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“To every action, there is an equal and opposite reaction”

 This means that if a body A applies a force F on body B, then the body B will also exert force –F on the body A. 

Illustration of Newton’s Third Law:

 1. Walking: While walking a person presses the ground in the backward direction (action) by his feet. The ground pushes the person in forward direction with an equal force (reaction). The component of reaction in the horizontal direction makes the person move forward.


2. Swimming: A swimmer pushes the water backwards (action). The water pushes the swimmer forward (reaction) with the same force. Hence the swimmer swims. 

3. Firing from a gun: When a gun is fired, the bullet moves forward (action). The gun recoils backwards (reaction). 

4. Horse and Cart Problem: Horse pulls the cart and in turn cart pulls the horse with equal and opposite force. Then how is horse able to move the cart?

The various forces acting on a system of horse and cart at rest are shown in the figure. Here, the weight (W1) of the cart C is balanced by the reaction (R1) of the ground on the cart. The weight W2 of the horse H is balanced by the reaction R2 of the ground on the horse. The horse pulls the cart with a force T in the forward direction. The cart, in turn, pulls the horse with the same force T in the backward direction. These two forces are balanced. While pulling the cart, the horse pushes the ground backwards with its foot by a Force F inclined at an angle q with the horizontal. As a reaction, the ground exerts force R on the horse equal and opposite to F.


R can be resolved into two rectangular components:

R sin q vertically upwards, and R cos q along the horizontal.

The component RCosq tends to move the cart forward. This motion is opposed by the force of friction f between the cart and the ground. The cart will move only when RCosq > f.

5. Apparent weight of a Man in an Elevator
Suppose a person of mass m is standing on weighing machine placed in an elevator. The actual weight of the person is equal to mg. This acts on the weighing machine which offers a reaction R given by the reading of the weighing machine. This reaction exerted by the surface of contact on the person is the apparent weight of the person. R will depend on the acceleration of the elevator.


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