What is the potential and kinetic energy of a falling object?

[SmallPub]

Homework Statement

A physics book weighing 20 N is dropped from a position 2.0 meters above the floor. a) what is the potential energy of the book before it is dropped? b) what is the kinetic energy of the book when it is 1.0 meter from the floor

Homework Equations

Ek = 1/2mv^2 , Ep= mgh

The Attempt at a Solution

a) Ep = mgh = 20N X 2.0 = 40N

B) I don't know what to do ):

 

[vela]

What forces act on the book? Are any non-conservative?

 

[SmallPub]

What forces act on the book? Are any non-conservative?

Non-Conservative forces acting on the book are , air resistance, sound and heat. The force acting on the book are gravity

 

[AJKing]

How far has the book traveled?

And what are the equations that tell you velocity under constant acceleration after a certain distance?

 

[SmallPub]

How far has the book traveled?

And what are the equations that tell you velocity under constant acceleration after a certain distance?

book has traveled 1.0 meters? ac= v^2/r ?

 

[vela]

Sound and heat aren't forces. They don't push or pull on the book, right? Air resistance, however, is a force, but you can neglect it. So gravity is the only force acting on the book, and it's conservative. So you can use energy conservation. That means the energy at the initial point is equal to the energy at the final point.

What's the total mechanical energy at the initial point? You already found the potential energy there. What is the book's kinetic energy initially? Add the two together to find the total energy.

At the final point, the book is one meter off the floor. What does the total energy consist of there?

 

[SmallPub]

Sound and heat aren't forces. They don't push or pull on the book, right? Air resistance, however, is a force, but you can neglect it. So gravity is the only force acting on the book, and it's conservative. So you can use energy conservation. That means the energy at the initial point is equal to the energy at the final point.

What's the total mechanical energy at the initial point? You already found the potential energy there. What is the book's kinetic energy initially? Add the two together to find the total energy.

At the final point, the book is one meter off the floor. What does the total energy consist of there?

So if I am right, Ep2 = 20N x 1.0 m = = 20N

Ep1 + Ep2 = 40N + 20N = 60N

Ek= 60N?

 

[vela]

You got the potential energy at the end almost correct. You have the wrong units.

Energy conservation says that
\begin{align*}
E_i &= E_f \\
KE_i + PE_i &= KE_f + PE_f
\end{align*} So you have the potential energies. What are the kinetic energies?

 

[SmallPub]

You got the potential energy at the end almost correct. You have the wrong units.

Energy conservation says that
\begin{align*}
E_i &= E_f \\
KE_i + PE_i &= KE_f + PE_f
\end{align*} So you have the potential energies. What are the kinetic energies?

Ek= 60 J?

 

[vela]

No. You have a formula for kinetic energy. Use that. And there are two, one at the beginning and one at the end.

 

[SmallPub]

No. You have a formula for kinetic energy. Use that. And there are two, one at the beginning and one at the end.

Thanks boss