VELAMMAL VIDYALAYA ALAPAKKAM
FORCE AND LAW OF MOTION
Force:
It is a push or pull on an object that produces acceleration in the body on which it acts.
S.I. unit of force is Newton.
What are the effects of force?
(a) change the speed of a body.
(b) change the direction of motion of a body.
(c) change the shape of a body.
• Balanced forces: Forces are said to be balanced forces if they nullify one another and their resultant
force is zero.
• Unbalanced forces: When two opposite forces acting on a body, move a body in the direction of the
greater force or forces which brings motion in a body are called as unbalanced forces.
• First law of motion:
An object remains in a state of rest or of uniform motion in a straight line unless acted upon by
an external unbalanced force.
• Inertia: The natural tendency of an object to resist a change in their state of rest or of uniform motion
is called inertia.
The mass of an object is a measure of its inertia.
Its S.I. unit is kg.
A body with greater mass has greater inertia.
• Frictional force: The force that always opposes the motion of objects is called force of friction.
• Second law of motion: The rate of change of momentum of an object is proportional to the applied
unbalanced force in the direction of the force. Mathematically.
F = k ma (where k is a proportionality constant)
F = ma (∵ k = 1)
• Momentum: The momentum of an object is the product of its mass and velocity and has the same
direction as that of the velocity. Its S.I. unit is kg m/s.
(p = mv)
1 Newton: A force of one Newton produces an acceleration of 1 m/s 2 on an object of mass 1 kg.
1N = 1kg m/s2
(F = ma)
• Third law of motion: To every action, there is an equal and opposite reaction and they act on two
different bodies.
• Conservation of momentum: If the external force on a system is zero, the momentum of the system
remains constant i.e., in an isolated system, the total momentum remains conserved.
1. Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle
and a train? (c) a five-rupee coin and a one-rupee coin?
Solution
It is dependent on the mass of the object, the object with the greater mass will hold greater inertia. The
following objects hold greater inertia because of their mass.
1.Stone
2.Train
3.Five-Rupee coin
2. In the following example, try to identify the number of times the velocity of the ball changes: “A
football player kicks a football to another player of his team who kicks the football towards the goal.
The goalkeeper of the opposite team collects the football and kicks it towards a player of his own
team”. Also identify the agent supplying the force in each case.
Solution
The velocity of football changes four times.
First, when a football player kicks a football to another player, second when that player kicks the football to
the goalkeeper. Third when the goalkeeper stops the football. Fourth, when the goalkeeper kicks the football
towards his team player.
3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
Solution
When the branch of the tree is shaken, the branch moves in a to-and-fro motion. However, the inertia of the
leaves in attached to the branch resists the motion of the branch. Therefore, the leaves that are weakly
attached to the branch fall off due to inertia whereas the leaves that are firmly attached to the branch remain
attached.
4. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards
when it accelerates from rest?
Initially, when the bus accelerates in a forward direction from a state of rest, the passengers
experience a force exerted on them in the backward direction due to their inertia opposing the
forward motion.
Once the bus starts moving, the passengers are in a state of motion in the forward direction. When the brakes
are applied, the bus moves towards a position of rest. Now, a force in the forward direction is applied on the
passengers because their inertia resists the change in the motion of the bus. This causes the passengers to
fall forwards when the brakes are applied.
5. If action is always equal to the reaction, explain how a horse can pull a cart.
When the horse walks forward (with the cart attached to it), it exerts a force in the backward direction on the
Earth. An equal force in the opposite direction (forward direction) is applied on the horse by the Earth. This
force moves the horse and the cart forward. As a result, the cart moves forward.
6. Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a
high velocity.
When a fireman holds a hose, which is ejecting large amounts of water at a high velocity, then a reaction
force is exerted on him by the ejecting water in the backward direction. This is because of Newton’s third
law of motion. As a result of the backward force, the stability of the fireman decreases. Hence, it is difficult
for him to remain stable while holding the hose.
7. When a carpet is beaten with a stick, dust comes out of it. Explain.
When the carpet is beaten with a stick, the stick exerts a force on the carpet which sets it in motion. The
inertia of the dust particles residing on the carpet resists the change in the motion of the carpet. Therefore,
the forward motion of the carpet exerts a backward force on the dust particles, setting them in motion in the
opposite direction. This is why the dust comes out of the carpet when beaten.
9. Why is it advised to tie any luggage kept on the roof of a bus with a rope?
When some luggage is placed on the roof of a bus which is initially at rest, the acceleration of the bus in the
forward direction will exert a force (in the backward direction) on the luggage. In a similar manner, when a
bus which is initially in a state of motion suddenly comes to rest due to the application of brakes, a force (in
the forward direction) is exerted on the luggage.
Depending on the mass of the luggage and the magnitude of the force, the luggage may fall off the bus due to
inertia. Tying up the luggage will secure its position and prevent it from falling off the bus.
