Question

If you are holding a book and you let go of it, why does it fall? Explain in terms of forces, potential energy, and kinetic energy.

Answer 1

Let the mass of the book be "m", acceleration due to gravity be 'g", velocity "v" and height be "h".

Now if we are holding a book at a certain height (h), the potential energy will be maximum which is equal to mass×acceleration due to gravity× height (=mgh).

(Remember: kinetic energy=0)

Now we consider that the book is dropped, in this case a force will act downward towards the centre of the earth, Force=mass×acceleration due to gravity (F=mg). It is equal to the weight of the book.

While the book is falling, the potential energy stored in the book converts into kinetic energy and strikes the floor with the maximum kinetic energy=(1/2)×mass×velocity² (=1/2mv²).

(Remember: potential energy=0)

Due to the process the whole energy is conserved.

As potential energy decreases kinetic energy increases.