Question

A man pushes a 15 kg box of books 2.0 m up a 37∘ incline into the back of a moving van. The box moves at a constant velocity if you push it with a force of 95 N. Find the work done by gravity on the box. Take (g=10 m/s2)

- −180 J
- 180 J
- −120 J
- Zero

Solution

The correct option is **A** −180 J

FBD of system is

From FBD of the block, we have the component of weight down the incline plane as mgsinθ and the displacment is up the incline plane d=2.0 m, the other component of weight will be perpendicular to plane mgcosθ.

Now here the work done will be due to the force component along the plane only which is, F=mgsinθ.

The angle between F and d is 180o

Hence work done by gravity will be only due to the force along the incline plane Wg=→F.→s=(mgsin37o)×2×cos180o

Wg=−180 J

Hence option A is the correct answer

FBD of system is

From FBD of the block, we have the component of weight down the incline plane as mgsinθ and the displacment is up the incline plane d=2.0 m, the other component of weight will be perpendicular to plane mgcosθ.

Now here the work done will be due to the force component along the plane only which is, F=mgsinθ.

The angle between F and d is 180o

Hence work done by gravity will be only due to the force along the incline plane Wg=→F.→s=(mgsin37o)×2×cos180o

Wg=−180 J

Hence option A is the correct answer

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