Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I am trying to do a simple pendulum problem but for some reason my answer is different from the book's answer and I don't know what I am doing incorrectly.

The question is:

Consider a simple pendulum that has a length of $50$cm and a maximum horizontal displacement of $8$ cm.

The first portion asks to find the period. So period of a pendulum can be calculated as $T=2\pi \sqrt{\frac{l}{g}}$ where $l$ is the length and $g$ is gravity = $9.81$. Therefore, period of my pendulum should be:

$$\begin{align} &T=2\pi \sqrt{\frac{l}{g}} \\ &T=2\pi \sqrt{\frac{50}{9.81}}\\ &T=14.185 \end{align}$$

But the back of the is suggesting that answer is $T=1.412$s

What am I doing wrong?

share|cite|improve this question
I think you are mixing units --- that figure for $g$ is in meters, but your lengths are in centimeters. – Gerry Myerson Apr 14 '13 at 12:34
up vote 1 down vote accepted

Units! The $l=50$ is for 50 cm, but $g=9.81 m/s^2$ (meters per second second). So you should convert them to the same units before you divide, say write $l=0.5 m$.

share|cite|improve this answer
What a silly mistake! Thanks! – Jeel Shah Apr 14 '13 at 13:01

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.