Question

Which of the following is NOT an example of action reaction pair?

• A. (a) To walk, we push the ground in the backward direction by feet
• B. (b) When a bullet is fired from a gun, the gun recoils
• C. (c) A rocket engine moves up by ejecting gases downwards
• D. (d) A book kept on a table is acted upon by two forces, the force exerted by earth (gravitational force) due to weight of the book and the normal force, applied by the table

Answer 1

The correct option is D

(d) A book kept on a table is acted upon by two forces, the force exerted by earth (gravitational force) due to weight of the book and the normal force, applied by the table

The explanation for the correct option: (d) A book kept on a table is acted upon by two forces, the force exerted by earth (gravitational force) due to weight of the book and the normal force, applied by the table

1. Newton's third law states that for every action, there is an equal and opposite reaction.
2. The force exerted on an object is the action, and the force experienced by the object as a consequence of Newton's third law is the reaction.
3. Action and reaction forces act on different bodies. They are opposite in direction.
4. In this case gravitational force is acting on the book and normal force is also acting on the book. But we know that action and reaction forces act on different bodies. So, here gravitational force and the normal force are not an example of action-reaction pair.

Thus, option (d) is correct.

The explanation for the incorrect options:

(a) To walk, we push the ground in the backward direction by feet

1. When we push the ground we apply a force on the ground and in reaction ground also applies force on our feet in the opposite direction. So, force on the ground is the action force and force on our feet is the reaction force.
2. Action and reaction forces are acting on different bodies and also they are equal in magnitude as well as in opposite directions.
3. So, this is an example of action-reaction pair.

(b) When a bullet is fired from a gun, the gun recoils

1. While firing the bullet, the gun applies action force on the bullet in the forward direction. In reaction gun recoils in the backward direction.
2. Action and reaction forces are acting on different bodies and also they are equal in magnitude as well as in opposite directions.
3. So, this is an example of action-reaction pair.

(c) A rocket engine moves up by ejecting gases downwards

1. Here engine applies action force on gas to eject the gas downward. In reaction ejecting gas also applies reaction force to the engine in an upward direction.
2. Action and reaction forces are acting on different bodies and also they are equal in magnitude as well as in opposite directions.
3. So, this is an example of action-reaction pair.

Thus, option (d) is correct.