Force: A physical cause which changes or tends to change
either the size and shape of the body or the state of rest or the state of
motion of the body.
Forces are categorized into two kinds : 1.
Contact Forces
2.
Non – contact forces
1. Contact
Force: The force acting between two bodies when the bodies are in physical contact.
It is also called Direct Force. The following forces are considered
contact forces.
a) Frictional
Force: When a body slides or rolls over the surface of another body, a
force resists the motion of the body and acts in the opposite direction. This
is called Frictional force (Force of Friction).
b) Normal
Reaction Force: When a body is placed on a surface the body exerts a force which
is equal to its weight and the surface also exerts an equal and opposite force
normal to the surface.
c) Tension
Force as applied through a string: When a body is suspended by a string,
the body exerts a force on the string and pulls it vertically downwards which
balances the weight of the body. This force is called the Tension Force(T).
d) Force
exerted by a spring: Restoring Force: The force exerted by a compressed or stretched spring on an
object attached to the spring.
This force is
directly proportional to its elongation or compression (the displacement of the
other end of the spring).
The elongation of a spring is directly proportional to the
weight suspended from it – Spring balance works on this principle.
e) Force exerted during collision: When two bodies collide, equal and
opposite forces act each body and they push each other.
When a body A moves towards another
body B and collides with it, the force exerted by A on B is called Force of
action and B exerts an equal and opposite force on A i.e. called Force of
Reaction.
2. Non-contact
Forces: The Force experienced by the bodies ven when they are not in
physical contact (Forces at distance).
a) Gravitational
Force: The force of attraction
existing between any two particles in the universe due to their masses.
The force on
a body due to Earth’s attraction is called Pull of Gravity or Force
of Gravity. Due to this force an object released from a height falls down.
b) Electrostatic Force : The force acting between two charges
when they are at a distance .
example: A
comb rubbed on a dry hair, attracts bits of paper due to electrostatic force
c)
Magnetic Force: The force
existing between two magnetic poles even when they are at a distance.
General
Characters of Non-contact Forcers:
i)
Gravitational
force is always an attraction force where as electrostatic and magnetic force
can be either attraction or repulsion force.
ii)
The
magnitude of non-contact force decreases with increase in distance (separation)
between the bodies.
F a 1/square of d
