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I have seen this example in several books.

The work done by a porter in carrying a luggage on head while walking on a horizontal force is $0$ as the gravitational force is at $90^0$ with the horizontal plain.

Now,I cannot accept it as I think that the gravitational is not causing the displacement.So,we cannot relate this force with another displacement caused by a different force (here,the muscular force of the porter).

So,am I correct?

Thanks for any answer!!

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  • $\begingroup$ Forces don't cause displacements. Forces cause changes in velocity. $\endgroup$ – march Nov 16 '15 at 17:33
  • $\begingroup$ @march-It may not be directly related but a subtle factor for displacement... $\endgroup$ – tatan Nov 17 '15 at 7:42
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When you compute the work of individual forces it does not matter what causes the displacement. As long as the force is acting while the object is being displaced, it has the potential to do work. Both gravity and the normal on the head are perpendicular to the displacement and do not perform work. Friction (a force made by the porter over the luggage) is horizontal and produces work

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  • $\begingroup$ So,you want to say that the example is correct but not the explanation. $\endgroup$ – tatan Nov 17 '15 at 7:25
  • $\begingroup$ the statement as you wrote it it is very wrong, because 1) the porter does work, 2) it only considers one force 3) gravity is not a force that the porter does. May I ask what book says things exactly that way? $\endgroup$ – user83548 Nov 17 '15 at 13:12
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The weight of the luggage is in downward direction. So to balance the gravitational force of the luggage, the porter have to apply the force in upward direction. That's why the angle between the force applied and the displacement is 90 degree. So the work is 0.

So in simple words, its not really the gravitational force but the force applied by us on the luggage.

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