Class 9 Science

Work Energy

Exercise Question Answer

Question 1. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’.

(a) Suma is swimming in a pond

Answer: Suma is applying force on water in backward direction. The reactionary force is being applied on Suma in forward direction which helps Suma in moving forward. Here, the direction of force applied by water and direction of displacement of Suma are in same direction. Hence, work is done in this case.

(b) A donkey is carrying a load on its back.

Answer: The donkey is applying force in upward direction but the donkey is moving in forward direction. Here; force and displacement are in mutually perpendicular directions. Hence, work is not done in this case.

(c) A wind-mill is lifting water from a well.

Answer: The wind mill is applying force in upward direction. Displacement of water is also in upward direction. Hence, work is done in this case.

(d) A green plant is carrying out photosynthesis.

Answer: No displacement is happening and no force is being applied during photosynthesis. Hence, no work is done in this case.

(e) An engine is pulling a train.

Answer: The force applied by the engine and displacement of train are in same direction. Hence, work is done in this case.

(f) Food grains are getting dried in the sun.

Answer: No force is being applied and no displacement is taking place during this process. Hence, no work is done in this case.

(g) A sailboat is moving due to wind energy.

Answer: The wind energy is applying force on the sail, and sailboat is propelled in the direction of force. Hence, work is done in this case.

Question 2. An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

Answer: Force of gravity is being applied in downward direction, but displacement is in horizontal direction. Hence, no work is done in this case.

Question 3. A battery lights a bulb. Describe the energy changes involved in the process.

Answer: Chemical energy changes to electrical energy in the battery. Electrical energy changes to heat and light energy in the bulb.

Question 4. Certain force acting on a 20 kg mass changes its velocity from 5 m s–1 to 2 m s–1. Calculate the work done by the force.

Answer: Given; m = 20 kg, u = 5 m/s and v = 2 m/s

Kinetic energy when velocity is `5 m//s = ½ mv^2`
`= ½ xx 20 kg xx (5 m//s)^2 = 250 J`

Kinetic energy when velocity is `2 m//s = ½ mv^2`
`= ½ xx 20 kg xx(2 m//s)^2 = 40 J`

Difference `= 250 J – 40 J = 210 J`

So, work done is – 210 J (Work done is negative because kinetic energy has reduced.

Question 5. A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.

Answer: Here, direction of gravitational force and that of displacement are perpendicular to each other. Hence, no work is done in this case.

Question 6. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Answer: When an object is in free fall, its potential energy decreases progressively. At the same time, the kinetic energy of the object increases progressively. At any given time, the sum of potential energy and kinetic energy is constant. Thus, it obeys the law of conservation of energy.

Question 7. What are the various energy transformations that occur when you are riding a bicycle?

Answer: The chemical energy from food is converted to kinetic energy while someone pedals the bicycle. This energy is then transferred to the wheels of the bicycle.

Chemical energy (food) → Muscular energy → Kinetic energy + Heat Energy

Question 8. Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

Answer: When we push a huge rock but the rock does not move, no energy is transferred to the rock. Here; the muscular energy is converted into heat energy which is evident from the rise in temperature of our body while doing so.

Question 9. A certain household has consumed 250 units of energy during a month. How much energy is this in joules?

Answer: 1 unit of energy = 1 kWh
`1 kWh = 3.6 xx 10^6\ J`
So, `250 kWh = 250 xx 3.6 xx 10^6\ J = 9 xx 10^8\J`