Physics · Laws Of Motion · NEET
No. This is the most tested point in NEET. Action and reaction always act on TWO DIFFERENT bodies, never on the same body. If body A pushes body B, the action is the force on B (by A) and the reaction is the force on A (by B). One force is on B, the other is on A. Because they act on different bodies, you can never add them or cancel them to get zero net force on one body.
Neither. There is no cause-and-effect order. NCERT states clearly there is no cause-effect relation in the third law. The force on A by B and the force on B by A act at the SAME instant. Because of this, any one of the pair may be called action and the other reaction. So do not think action happens first and reaction follows later.
Yes, always. The two mutual forces between two bodies are always equal in size and opposite in direction: F(AB) = -F(BA). This is true even if the two bodies have very different masses. A truck hitting a small car pushes the car with the same force that the car pushes the truck. What differs is the ACCELERATION each gets, because a = F/m, and the smaller mass gets a much bigger acceleration.
Because action and reaction act on DIFFERENT bodies, they do not cancel on one body. When you walk, your foot pushes the ground backward (action), the ground pushes your foot forward (reaction). The forward force is on YOU, so you move forward. The backward force is on the ground, not on you. Since the two forces are on different bodies, the pair never sums to zero on your body.
Yes. A key rule: action and reaction are always the SAME type of force. If the action is gravitational, the reaction is also gravitational. If the action is a normal contact push, the reaction is also a normal contact push. The Earth pulls you down by gravity, and you pull the Earth up by gravity with the same force. They are a matched pair of the same nature.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
To every action, there is always an equal and opposite reaction. In symbols, the force on body A by body B equals the negative of the force on B by A: F(AB) = -F(BA). The two forces are equal in magnitude and opposite in direction.
When a gun is fired, the gun pushes the bullet forward (action) and the bullet pushes the gun backward (reaction), which is the recoil. Other examples: walking (you push ground back, ground pushes you forward), swimming (you push water back, water pushes you forward), and rocket propulsion (rocket pushes gas down, gas pushes rocket up).
Because they act on two different bodies. Forces can only cancel when they act on the same body. Since the action is on one object and the reaction is on the other object, neither body feels both forces, so there is nothing to cancel out on a single body.
Yes. It works for contact forces like pushing and normal reaction, and also for non-contact forces like gravity, electric, and magnetic forces. For example, the Earth and an apple pull each other with equal and opposite gravitational forces.
The Third Law tells us the force on the bullet by the gun equals the force on the gun by the bullet in size (opposite direction). The Second Law then tells us the acceleration: since the bullet has small mass it gets a large acceleration, while the heavy gun gets a small acceleration (small recoil velocity). Together they lead to conservation of momentum.
