# Homework Help: Conservation of Energy,pendulum question

1. Jun 25, 2010

### missileblitz

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

A pendulum consists of a mass on a 50 cm long string. It is being held stationary at a 30°angle with the vertical. After release, how fast will it be going at the bottom? (Hint: Use trigonometry to find how high the ball is initally)

2. Relevant equations

Ek = 1/2mv^2 for sure to find the velocity
Eg = mgh

3. The attempt at a solution
I'm basically brain-dead on this question, I don't know where to start.

2. Jun 25, 2010

### Chewy0087

Draw a diagram first, that always helps.

Hint: Trigonometry, you're going to have to consider cos of the angle in this situation. It's like a force, you need to consider the vertical component of the string. In forces that would be the vertical force, but here it's the length.

Last edited: Jun 25, 2010
3. Jun 25, 2010

### graphene

Yes, draw a diagram with both the initial position and the final postion(bottom-most).
Then find the height difference between the two positions. It will be a function of the angle.
And then apply energy conservation.

4. Jun 25, 2010

### missileblitz

So basically I've had a go on the question with the help of the previous posts:
[PLAIN]http://img532.imageshack.us/img532/6052/physicsi.png [Broken]

So to find the left side of the triangle (lets say n):

50 x cos30 = n
n = 43.3

The string is 50cm
50 - 43.3 = 6.70 cm
x = 6.70cm
=0.0670m

Since Ep @ top = Ek @ bottom
m x 9.80 x 0.0670 = 1/2mv^2, m's cancel out I think?

0.6566 = 1/2v^2
sqrt 1.3132 = sqrt v^2
1.15 m/s = v

I'm not sure about the significant digits, but this is how I tried to solve the question, can anyone confirm the answer?

Last edited by a moderator: May 4, 2017
5. Jun 25, 2010

### Agent M27

Assuming your arithmetic is all correct that looks to be correct, and yes for this relationship your mass is negligible.

Joe