Download NCERT Solutions for Class 9 Science Chapter 11 Work and Energy in PDF form. Solutions of other chapters of **Class 9 Science** are also in PDF form to free download. Download **NCERT Solutions** for class 9 all subjects.

## NCERT Solutions for Class 9 Science Chapter 11

### Class 9 Science – Physics – Solutions

#### Work and Energy – Important Questions with answers

**Question 1**:

A boy of mass 55 kg runs up a flight of 40 stairs, each measuring 0.15 m. calculate the work done bye the boy.**Answer 1**:

3300 J**Question 2**:

A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground. Calculate the work done by him on the luggage.**Answer 2**:

225 J**Question 3**:

A bullet of mass 20 g is found to pass two points 30 m apart in 4 s? Assuming the speed to be constant, find its kinetic energy.**Answer 3**:

0.5625 J**Question 4**:

A bullet of mass 5 g travels with a speed of 500 m/s. if it penetrates a fixed target which offers a constant resistive force of 1000 N to the motion of the bullet, find (a) the initial kinetic energy of the bullet (b) the distance through which the bullet has penetrated.**Answer 4**:

(a) 625 J (b) 0.625 m**Question 5**:

What is the work done to increase the velocity of a car from 30 km/h to 60 km/h if the mass of the car is 1500 kg?**Answer 5**:

156250 J**Question 6**:

A ball of mass 1 kg is dropped from a height of 5 m. (a) Find the kinetic energy of the ball just before it reaches the ground (b) what is the speed at this instant?**Answer 6**:

(a) 50 J (b) 10 m/s**Question 7**:

An object of mass 12 kg is at a certain height above the ground. If the potential energy of the object is 480 J, find the height at which the object is with respect to the ground. Given, g=10 m/s²?**Answer 7**:

4 m**Question 8**:

A man does 200 J of work in 10 s and a boy does 100 J of work in 4 s. (a) who is delivering more power? (b) Find the ratio of the power delivered by the man to that delivered by the man to that delivered by the boy.**Answer 8**:

(a) The boy delivers more power

(b) 4/5 J**Question 9**:

Two girls, each of weight 400 N, climb up a rope through a height of 8 m. We name one girl A and the other girl B. Girl A takes 20 s while b takes 50 s to accomplished this task. What is the power expended by each girl?**Answer 9**:

160 W, 64 W**Question 10**:

Why do some engines require fuels like petrol and diesel?**Answer 10**:

Internal combustion heat engines use the chemical energy of fossil fuels (petrol and diesel) for their operation. These engines first convert the chemical energy of the fuels into heat energy which is later on converted into mechanical energy.**Question 11**:

What is the difference between potential energy and kinetic energy?**Answer 11**:

Potential energy of a body is by virtue of its position or shape whereas kinetic energy of a body is by virtue of its motion.