Imagine trying to walk on a smooth icy surface. The moment you try to move forward, your feet slip backward. But on a normal road, you can walk easily without even thinking about it. Have you ever wondered why?
The answer lies in one of the most important laws of motion given by Sir Isaac Newton — Newton’s third law of motion. This law explains not only walking, but also swimming, flying, rocket launches, jumping from boats, gun recoil, and many other real-life phenomena.
To understand motion properly, we must first understand that forces never act alone.
Newton’s Third Law of Motion:
Newton’s third law states that, “For every action, there is an equal and opposite reaction.”
This means whenever one object exerts a force on another object, the second object also exerts an equal force back on the first object in the opposite direction.
The two forces are called:
- Action force
- Reaction force
These forces:
- are equal in magnitude,
- opposite in direction,
- and act on different objects.
Because they act on different objects, they do not cancel each other.
Understanding the Law Through Walking:
When we walk, it may look like our legs simply move us forward. But actually, walking happens because of action and reaction forces.
When your foot pushes the ground backward, the ground pushes your foot forward with equal force. This forward push from the ground moves your body ahead.
Walking on the road
Without this reaction force, walking would not be possible.
This is why walking becomes difficult on ice. Ice provides very little friction, so the ground cannot push your feet forward properly. As a result, your feet slip.
Here we also understand the importance of frictional force. Friction opposes motion, but it is also necessary for walking, running, and driving.
Applying Newton's Law in real life:
| Real life situation | Newton's law applied | Explanation |
| Passenger jerks forward when a bus stops suddenl | Newton’s First Law | The passenger’s body tends to continue moving due to inertia even after the bus stops. |
| A football moves faster when kicked harder | Newton’s Second Law | Greater force applied on the football produces greater acceleration. |
| Rocket launching into space | Newton’s Third Law | The rocket pushes gases downward, and gases push the rocket upward with equal and opposite force. |
Analysing Forces Acting on Objects:
In real life, many forces act together on objects.
Consider a cyclist riding a bicycle.
Several forces act simultaneously:
| Force | Effect |
| Pedaling force | Pushes bicycle forward |
| Friction | Opposes motion |
| Gravity | Pulls downward |
| Normal reaction force | Pushes upward from road |
| Air resistance | Slows motion |
Forces acting on a cyclist
If pedaling force becomes greater than friction and air resistance, the bicycle accelerates.
If all forces become balanced, the bicycle moves with constant speed.
Why Action and Reaction Forces Do Not Cancel:
Students often ask:
“If action and reaction forces are equal and opposite, why doesn’t everything remain stationary?”
The answer is simple:
Action and reaction forces act on different objects.
For example:
- rocket pushes gases downward,
- gases push rocket upward.
Since the forces act on separate objects, they do not cancel.
Forces cancel only when:
- they act on the same object,
- and are equal and opposite.