Inertia: The property of a body which tends to keep the body in its state of rest or of uniform motion is called inertia.
Momentum: Measure of motion is called momentum i.e. momentum = mass × velocity.
p = mv
Laws of Motion
First Law: 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: 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 = ma
The S.I. unit of force is Newton.
Third Law: 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.
Disadvantages of Friction
- Force of friction causes lot of wear and tear in the moving parts of machine.
- Efficiency of machine decreases due to presence of force of friction.
- Force of friction restricts the speed of moving vehicles.
Advantages of Friction
- The force of friction between chalk and blackboard helps to write on the board.
- Moving belt remain on the rim of a wheel because of friction.
- Force of friction helps us to move on the surface of the earth.
Methods of reducing friction
- Polishing of surfaces
- Lubrication
- Use of ball bearings and roller bearings
Thrust and Pressure
Thrust: The force acting upon the surface of a body perpendicular to it is called thrust.
Pressure: Thrust per unit area is called pressure. The unit of pressure is Nm-2 or pascal.