Force and Pressure
State of Motion
The state of motion of an object is described by its speed and direction of motion. The
state of rest is the state of zero speed. An object may be at rest or in motion, both are its
state of motion.
Force
A push or pull on an object is called force.
Push: A push is a force directed away from the body. Eg kicking a football, repulsion from a
magnet.
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Pull: A pull is a force exerted towards the body. Eg pulling a bucket, attraction of a magnet.
SI unit of force is NEWTON (N)
Forces are due to Interaction
At least two objects must interact with each other for a force to come into play. No force can show
its effect without the interaction between two objects.
Examples:
a. A player standing in front of a football.
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b. A player kicking a football
The ball will only move when the player kicks it, hence a force comes into play.
Magnitude and Direction
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Force has both magnitude and direction. The strength of a force is expressed by its magnitude.
● If the two forces applied on an object are in the same direction, then the resultant acting
force on the object is the sum of the two forces.
● If the two forces applied on an object are in the opposite directions, then the resultant
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acting force on the object is the difference of the two forces.
● If the two forces acting on an object are in the opposite directions, having the same
magnitude, the resultant acting force will be zero.
The quantities having magnitude as well as direction are known as vector quantities. Eg
Force, velocity, displacement etc.
Effects of Force
A force can produce the following effects:
1. A force can move a stationary object.
Eg when you kick a stone laying at rest on the ground.
2. A force can stop a moving object.
● Eg when a fielder stops a ball of cricket.
3. A force can change the speed of an object.
Eg when you pedal your bicycle for more speed.
4. A force can change the direction of an object.
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Eg when you hit the shuttle with a badminton racket.
5. A force can change the shape of an object
Eg blowing air in a balloon.
Types of Forces
All forces can be divided into two main groups as:
Contact Forces
A force which can be exerted by an object on another object only through physical touch, is called
a contact force.
Muscular Force
A force resulting due to the action of muscles is known as the muscular force. Eg.
a. Inhalation and exhalation
b. Movement of food in the alimentary canal.
c. Animals pulling carts
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Friction
The force which always opposes the motion of one body over another body is called frictional
force.
● It always acts on all moving objects.
● Its direction is always opposite to the direction of motion. So Frictional force always
opposes motion.
Non Contact Forces
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A force which can be exerted by an object on another object even from a distance (without
contact) is called a non contact force.
Magnetic Force
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The force exerted by a magnet is called a magnetic force. A magnet can apply both a push or pull.
● Opposite poles attract each other (Pull)
● Same poles repel each other (Push)
Some examples of magnetic materials are iron, steel and nickel.
Uses of magnetic force:
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1. Used in cranes
2. Used in locks
3. Used to separate magnetic materials
Electrostatic Force
The force exerted by an electrically charged object is called electrostatic force. An electrically
charged object can exert a force on an uncharged object or a charged object.
Activity 8.7 Rubbing a piece of straw with a paper induces electrostatic charge in it,
and the straw is said to be electrically charged.
Gravitation Force
A force of attraction between any two objects having mass is known as gravitational force.
● Gravitational pull of earth keeps the moon in its orbit.
● Gravitational pull of the sun keeps the earth in its orbit.
The force with which the earth pulls every object towards it, is called gravity. It is maximum at the
surface of the earth and zero at the centre of the earth.
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Pressure
Force per unit area is called pressure./ Force acting on a unit area is called pressure.
Pressure= Force / Area
So the pressure depend on two factors:
1. Force applied: The higher the magnitude of the force, higher is the pressure.
2. Area: The larger the area, the smaller is the pressure.
Some examples:
● School bags have wide straps to minimise pressure.
○ Tip of the nail is sharper.
Pressure Exerted by Liquids
All liquids exert pressure in all directions. The pressure in a liquid acts equally in all
directions (even upwards).
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1. The pressure is created due to the moving particles in the liquid.
2. Pressure exerted at the bottom of the container depends on the height of the liquid (Activity
8.8).
3. Liquid exerts pressure on the walls of the container (Activity 8.9).
4. Liquids exert the same pressure at all points at the same depth (Activity 9.10).
5. A liquid exerts pressure in the upward direction also.
Pressure Exerted by Gases
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Same properties as liquids above.
Atmospheric Pressure
The pressure exerted by the weight of air present in the atmosphere is known as atmospheric
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pressure. It is maximum at sea level and keeps on decreasing with increasing height.
Activity 8.11 We can not pull the rubber sucker because of the atmospheric pressure acting on it.
To pull it off the surface we have to apply a force equal to the atmospheric pressure.
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