NEWTON’S THIRD LAW OF MOTION
“To every action, there is an equal and
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:
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
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
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
vertically upwards, and R cos
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