# Homework Help: Physics 11 help please ?

1. Dec 19, 2008

### roxxyroxx

1. The problem statement, all variables and given/known data

A pendulum bob is pulled sideways to the left. When released, the pendulum string will contact a horizontal rod causing the pendulum to swing in an arc of smaller radius.

the height to which the bob swings on to the right is equal to the original height but i don't understand why-- please help ?

2. Relevant equations

N/A

3. The attempt at a solution

???

2. Dec 19, 2008

### Staff: Mentor

The relevant equations have to do with kinetic energy KE, potential energy PE, and total energy TE.

Show us the equations, and tell us how they could be interpreted to give you your answer.

3. Dec 19, 2008

### roxxyroxx

KE = 0.5mv^2
PE = mgh
TE = 0.5mv^2 + mgh
= mv^2 + gh

4. Dec 19, 2008

### Staff: Mentor

Good start. (Not sure what you mean by the last line, but the other lines are correct.)

Now, what common characteristic does the ball have when it is first released, and when it stops at the top of its arc after the string contacts the peg? What is the KE in each of those two cases? Answer that, and then answer the whole problem!

5. Dec 20, 2008

### roxxyroxx

umm im not 2 sure but:
mgh = 0.5mv^2 + mgh

??

6. Dec 20, 2008

### Staff: Mentor

No, how fast is the object moving when FIRST released? How fast when it reaches the top of its arc? What do those velocities tell you about the KE at those two points? Can you then write the equation that answers the original question?