1. Statement of Newton’s Third Law
Newton’s third law of motion states:
“For every action, there is an equal and opposite reaction.”
This means that forces always come in pairs. When one object applies a force on another, the second object applies an equal force back on the first object.
1.1. Understanding Action and Reaction
The action force and reaction force act on different objects. They are equal in strength but opposite in direction.
These forces happen at the same time — not one after the other.
2. How Action–Reaction Forces Work
Whenever two objects interact, they push or pull each other with equal force. This happens whether the objects touch each other or act from a distance.
2.1. Equal in Magnitude
If object A pushes object B with a force of 10 N, then object B pushes object A back with a force of 10 N.
2.2. Opposite in Direction
The direction of the reaction force is opposite to that of the action force.
3. Examples of Newton’s Third Law
The third law can be seen in many simple activities we do daily.
3.1. Walking
When you walk, your foot pushes the ground backward (action). The ground pushes your foot forward (reaction). This forward push makes you move.
3.2. Rocket Motion
The gases expelled downward from the rocket act as the action force. The rocket moves upward due to the reaction force.
3.3. Jumping
When you jump, you push the ground downward (action). The ground pushes you upward (reaction), lifting you into the air.
3.4. A Boat Moving Backward When You Step Forward
When you step forward inside a small boat, your foot pushes the boat backward. The boat moves backward due to the reaction force.
4. Action–Reaction Does Not Mean Balanced Forces
Action and reaction forces are equal and opposite but they act on different objects, so they do not cancel each other.
4.1. Why They Don’t Cancel
Balanced forces act on the same object. But action and reaction forces act on two different objects, so their effects are different.
4.2. Example
When a ball hits a bat, the bat exerts a force on the ball and the ball exerts an equal force on the bat. Both objects experience acceleration depending on their mass.
5. Third Law in Non-Contact Forces
Newton’s third law also applies to forces that act without touching.
5.1. Gravitational Force
Earth pulls you downward (action), and you pull the Earth upward with an equal force (reaction). But because Earth’s mass is huge, it does not move noticeably.
5.2. Magnetic Force
A magnet attracts an iron piece, and the iron piece pulls the magnet with the same force in the opposite direction.