Newton’s Third Law of Motion

Introduction

Newton’s laws of motion were initially published in Philosophiae Naturalis Principia Mathematica, commonly referred to as the ‘Principia’, 1687. Newton developed the laws of motion to explain the elliptical shape of the planetary orbits. They proved to be of great scientific importance for centuries after.

Newton’s Laws of Motion

1. Newton’s first law explains inertia: A body continues in its state of rest, or in uniform motion in a straight line, unless acted upon by a force.
2. Newton’s second law explains force: A body acted upon by a force moves in such a manner that the time rate of change of momentum equals the force.
3. Newton’s third law explains action and reaction: If two bodies exert forces on each other, these forces are equal in magnitude and opposite in direction.

Newton’s Third Law of Motion

Newton’s third law of motion states that when one object exerts a force on another, the second object responds by exerting an equal and opposite force on the first.

In other terms, every action has an equal and opposite reaction.

There are two forces acting on the two interacting objects in every interaction. The forces acting on the first object are equivalent to the forces acting on the second object. The force on the first object acts in the opposite direction to the force on the second object. 

There are always equal and opposing action-reaction force pairings.

A force can be defined as the push or pull that occurs as a result of one object’s interaction with another object. Some forces are the result of contact interactions such as frictional, tensional, and applied forces, while others are distance interactions such as gravitational, electrical, and magnetic forces.

Consider two objects, A and B. When objects A and B interact with each other, Newton’s third law says they exert forces on each other. 

Newton’s third law of motion deals with these two forces, which are called action and reaction forces.

Examples of Interacting Force Pairs

Nature contains a wide range of action-reaction force pairs. 

Movement of fish in water

When a fish swims, it uses its fins to push the water backwards, and water exerts a force on the fish in return, which causes the fish to move forward. The force produced by the water is equal to the force exerted by the fish.

There is an equal and opposite (with respect to the direction) response force for every action.

The direction of forces in this example is forward (the fish’s fins) and backward (the force on water).

The fish is able to swim in the water because of these action-reaction force pairings.

Flying birds

When birds fly, their wings push air downwards. The air, in return, pushes against the bird’s wings and causes it to fly higher. 

The force exerted on the air is equal to the force exerted on the bird, and the force on the air is opposite the force on the bird. The wings are forced upwards, and the air is forced downwards.

Birds can fly because of action-reaction force pairings.

The motion of a moving car

When a car is in motion, its wheels hold the road and push it backwards, and the road pushes the wheel forwards. 

The force on the road is equal to the force on the wheels of the vehicle, and the force on the road is opposite the force on the wheels. 

Cars can travel on the road due to action-reaction force pairings.

Aerodynamics and Action-reaction Forces

Newton’s third law, in combination with Bernoulli’s principle, can be used to describe how a wing produces lift. For aircraft, the concept of action and reaction play an important role. 

The principle of action and reaction is crucial in aircraft. It aids in the understanding of how an airfoil creates lift. The motion of the airfoil bends the air downward, and the wing is pushed upward.  

Action and reaction are also used to produce thrust in a jet engine. The engine creates hot exhaust gases, which exit the engine from the back. A thrusting force is produced in the opposite direction as the reacting force.

Bernouille’s Principle 

Bernoulli’s principle states that when a fluid travelling through a tube reaches a constriction or narrowing, the speed of the fluid increases while its pressure decreases. The inclined (curved) surface of an airfoil (wing) influences airflow in the same way that a tube constriction does.

As air passes over the upper surface of an airfoil, its velocity rises, but its pressure falls, forming a low-pressure zone. On the lower surface of the airfoil, there is a zone of higher pressure, and this higher pressure causes the wing to rise. Lift is caused due to the pressure difference between the upper and bottom surfaces of a wing. The decrease in pressure across the upper surface accounts for three-quarters of an airfoil’s overall lift. The other one-fourth of total lift is produced by the impact of air on the undersurface of an airfoil.

Examples involving Reaction and Action in Aerodynamics

1. When an airfoil generates lift, the air is forced downward by the airfoil’s action, and the wing is pushed up in response.
2. When the air is deflected to one side by a spinning ball, the ball reacts by travelling in the other direction.
3. A jet engine’s motion provides thrust, while hot exhaust gases rush out the back of the engine, producing thrust in the opposite direction.

Conclusion

Newton proposed three main laws of motion, which had a significant impact on the scientific world as they provided a wealth of information on the movement and arrangement of the solar system. Newton’s third law of motion states that every action has an equal and opposite reaction.