# CBSE Class 11-science Physics Work, Energy and Power

#### Work and Energy

Work is given by dot product of force and displacement

Work done is a scalar quantity. It can be positive as well as negative. Work done by air friction is always negative.

Work Energy Theorem: The change in kinetic energy of an object is equal to the net work done on it by the net force:#### Potential Energy

Energy possessed by a body: Changes in height with respect to the surface of earth

Law of conservation of mechanical energy: Total mechanical energy of the system always remains constant in the absence of dissipative forces.#### Conservative Forces

Work done by Conservative Force: a) in a round trip = Zero b) depends only on the initial and final positions of the object and not on the path.

Work done by Non Conservative force: a) in a round trip ¹ Zero b) depends on the path taken.

Conservative force F = negative of change in PE with respect to change in position. F (x) = - D U/Dx.#### Energy and Power

Energy comes in many forms which transform into one another.

Laws of conservation of energy states that the total amount of energy in the universe or in any isolated system remains constant

Power is the rate at which work is done or energy transformed.

The unit of power is Watt 1 watt = 1joule/second and 1hp = 746 watt#### Collisions

Linear momentum of an isolated system is always conserved in a collision. i) Elastic collision: Kinetic energy (KE) of the system is conserved. ii) Inelastic collision: Total KE of the system is not conserved. iii) Completely inelastic collision: Two bodies collide, stick together and have a common final velocity thereafter.