NCERT Class 9 Science Solutions: Chapter 11 – Work and Energy Part 4

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Question 1:

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

  • Suma is swimming in a pond.

  • A donkey is carrying a load on its back.

  • A wind mill is lifting water from a well.

  • A green plant is carrying out photosynthesis.

  • An engine is pulling a train.

  • Food grains are getting dried in the sun.

  • A sailboat is moving g due to wind energy.

Answer:

Work is done whenever the given two conditions are satisfied:

(i) A force acts on the body.

(ii) There is a displacement of the body by the application of force in or opposite to the direction of force.

(a) While swimming, Suma applies a force to push the water backwards. Therefore, Suma swims in the forward direction caused by the forward reaction of water. Here, the force causes a displacement. Hence, work is done by Seema while swimming.

(b) While carrying a load, the donkey has to apply a force in the upward direction. But displacement of the load is in the forward direction. Since, displacement is perpendicular to force, the work done is zero.

(c) A wind mill works against the gravitational force to lift water. Hence, work is done by the wind mill in lifting water from the well.

(d) In this case, there is no displacement of the leaves of the plant. Therefore, the work done is zero.

(e)An engine applies force to pull the train. This allows the train to move in the direction of force. Therefore, there is a displacement in the train in the same direction of force. Therefore, there is a displacement in the train in the same direction. Hence, work is done by the engine on the train.

(f) Food grains do not move in the presence of solar energy. Hence, the work done is zero during the process of food grains getting dried in the Sun.

(g) Wind energy applies a force on the sailboat to push it in the forward direction. Therefore, there is a displacement in the boat in the direction of force. Hence, work is done by wind on the boat.

Question 2:

Q. 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?

Q 2 Image Shwoing Work Done.

Q 2 Image Shwoing Work Done

Answer:

Work done by the force of gravity on an object depends only on vertical displacement. Vertical displacement is given by the difference in the initial and final positions/heights of the object, which is zero.

Work done by gravity is given by the expression,

Where,

Therefore, the work done by gravity on the given object is zero Joule.

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