Third law of motion

Newton's Third Law of Motion

In the study of physics, Sir Isaac Newton's laws are fundamental to understanding how motion works. The Third Law, in particular, holds great importance as it explains that every action is met with an equal and opposite reaction. This principle plays a key role in both scientific applications and daily experiences.

The Principle Behind Newton's Third Law

This law centers around the interaction between two bodies. When an object A exerts a force on object B, object B instantly responds by exerting a force of the same magnitude but in the opposite direction on object A. In simple terms, any force exerted results in a reaction force of equal magnitude in the reverse direction. This mutual exchange of forces defines Newton's Third Law.

We use the formula: F = m × a
where F = force, m = mass, and a = acceleration.

Units of Force:

In the CGS (centimeter-gram-second) system:
- Mass is measured in grams
- Acceleration is in cm/s²
- Hence, unit of force = gram × cm/s² = 1 dyne

In the SI (International System) system:
- Mass is measured in kilograms
- Acceleration is in m/s²
- Therefore, unit of force = kg × m/s² = 1 newton (N)

Conversion: 1 N = 10³ g × 10² cm/s² = 10⁵ dyne

Understanding Action and Reaction

Newton's Third Law is often expressed as: "For every action, there is an equal and opposite reaction." For instance, when you press your hand against a wall, your hand exerts a force on the wall (action), and the wall responds with an equal force in the opposite direction (reaction). This opposing force is what causes the sensation of pressure or discomfort. It’s important to note that these forces act on separate objects, so they do not balance each other or lead to equilibrium.

If two equal and opposite forces—say F₁ and F₂—act at the same point on a single object, they can cancel out and bring the object into equilibrium. However, in Newton's Third Law, action and reaction forces act on separate bodies, so they do not result in balance.

Real-Life Examples of Newton's Third Law

• Diving Board: A diver pushes down on the board; the board pushes back and propels the diver upward.
• Rocket Launch: Exhaust gases are forced downward; in return, the rocket is pushed upward into space.
• Balloon Rocket: When air escapes from the balloon in one direction, it propels the balloon in the opposite direction.

The Role of Newton's Third Law in Sports

This law is highly relevant in athletic performance. When a footballer kicks the ball, their foot exerts a force on it, and in return, the ball pushes back with an equal force, allowing the player to feel the contact. Swimmers push water backward with their limbs, and in response, the water pushes them forward. Similarly, in skiing or skateboarding, pushing against the ground generates a reaction that moves the athlete ahead.

Applications in Daily Life

Newton’s Third Law is evident in many everyday activities. Walking involves pushing the ground backward with our feet, and the ground pushes us forward. Rowing a boat, driving a car, or even using a skateboard all depend on the reaction force to generate movement. As the car's tires press against the road surface, the road exerts an equal and opposite force, and this interaction generates the friction needed to propel the car forward.

Do Action and Reaction Forces Create Equilibrium?

No, action and reaction forces do not create equilibrium. Even though they are equal in size and opposite in direction, they act on different bodies, not the same object. As a result, these forces do not neutralize one another.