2015 Dec

Newton's Law of Reaction

December 10, 2015

Newton's Third Law

In the picture, who will exert a larger force on the other? The large sumo wrestler or the small one?

Newton’s third law states that “all forces between two objects exist in equal magnitude and opposite direction”. This means that if one object let’s say L exerts a force FL on a second object M, then M concurrently exerts a force FM on L, and the two forces (FL and FM) are equal and opposite.

So, FL = – FM or FM= – FL.

Going by this third law of motion, every force is as a result of an interaction between two or more bodies. For there to be a force acting on a body, then there have to be another body interacting with it. In other words, there is always an action force and a reaction force.

Practical applications of the Newton’s third law of motion

The applications of this law of motion are numerous. In almost everything that we do we apply this law. Below are several examples of the applications of the law of reaction.

i. If you place a box on a table, the box will exert some force on the table due to its weight. The table will also exert back the same amount of force on the box.

ii. Another example is between the tires of a vehicle and the road. There is a friction force that pushes on the tires as they exert a force on the road when the vehicle is accelerating.

iii. When walking, you push against the floor and the floor pushes against you.

iv. When swimming, the water pushes you forward as you push it backwards. Therefore the action and reaction forces are the ones that enable you to move through the water as you swim.

v. When you are jumping, the force that you exert on the ground is the same amount of force that the ground will exert on you thus determining how far you will be catapulted into the air.

vi. This law is taken into consideration when analyzing forces in structures especially at fixed joints. At fixed joint, you have to incorporate the two forces; action and reaction forces.

vii. When a plane is cruising through the atmosphere, it emits compressed air from its engines backwards. This compressed air exerts a force on the surrounding air which also exerts a similar force on the plane and hence propelling the plane forward.

There are many other applications which we will cover in our Physics tuition classes, such as those in the chapters of Gravitation and Electromagnetism.

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