# Simple pendulum problem - having trouble with the math

1. Jun 20, 2011

### Confused20

Simple pendulum problem - having trouble with the math!!!

1. The problem statement, all variables and given/known data
A simple pendulum is set up on Mars where g=3.72m/s^2. How long should pendulum be if period T is to be 2.00 seconds?

2. Relevant equations
T=2pia(3.14) times square root of l(length)/gravity

3. The attempt at a solution
I placed in 2 seconds for T and divided it by 2pia(3.14). Then I squared this number and then multiplied it by 3.72 to solve for l(length). The correct answer is supposed to be .965 but I did not get this answer. Can anyone please help. I have an exam in 2 days!!!!!!!! Any input will be greatly appreciated.

2. Jun 20, 2011

### lewando

Re: Simple pendulum problem - having trouble with the math!!!

So, what did you come up with? I think the "correct answer" is wrong.

3. Jun 20, 2011

### Confused20

Re: Simple pendulum problem - having trouble with the math!!!

Thank you for your response. The answer I got was .377 meters. My professor stated in class that the answer was .965. Is this the answer that you got as well? Is the manner in which I approached the algebra correct?

4. Jun 20, 2011

### lewando

Re: Simple pendulum problem - having trouble with the math!!!

Yes, that's what I got. Your algebra looks immaculate! As a check, you can always plug your result back into the equation and see it you get the right T. Good luck with your test (and welcome to PF!)

5. Jun 20, 2011

### Confused20

Re: Simple pendulum problem - having trouble with the math!!!

Thank you very much for confirming that I did the algebra correctly. My professor also suggested that we do a variation of the problem where length=.965m and T=2.00 and then attempt to solve for gravity (answer should be 3.72 m/s^2). Can you assist me in solving this problem. We are of course using the same equation of T=2pia(3.14) times the square root of length over gravity. I began to attempt to solve this problem in a similar manner by dividing 2.00 by 2pia(3.14) and then squaring that number. The next step is puzzling for me because of my lack of algebra. I decided to divide that number by .965. Can you please assist me in this last step. The answer I get is .105 m/s^2. Any help on how to solve this problem will be greatly appreciated.

6. Jun 20, 2011

### lewando

Re: Simple pendulum problem - having trouble with the math!!!

Notice in this equation that if you increase l (say, from 0.377 to .965), then g must also increase for T to remain constant.

$$T = 2\pi\sqrt{\frac{l}{g}}$$

Here is what you did:

$$\frac{T}{2\pi} = \sqrt{\frac{l}{g}}$$

$$\frac{T^2}{4\pi^2} = \frac{l}{g}$$

$$\frac{T^2}{4l\pi^2} = \frac{1}{g}$$

I think you solved for 1/g. Can you think of what to do to bring up g to the numerator?

7. Jun 20, 2011

### SteamKing

Staff Emeritus
Re: Simple pendulum problem - having trouble with the math!!!

If you use g = 9.81 m/s^2 in the formula for the period of a pendulum, then the length of 0.965 m gives a period of 2 s. An l = 0.377 m and a g = 3.72 m/s^2 also gives a period of 2 s. Your 'professor' owes you an apology and he is also due a head slap for not checking his math.

8. Jul 3, 2012

### dinyatta32

Re: Simple pendulum problem - having trouble with the math!!!

Im also having problems with this simple pendulum equation if you have a 1m long pendulum with a 2kg mass is pulled back 0.1 m and then released so that it swings back and forth predict what the horizontal position x will look like as a function of time i cant figure out how this equation goes i have been on the same thing for 2 days im stuck can someone help me please