Laws of Motion Standard 9

Force A push or pull that can cause an object to change its velocity or which can cause a flexible object to deform.

Balanced Forces Force F 1 Force F 2 F1 and F2 are acting on the rope. If both teams are evenly matched then F1 = F2. But F1 and F2 are in the opposite direction and hence cancel each other. The net force on the rope = 0. When a number of forces act on a body such that the net force acting on the body is 0, then such a combination of forces are known as balanced forces.

Types of Forces Muscular Frictional Contact Forces Non-Contact Forces Magnetic Gravitational Electrostatic

Objects are Lazy – 1 Smooth tiled floor Rough Ground Football will remain at the same place unless someone kicks it. The football stops rolling after sometime as friction opposes its motion. The football travels more distance on a smooth surface as the frictional force is smaller. If there was no friction, the football would keep on rolling. First the football does not want to move and once moving it does not want to stop (unless some force is applied).

Objects are Lazy – 2 Stack of books Toy vehicle Little guy Wall What will happen when the bus hits the wall? The bus stops moving when it hits the wall. However the little guy on top of the bus continues moving and topples over.

Newton’s First Law of Motion

Newton’s Second Law of Motion

The rate of change of momentum is directly proportional to the impressed force and takes place in the direction in which the force acts.

Newton’s Third Law of Motion As the air escapes from the balloon it pushes the balloon forward. Atmospheric pressure pushes the air out Force of air on balloon pushes it forward Wall StrawString

Newton’s Third Law of Motion Ball pushes the ground downwards Ground pushes the ball upwards When moving down, gravity acts in the direction of motion accelerating the ball. When moving up, gravity opposes the motion decelerating the ball.

Newton’s Third Law of Motion For every action there is an equal and opposite reaction. Action and Reaction act on different bodies and hence do not cancel each other. When you take off your hand the force of the water pushes the block upwards. The harder you push the block downwards, more the force with which water pushes the block back. Block pushes the water downwards Water pushes the block upwards

Conservation of Momentum If there is no net force acting on two interacting bodies then their total momentum is conserved.

Units of Measurement Physical QuantitySI UnitsCGS Units Forcekg m/s 2 or Newtong cm/s 2 or dyne Momentumkg m/sg cm/s