Question

A student of Physics applies a constant force of 25Newtons while lifting a heavy Physics book up into the air a distance of 2.0meters. How much Work does the student do on the book?
1) 0joules, because gravity pulled down on the book
2) 5.0joules
3) 50 joules
4) The answer cannot be found without knowing the mass of the book.
5) The answer cannot be found without knowing the net Force on the book.
*Write the detail reason of your above selected choice as well.

Answer 1

Work = Force • Displacement • Cosine(theta)

If we assume the lift was vertical (theta = 90 degree )
then work done = 25 newtons (force) * 2.0 meter (displacement) * Cosine (90 degree)
therefore work done = 0 joule ( as cosine (90 degree) is 0 )

So the answer is 1) 0 joules

If the lifting angle varies the workdone varies

Answer 2

can we take theta 0 ?

Answer 3

Yes you can take thera 0 when theta is 0 direction of displacement vector and force vector are in same direction (just refer the image )

Answer 4

so for above question we can take 0 ??

Answer 5

@Jose4L

Answer 6

Yes you can take theta = 0 if the displacement vector (due to) force vector both having same direction
If you apply a force of 25 newtons to lift an object and if you lift the object 2.0 meters vertically above then both displacement vector and force vector have same direction so you can take theta = 0 degree