Materials: Rounded pebble, string, thumb tack, ruler and watch.

1. Tie a pebble to a 2 m long string, hang it loosely to a thumb tack and press it onto a door or window frame or onto one side of your desk. Adjust the length L of the string to be 0.5 m. Make sure the pendulum swings freely.

2. Gently tap the pebble or pull it aside a few centimeters . The oscillations of the pendulum ought to be small. With a watch find the time T to complete 20 full cycles. Repeat the same measurement three times and find the period P in each trial, the period being P = T/20 (in seconds).

3. From Newton's second law it is possible to deduce a formula for the acceleration of gravity g ,

g = (4*L*Pi ^2 )/ P^2

Use this formula to find the acceleration of gravity g for each of the trials, its mean value and the  % error in the mean . Is the mean close to 9.8 m/s^2 ?

4. To appreciate the validity of the formula for P, change the length of the pendulum to another length L = 0.75 m and repeat steps 1-3. Do you get the same value of g?

5. If you wish to make a pendulum with a period exactly equal to P = 1 second, what length L would you need ?

6. Provide details of your set-up, and explain any difficulties you did encounter. Also discuss sources of experimental error.