Question

A body thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and 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? (b) You lift a heavily packed carton of mass m in vertically upward direction through a height h. What is the work done (i) by you on the carton, (ii) by force of gravity on the carton? (c) Anil is doing work at a rapid rate but works for only one hour. Ashok does work at a somewhat slower rate but continues to work for six hours. Who has greater power? Who has more energy?

  • Work and energy CBSE Class 9
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Answer

(a) Work done is zero. This is because equal and opposite work is done in the two paths.

(b) (i) Work done by me is positive and having a value = mgh. This is because I am applying force in vertically upward direction on the carton to hold it and displacement is also in the same direction.

(ii) Work done by the force of gravity on the carton = – mgh. This in because force is vertically downward but motion is vertically upward.

(b) Anil has greater power because his rate of doing work is more. Ashok has more energy as he worked for a longer time and the total work done by him in definitely more.

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