## Chapter 4 Work And Energy Very Short Question and Answers

**Question 1. Define energy and its commercial unit. Express it in terms of Joules.**

**Answer:**

**Energy and its commercial unit.:**

Energy is defined as the capacity of doing work. The commercial unit of energy is kilowatt-hour.

1 kWh = 1000 W × 3600 s = 3.6 × 10^{6} J

**Question 2. How are resources classified?**

**Answer:**

**Classification Of resources:**

When energy source used is easily replenished in a short period and the reserve is limitless is called renewable source. The sources of energy which are limited in quantity and are exhaustible are called non-renewable sources of energy.

**Question 3. Name the different sources of energy.**

**Answer:**

**The different sources of energy**

The five sources of energy are solar energy, wind energy, geothermal energy, hydroelectric energy and biomass energy

**Question 4. What are the different forms of energy?**

**Answer:**

**Different forms of energy:**

The various forms include mechanical energy (potential energy + kinetic energy), chemical energy, heat energy, light energy and electrical energy.

**Question 5. Define mechanical energy and its types.**

**Answer:**

**Mechanical energy and its types:**

Mechanical energy is of two types i.e., the energy possessed by a body due to its motion is called kinetic energy. The energy possessed by a body due to its position or height is called potential energy.

**Question 6. Define conservative and non-conservative sources of energy.**

**Answer:**

**Conservative and non-conservative sources of energy:**

If the work done by a force depends only on initial and final positions but not on the path taken, then the force is said to be conservative. If the work done by a force depend on the path taken then the force is said to be non-conservative.

**Question 7. Define law of conservation of energy.**

**Answer:**

**Law of conservation of energy:**

Energy can neither be created nor destroyed. It can only be changed from one form to another.

**Question 8. A car is moving with the uniform velocity of 50 km/h. What is the kinetic energy of the object of mass 40 kg kept in the car?**

**Answer:**

**Given:**

A car is moving with the uniform velocity of 50 km/h.

We know that both objects and car are moving with the same velocity

i.e., v = 50 km/h = 50 × 103/(60 × 60)

= 13.88 m/s^{2}

Mass of the object,

m = 40 kg

K.E. = \(\frac{1}{2} m V^2\)

K.E. = \(\frac{1}{2} \times 40 \times(13.88)^2\)

K.E. = 3853.08 J

**Question 9. The acceleration due to gravity is 20 m/s ^{2}, what will be the potential energy of a body of mass 1 kg kept at the height of 10 m?**

**Answer:**

**Given:**

The acceleration due to gravity is 20 m/s^{2},

Potential Energy = mgh

Mass, m = 1 kg

Acceleration due to gravity,

g = 20 m/s^{2}

Height, h = 10 m

Potential Energy = 1 × 20 × 10

= 200 J

Hence, potential energy is 200 Joules.

**Question 10. Calculate potential energy of water in a tank having l = 4 m, b = 3 m and h = 2, situated on top of a house whose height is 9 m.**

**Take rw = 1 x 10 ^{3} kg/m^{3} and g = 10 ms^{-2}**

**Answer:**

Volume of water = Volume of tank

= lbh

= 4 m × 3 m × 2 m

= 24 m^{3}

Mass of water = Volume of density

= 24 × 10^{3} kg

Mean height of water above the ground

H = h_{1 }+ h_{2}

= 9 m + 1 m

= 10 m

Therefore, Potential energy = MgH

= 24 × 10^{3} × 10 × 10

= 24 × 10^{5} J

**Question 11. A body of mass 0.5 kg is in rest at a height of 20 m above the ground.**

**Find its total energy.****What is the total energy, if it starts falling freely and has fallen through a height of 5 m? (Take g = 10 ms**^{-2})**What conclusion can be drawn from these calculations?**

**Answer:**

(1) P.E. of the body = mgh

= 0.5 × 10 × 20

= 100 J

K.E. of the body = \(\frac{1}{2} m V^2\)

= \(\frac{1}{2} \times 0.5 \times 0\)

= 0

Total energy = P.E. + K.E.

= 100 + 0

= 100 J

(2)

Velocity of the body at point B is given by

v^{2} = u^{2} + 2aS

= 0 + 2 × 10 × 5

= 100

∴ v = 10 ms^{-1}

∴ Its K.E. at point B = \(\frac{1}{2} m V^2\)

= \(\frac{1}{2} \times 0.5 \times(10)^2\)

= \(\frac{1}{2} \times 0.5 \times 100\)

= 25 J

Its P.E. at point B = mgh2

= 0.5 × 10 × 15

= 75 J

Its total energy at point B = K.E. + P.E.

= 25 J + 75 J

= 100 J

Thus, we find that the total energy of a freely falling body remains constant, though its potential energy gradually change into kinetic energy.

**Question 12. Calculate the kinetic energy of a body of mass 4 kg moving with the velocity of 0.2 meter per second.**

**Answer:**

Kinetic Energy = \(\frac{1}{2} m V^2\)

Here mass = 4 kg

Velocity = 0.2 m/s

So, by putting the values in formula:

Kinetic energy = \(\frac{1}{2} \times 4 \times(0.2)^2\)

= \(\frac{1}{2} \times 4 \times 0.2 \times 0.2\)

= 0.08

**Question 13. Define work and its types.**

**Answer:**

**Work and its types:**

If a force is applied on a body which led to displacement in the body then work is done. Work is the product of magnitude of force and displacement.

**Positive work:** If a force is applied in the same direction to displacement, then work done is said to be positive.

**Negative work: **If a force is applied in the opposite direction of displacement, then work done is said to be negative.

**Question 14. A swimmer makes a swing jump between two points, by holding one end of a rope, other end of which is tied to some higher point. What type of work is done by rope in jumping of the swimmer from one point to another?**

**Answer:**

Zero work done.

**Question 15. Calculate the work done in operating the crane, if it lifts a mass of 1600 kg through a vertical height of 40 m. **

**Answer:**

g = 9.8 m/s^{2}

F = mg = 1600 kg × 9.8 m/s^{2}

= 15680 N

W = F × S = 15680 N × 40 m = 627,200 J

**Question 16. How much work is done by a force of 15 N in moving an object through a distance of 2 m in the direction of force?**

**Answer:**

W = F × S

Hence force, F = 15 N

Distance, S = 2 m

Work done, W = 15 × 2

W = 30 Joules