1.

In which of the following cases is the work done positive or zero or negative?a) Work done by the porter on a suitcase in lifting it from the platform on to his head.b) Work done by the force of gravity on suitcase as the suitcase falls from porter’s head.c) Work done by the porter standing on platform with suitcase on his head.d) Work done by force of gravity on a ball thrown up vertically up into the sky.e) Work done by force applied by hands of a man swimming in a pond.

Answer»


(a) Positive :
SINCE, Porter pulls the suitcase up to the platform, and suitcase lifts up.
So, Both Force and dusplacement are in same direction.
So, Work Done is positive.

(b) Positive :
As suitcase falls down, it displaces in downward direction and force of gravity also acts in the same direction.
=> Work done by gravity force is positive.

(c) Zero
When Porter stands on the platform with suitcase in his head,then there is no moment in suitcases.
So, work done by Porter on suitcase is 0.

(d) Negative :
When ball is thrown UPWARD, it displaces in upward direction but Force of gravity acts in downward direction, so Work Done by Force of gravity is negative.


(e) Positive:
While swimming in still water, swimmer push water in backward direction and it displaces water back ward while GIVING swimmer an EQUAL and opposite force in forward direction to SWIM.
So, Work done by swimmer on water is positive and also work done by water on swimmer.



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