# Consider a pendulum that that has a length of $50$cm …

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?

• 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

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 $l=0.5 m$.