Physics Force & Motion-potential And Kinetic Energy

Energy

Energy • Energy is the capacity of a system to do work. • The SI unit for energy is Joule (J) • Energy can exist in various form : – – – – –

Potential energy Kinetic energy Heat energy Sound energy Electric energy

• When energy change from one form to another form, work is done.

Potential energy

Kinetic energy

Chemical energy

Potential Energy • The potential energy of an object is the energy stored in the object because of its position or state. • Two main types : – Gravitational potential energy – Elastic potential energy

Gravitational potential energy

• The gravitational potential energy of any object is the energy stored in the object because of its height above the earth’s surface. • When an object of mass, m kg is raised to a height, h metres above the earth’s surface, the object possesses gravitational potential.

Gravitational potential energy

Final position

hm

mass = m kg

Initial position

The gravitational potential energy is equal to the work done to raise an object to a particular height. Work done = F x s = mg x h = mgh Ep = W Ep = mgh

Elastic potential energy

• The elastic potential energy of an object is the energy stored in the object as a result of stretching or compressing it.

Kinetic Energy • Kinetic energy is the energy possessed by a moving object. • When a stationary object of mass, m kg moves and reaches a velocity of v ms-1, the kinetic energy of the object is the same as the work done to change the velocity of the object from 0 ms-1 to v ms-1.

Kinetic energy

Assume a force, F is acting on a stationary object of mass m kg moving on a smooth surface. The force acting over a distance of s causes the trolley to achieve a v=v ms-1

u=0 ms-1

F s

Kinetic energy

v=v ms-1

u=0 ms-1

F s • • • • •

Initial velocity = 0 ms-1 Final velocity = v ms-1 Displacement = s m Work done = F x s = mas …….. (1) From equation;

Principle of Conservation of Energy • State : – The total energy in a system is constant.

• Energy can’t be created or destroy. • Energy can be converted from one form to another form.

Principle of Conservation of Energy

Before falls, Ep = mgh Ek = 0

Ep = Ek

Ep = 0 Ek = ½ mv2