Force : - A force is an influence which tends to set a stationary body in motion or which tends to change the shape of a body.

Balanced Forces : - If the resultant of all the forces acting on the body is zero, the forces are called balanced forces.

Unbalanced Forces : - If the resultant of all the forces acting on the body is not zero, the forces are called unbalanced forces.

Newton's first law of motion : - A body continues to be in a state of rest or of uniform motion along a straight line unless an external force act upon it. (Law of Inertia)

Inertia : - The tendency of a body to resist change in the state of their uniform motion or rest is called inertia.

Newton's second law of motion : - The rate of change of momentum of a body is directly motion proportional to the force and takes place in the same direction as the force.

Momentum : - The momentum of a moving body is defined as the product of its mass and velocity.

Newton's third law of motion : - To every action there is an equal and opposite reaction.

Conservation of momentum : - If a group of bodies are exerting force on each other , i.e. interacting with each other, their total momentum remains conserved before and after the interaction provided there is no external force acting on them.

Friction : - The force, which always opposes the motion of one body over another body in contact with it, is called the friction (force of friction).

Limiting friction : - The magnitude of the force, which is sufficient to move the body, is called limiting friction(static friction).

Sliding Friction : - When a body slides over another, the friction between them is known as sliding friction(dynamic friction).

Rolling Friction : - The friction in the case of bodies that move on rollers is known as rolling friction.

Thrust : - Force acting on the body perpendicular to its surface is called thrust.

Pressure : - The trust per unit area is called pressure.

Force of Buoyancy : - The upward force exerted by liquids on all bodies when they are immersed in a fluid is known as force of buoyancy.

Archimedes principle : - When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Density : - Density of a substance is defined as mass of a unit volume.

Relative Density : - The relative density of a substance is the ratio of its density to that of water.