Class 8, Science, Chapter-8, Lecture-1, Force (Notes)
FORCE:
An external influence which when acting on a body, changes or tends to change its state of rest or motion in a straight line.
Force is a VECTOR quantity
S. I. Unit of force: newton ( N )
POINT OF APPLICATION OF FORCE:
The point in a body at which applied force is exerted, is called the point of application of force.
LINE OF APPLICATION OF FORCE
The straight line through the point of application of force in the direction of force is called the line of force.

Line of action of force
RESULTANT FORCE:
- In the same direction: F1 + F2
- In opposite direction: F1 – F2
INTERACTION:
When one body influences another body by applying force, the first body is said to be interacting with the second body.
Types of Forces on the basis of Interaction:
- Contact force
A force applied by one object on another when they are in contact with each other is termed as contact force. - Non-contact force
A force applied by one object on another when they are not in contact with each other is termed as non-contact force.
Types of Forces:
- Contact Forces
- Muscular Force
- Impact Force
- Frictional Force
- Non-contact forces
- Force of Gravity
- Gravitational Force
- Electrostatic Force
- Magnetic Force
BALANCED FORCES:
A number of forces acting on a body which do not produce any change in state of rest or uniform motion or direction of motion are called Balanced Forces.
CONDITION FOR TWO FORCES TO BE BALANCED:
- Forces must be equal and opposite in direction.
- They must have same line of action of force,
Effect of Balanced Forces: Change in shape
UNBALANCED FORCES:
A number of forces acting on a body which produce some change in state of rest or uniform motion or direction of motion are called unbalanced forces.
Effect of Unbalanced Forces: Change in state of rest or motion.
Effects of a Force:
- Force can produce motion
- Force can stop motion
- Force can increase or decrease the speed of moving objects.
- Force can change the direction of motion.
- Balanced forces can change the shape or size of an object.
UNITS OF FORCE:
S.I.Unit – newton ( N )
C.G.S. unit – dyne ( dyn )
Relation between 1 N and 1 dyn : ${\rm{1}}\;{\rm{N = 1}}{{\rm{0}}^{\rm{5}}}{\rm{dyn}}$