A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction
Force has both direction and magnitude.
SI unit of force is newton (N)
a) Force can make a stationary object move.
b) Force can stop a moving object.
c) Force can change the direction of a moving object.
d) Force can change speed of a moving object.
e) Force can change the shape of an object.
Examples: the force involved in kicking a ball, pulling a wagon, compressing a
spring, etc
Examples: the gravitational force and the electromagnetic force
Gravitational force is the weakest force among all forces existing in nature. It is also
the easiest to observe, because it acts on all matter and it is always attractive.
Contact Forces
a) Muscular Force
b) Normal Force, Air Resistance Force
c) Applied Force
d) Spring Force
b) Electrostatic Force
c) Magnetic Force
1. If forces applied on an object are in the same direction
Net force=addition of all forces
When forces acting on a body do not bring about any change in its state of motion, then the forces acting on the body are said to be balanced.
Net resultant force acting on body = zero
Unbalanced Forces:
When forces acting on a body bring about a change in its state of motion, then the forces acting on the body are said to be unbalanced.
Net resultant force acting on body is not equal to zero
Comparision between Balanced and Unbalanced Forces
Q. For each situation, determine the net force acting upon the object.
Q. The net force is known for each situation. However, the magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude of the unknown forces.
Contact Forces:
1. Muscular Force
The force resulting due to the action of muscles is known as the muscular force.
2. Friction
The force that tries to stop materials sliding across each other. Direction of frictional force is always opposite to the direction of motion.
Non-contact Forces:
1.Magnetic Force
Magnetic force is the force exerted by a magnet on another magnet or any magnetic material. Magnetic force can be either attractive or repulsive.
2. Electrostatic Force:
Some material becomes charged when they are rubbed. Their charge is called electrostatic charge.
The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
3. Gravitational Force
If you hang an object from a spring balance, you measure a downward pull from the Earth.
This pull is called the force of gravity or just a gravitational force.
Main features of gravitational force
a) All masses attract each other
b) The greater the masses, the stronger the force.
c) The closer the masses, the stronger the force.
PRESSURE
Force (at right angles to a surface) acting on a unit area is called pressure.
Factors Effecting Pressure
1. Force: Pressure is directly proportional to force.
More the force, the more is the pressure and vice-versa
2. Area: Pressure is inversely proportional to the area
Lesser the area of contact, the more is the pressure and vice-versa.
Application of Pressure
1. Nails and pins have pointed ends
Nails and pins have very small are at the tips due to their pointed ends. A small force on the head of the pin or nail will exert considerable large pressure to drive pin into the surface easily.
2. Heavy trucks are fitted with six to eight wheels
Due to large number of wheels, the area of contact on the ground over which the weight of the heavy truck acts is increased. This reduces pressure exerted by the heavy truck on the ground.
3. Skiers use flat skies to slide over snow:
Skies have large area to reduce the pressure on the snow; enabling the skier to slide over the snow without sinking.
4. High rise buildings have wide foundations:
Wall foundations have a large horizontal area. This reduces the pressure underneath so that the walls do not sink further into the ground
5. Potters place a round piece of cloth on their head while carrying heavy load
By this they increase area of contact with their head. So, the pressure on their head is reduced and they find it easier to carry load.
PRESSURE EXERTED BY LIQUID 
Liquids exert pressure due to distribution of their own weight.
Pressure in a liquid can be measured by a manometer.
* Water stream will start pouring out through the holes. This means that water is exerting pressure in all direction.* In tin I, the water stream comes out evenly irrespective of the direction of the hole. This means that the pressure is equal at the same height or depth.
* In tin II and III, the water stream coming out of the lowest hole reaches the farthest. This shows that the pressure exerted by liquid increases with depth.
* Since there is no difference between the streams coming out of tins II and III, the pressure exerted by liquid is independent of the size of the container, but depends only on the height or the depth of the liquid.
When dams are built to stop and store water, it becomes necessary to make the base of the dam broader than the top. The base has to sustain greater pressure from the pressure of the water column as compared to the pressure exerted by the water on the surface.
Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface.
In terms of air molecules, if the number of air molecules above a
surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases.
Atmospheric pressure is measured with an instrument called a "barometer".
Standard Atmospheric Pressure
1 atm=760 mmHg =101 300 Pa
At sea level the mass of air on 10cm × 10cm area is approximately equal to 1000kg.
We aren’t crushed by huge atmospheric pressure because it is matched by the pressure in our lungs and blood system and cancels the pressure from outside.
We expand our lungs to reduce the air pressure inside the straw. As a result atmospheric pressure pushes the liquid up the straw.
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