Work and Energy Test - 7

(1) The SI unit of work is joule and is denoted as ‘J’, which is named after an English physicist James Prescott Joule. The 1 joule of work done is equal to 1N x 1 m.  Or, 1 joule = Nm
(2) 1 kilowatt = 1000 watt
(3) 1 kWh = 1 kW x 1 h

Or, 1kWh = 1000 joule x 3600 s

Or, 1 kWh = 3600000 joule

Or, 1kWh = 3.6 x 10⁶ joule

(4) 1 HP = 746 W

Joule is unit of energy. Large quantity of energy is expressed in kilo watt hour and is written as kWh.
Energy is capacity of doing work, therefore, the SI unit of energy is same as of work. So, 1 joule = Nm is also unit of energy.

When force is exerted on an object and object is displaced, work is said to be done.
Work = Force x Displacement
Essential condition for work to be done are
1. A force should act on an object.
2. The object must be displaced.

Kinetic energy of a body is K.E  = 1/2 mv². Kinetic energy depends upon mass of the body and velocity. Since mass and velocity never be negative, kinetic energy is always positive.

The momentum of an object with mass m and traveling at a velocity v is given by M = m*v and the kinetic energy is given by K.E. = 1/2 mv²
The momentum of the object can be made double by either increasing its mass to twice the initial mass with the velocity remaining the same or by increasing the velocity to twice the initial value.
Assuming the mass of the object is not altered and only the velocity is increased to twice the initial value, the kinetic energy of the object is increased to 1/2 m (2v)² = 4 (1/2) mv², or its changes of four time the initial value.
So, If the momentum of an object is doubled by increasing its velocity, its kinetic energy becomes 4 times the initial value.

A wound up watch spring has elastic potential energy due to its wound up state. As the spring unwinds, the PE stored in it changes into kinetic energy which does work in moving the hands of the watch.

When a body is thrown vertically upwards, during its upward movement the potential energy is increase due to increase in height and kinetic energy decreases due to decrease in velocity.

Potential energy is maximum at the extremes .when it again starts its motion towards the mean position then the potential  energy is converted into kinetic energy. Thus at the mean position the kinetic energy is maximum but potential energy is minimum. At the extreme position the P.E is maximum but the K.E is minimum.

The pendulum shows the principal of conservation of energy in action. Gravitational potential energy is converted to kinetic energy and back, over and over again, as the pendulum swings.

The gravitational potential energy of an object depends on its mass, how high it is above the ground and the strength of the gravitational field.  P.E = mgh.