# Force and Motion

Inertia: The property of a body which tends to keep the body in its state of rest or of uniform motion is called inertia. Inertia is measure of mass.

Momentum: Measure of motion is called momentum.

Momentum = mass × velocity

p = mv

First Law of Motion: Whenever a body is at rest or uniform motion it will continue in rest or in uniform motion until or unless an external unbalanced force is applied on the body.

Second Law of Motion: The rate of change of momentum of a body is directly proportional to the force acting on it and takes place in the same direction as the force.

F ∝ dp/dt

F = ma

The S.I. unit of force is Newton.

Third Law of Motion: To every action there is an equal and opposite reaction but they act on different objects.

Law of Conservation of Momentum: It states that the momentum of a system remains unchanged (conserved or constant) if no external force is acting on the system.

Friction: Whenever a body slides over the surface of another body, an opposite force comes into play which resists the motion of the body and is called force of friction.

Static friction: The resistive force before the body starts moving over the surface of an other body is called static friction.

Thrust: The force acting upon the surface of a body perpendicular to it is called thrust.

Pressure: Thrust per unit area is called pressure.

Pressure = Thrust / Area = F/A

The unit of pressure is N/m2 or pascal.