket motion and free body diagrams.
