Work and Energy

Question (1) A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?

Answer:

Given;

Force (F) = 7N and

Displacement (s) = 8 m

Then work done (W) = ?

We know that, Work Done (W) = Force (F) × Displacement (s)

∴ W = 7 N × 8 m

⇒ W = 56 Nm or 56 J

Thus, work done (W) = 56 N m or 56 J Answer

Question (2) When do we say that work is done?

Answer

After applying a force on a object, if object is displaced along the direction of the force applied, we say that work is done.

Question (3) Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Answer:

Work Done = Force x Displacement

Or, W = F × s ---------- (i)

This is the expression for the work done when a force is acting on an object in the direction of its displacement.

Question (4) Define 1 J of work.

Answer: I J is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.

Question (5) A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Answer:

Given, Force (F) = 140 N

Displacement (s) = 15 m

Thus, work done (W) = ?

We know that, Work done (W) = Force (F) × Displacement (s)

⇒ W = 140 N × 15 m

⇒ W = 2100 Nm or 2100 J

Thus, work done = 2100 Nm or 2100 J Answer

Question (6) What is the kinetic energy of an object?

Answer:

The energy possessed by an object due to its motion is called Kinetic Energy of an object.

If an object of mass m is moving with a velocity of v, then kinetic energy of the object,

Question (7) Write an expression for the kinetic energy of an object.

Answer:

If an object of mass m is moving with a velocity of v, then kinetic energy of the object,

Question (8) The kinetic energy of an object of mass, m moving with a velocity of 5 m s^–1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

Answer:

Given,

We know that

Thus, it is clear that kinetic energy varies directly as square of velocity of the object.

Thus, when velocity becomes double, the kinetic energy will be increased by 2² = 4 times

And, when velocity becomes triple, the kinetic energy will be increases by 3² = 9 times

Here, given, kinetic energy = 25 J and velocity of the object = 5 m s^–1

Thus, when velocity will be doubled, i.e. 5 × 2 = 10 m s^–1

Then, kinetic energy will becomes 4 times. i.e. = 25 J × 4 = 100 J Answer

And, when velocity will be tripled, i.e. 5 × 3 = 15 m s^–1

Then, kinetic energy will becomes 9 times, i.e. 25 J × 9 = 225 J Answer

Question (9) What is power?

Answer:

The rate of doing work or rate of transfer of energy is called power.

If W is the work done in time t; then

P = W/t

The unit of power is Watt (W).

Question (10) Define 1 watt of power.

Answer:

When work is done at the rate of 1 joule per second, the power is called 1 Watt.

Or, It can be said that power is 1 W when the rate of consumption of energy is 1 joule per second.

Or, 1 W = 1 J s^–1

Question (11) A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

Answer:

Given, electrical consumption, i.e. work done, W = 1000 J

And time, t = 10 s

Then, power, P = ?

P = W/t

⇒ P = 1000 J /10 s = 100 W

Thus, power = 100 W Answer

Question (12) Define average power.

Answer:

The agent may be doing work at different rates at different intervals of time. Therefore, the concept of Average power is useful.

The Average power is equal to total energy consumed divided by total time taken.

i.e. Average power = total energy consumed/total time taken