48 Science -- Work, Energy and Power

Enlist various types of works and differentiate clearly between work against friction and work against gravity in two with examples.

1 Answers

Various types of works are:

Work done against friction
Work done against gravity

Work done against friction: When a body is moved in the direction of application of force against friction, then the work done is said to be work done against friction. It is not necessary to raise the height of a body in this condition, the body needs to slide over another surface. Example: pushing or pulling a book on the table, etc.

Work done against gravity: When a body is raised to a certain height against the Earth's gravity, the work done in such condition is said to be work done against gravity. An object must be raised to some height for this type of work. Example: lifting a stone upwards, etc.

Which type of energy is stored while stretching the catapult? How this energy is obtained?

While stretching the catapult, potential energy is stored in it. This energy is obtained by doing work against the restoring force of the string.

What happens to the kinetic energy when the velocity of a moving body is trippled?

The kinetic energy increases by 9 times when the velocity of a moving body is trippled.

Mathematical expression for Kinetic Energy (KE) = 1/2 m(v)²

When the velocity of body is trippled, new velocity (v') = (3v)

So, new Kinetic Energy (KE') = 1/2 m(v')²

= 1/2 m(3v)²

= 1/2 m(9v²)

= 9 . 1/2 m(v)²

= 9 KE

The potential energy of an object kept on the surface of the earth is zero, why? Give reason

The potential energy of an object kept on the Earth's surface is 0 because it has not been displaced to any height.

We know, Potential Energy (PE) = mgh

At the surface of the Earth, h = 0

So, PE = mg * 0

= 0

A car of mass 1500 kg is moving with the speed of 72km/hr. calculate its kinetic energy.

Solution:

Given,

mass of car (m) = 1500 kg

velocity of car (v) = 72 km/hr = 72 * 1000/3600 (m/s) = 20 m/s

To find: Kinetic Energy of car (KE) = ?

We know,

KE = 1/2 m(v)²

= 1/2 (1500) (20)²

= 300 000 J

= 3 x 10⁶ J

Hence, the required Kinetic Energy of the car is 3 x 10⁶J.

No work is done when a person is standing by carrying a load of 50kg for one hour, why?

When a person carries load and stands for any interval of time, total work done is zero. By definition, work is defined as the scalar product of Force and displacement. However, in this case, there is no displacement because the person is stationary which results in 0 work done.

If a machine lifts a load of 500N upto 40m in 2 minutes, how much is the power of the machine?

Solution:

Here,

Load lifted (F) = 500 N

Height (h) = 40 m

Time (t) = 2 minutes = 2 x 60 s = 120 s

Work done (W) = F x h = 500 x 40 = 20 000 J

To find: Power (P) = ?

We know,

Power (P) = W / t = (20000 / 120)

= 166.67 W

Hence, the required power of the machine is 166.67 Watt.

ask mattrab