Chapter 10

NCERT
Class 9
Science
Solutions
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". Suma is swimming in a pond. A donkey is carrying a load on its back.

Question:

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 windmill 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 due to wind energy.

Answer:

Work done in both negative and positive conditions is considered to be work done. Explanation is given below:

  • Yes, work is done, but it is negative because Suma is pushing water backward.
  • No, the work is not done, because the donkey is standing and the load placed on it's back is perpendicular to the horizontal displacement.
  • Yes, work is done, because the water is being lifted against the force of gravity.
  • No, work is done, because the leaves of the plant are in rest.
  • Yes, it is working, because the engine is pulling the train and the wagons are being displaced in the same direction.
  • No, because the grains are at rest.
  • Yes, the work is done, because the wind is pushing the sail-boat in favorable directions.

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