How Fast Does a Book Fall from 4.20 Meters?

[kissafilipino]

A physics book of unknown mass is dropped 4.20 m. What speed does the book have just before it hits the ground? Unless otherwise directed, assume that air resistance is negligible.

Height: 4.20
Acceleration: 9.81
Mass: ?
Velocity: ?

The answer box is for velocity: ___ m/s

Homework Equations

1)Kinetic Energy: 1/2*m*v^2
2)Kinetic Energy: (p^2)/2*m

3)Potential Grav. Energy: m*g*(delta or change in)h

The Attempt at a Solution

I thought the whole point was to look at how the Potential energy becomes kinetic energy, and so I went about it by using the Potential energy formula. m(9.81)(4.40) = 43.164 however I don't know if I that gives me mass or anything, I am stumped because I think I do not have enough information to answer this question. Plus going about it by using another formula but that does not seem to be working either, such as the Force = mass times acceleration, again I do not know mass, or the power = force times velocity, again I need to find force, which I do not know. Please help me!

 

[MillerGenuine]

U= potential energy
K= kinetic energy
Conservation of energy => Kf + Uf = Ki + Ui
What is the initial Kinetic Energy? (it has v=0)
What is the final potential energy? (it has h=0)
this should simplify this equation so that we have our kinetic = potential
And when you solve..the mass should cancel out.

 

[a.ratnaparkhi]

I'm satisfied with MillerGenuine's method, But I have another method. In the problem you know initial velocity which is zero, total displacement i.e. height, acceleration.
So you can think of
vf^2=vi^2+2as.

 

[kissafilipino]

THANKYOU, both your equations and formulas got the same answer (of course) but now I understand how it is possible and two ways of solving this type of question.