Table of Contents
Force and pressure class 8
Force and pressure class 8
force is a physical quantity that
changes the state of rest or of uniform
motion of a body in a straight line.
1. Newton’s first law of motion :
A stationary object on which no force is
acting, remains stationary. An object in
motion continues to move with the same
speed and direction when no force is
acting on the object.
2. A force acts on two bodies through an
interaction between them.
[Note: a body = an object.]
3. Contact force: A force that acts
through a direct contact of two objects
or via one more object, is called a contact
force.
4. Non-contact force: A force that acts
between two objects even if the two
objects are not in contact, is called a non
contact force.
5. Balanced forces : If a body is acted
upon by two forces, equal in magnitude,
opposite in direction and having the
same line of action, the forces are called
balanced forces. Here, the net force
acting on the body is zero.
6. Unbalanced force : If two or more
forces act on a body such that their
resultant is not zero, the resultant is an
unbalanced force. It produces
acceleration in the body.
Force and pressure class 8
7. Inertia : The tendency of an object to
remain in its existing state is called its
inertia.
8. The effects of a force acting on a
body :
(1) A force can set a body in motion. For
example, if a ball at rest on the floor is
pushed, it rolls on the floor.
(2) A force can stop a moving body. For
example, a moving bicycle can be
brought to rest by application of brakes.
(3) A force acting on a body can change the
speed of the body. For example, when
brakes are applied to a moving bicycle,
its speed decreases due to the friction
between the brakeshoes and the rim of
the tyre.
(4) A force can change the direction of
motion of the body. For example, in
uniform circular motion of a body, the
direction of motion of the body keeps on
changing due to the applied force.
(5) A force can change the speed as well as
the direction of motion of the body. For
example, when a ball bowled by a bowler
is hit by a batsman, there occurs a
change in the speed as well as the
direction of motion of the ball.
(6) A force can change the shape and size of
the body on which it acts. For example,
when a rubber ball is pressed, it gets
deformed and hence no longer remains
spherical. Also, there can be a decrease
in its volume.
Force and pressure class 8
9. Pressure : The
force exerted
perpendicularly on a unit area is called
pressure. The SI unit of pressure is
N/m2. It is called the pascal (Pa). In
atmospheric science, the unit for
pressure is called the bar.
1 bar = 105 Pa.
The air pressure at the sea level is called
1 atmosphere.
1 atmosphere = 101 x 103 Pa.
10. At a depth h below the free surface of a
liquid, the pressure exerted by the liquid
is hpg
11. Buoyant force : The upward force
acting on the object in water or other
fluid or gas is called the buoyant force.
It depends on the volume of the object
immersed in the fluid (V), the density of
Force and pressure class 8
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