1.  Open the Worksheet and fill in the header information.

2.  Examine the centripetal force apparatus to see how it is constructed. Notice that the spring tension is comparatively strong. Practice rotating the bob at a steady rate by maintaining its alignment with the pointer. Each lab partner should try this operation. If there are any questions, consult the Instructor.

3.  Remove the bob. If the spring is attached, remove it. Measure the mass of the bob using the balance, and record it in cell D9 of the Worksheet. Be sure to estimate the measurement uncertainty and record the value in cell F9.

4.  Adjust the position of the bob so that it is located as close to the axle as possible. Notice that there is a flat on the support rod where the thumbscrew can clamp it. Do not go beyond the flat portion on the rod. Make sure that the bob is clamped securely.

5.  Adjust the pointer so that it is accurately lined up with the tip of the bob. Measure the distance from the pointer to the center of the axle, and record the value in cell D12. The uncertainty in the measurement should be entered in cell F12. Be sure to use SI units! This is the value of r, the radius of rotation.

6.  Adjust the position of the counterweight so that it is located as close to the axle as you can get it. Do not go beyond the flat portion on the support rod.

7.  Attach the spring to the bob and to the axle support.

One partner (the rotator) will turn the rod and try to maintain a constant rotational speed in alignment with the pointer, while the other partner (timer) will be responsible for the timing. The rotator should take a few practice turns to see what is necessary to maintain the alignment of the bob with the pointer. The timer should prepare the stopwatch and get ready to count the number of revolutions.

8.  When both partners are ready, begin the first run by spinning the axle. Make sure the rotating bob is lined up with the pointer before beginning the timing. It is easiest to start counting turns and to simultaneously start the stopwatch; then when a certain number of turns (e.g. 100) is reached hit the stop button on the stopwatch.

9.  Record the number of turns N in cell D14. Record the elapsed time Δt with its uncertainty in cells D16 and F16, respectively.

10.  Calculate the period T and record the value along with its uncertainty in cells D18 and F18.

11.  With the bob at rest, attach a cord to the outer end of the bob and string it over the pulley.

12.  Add enough weights to stretch the spring and bring the bob into alignment with the pointer. Record the value of the mass required to balance the spring force in cell D20. (Don't forget the mass holder.) Include the uncertainty in this measurement in cell F20. Multiply the mass by the acceleration due to gravity to get the force required, and enter the result in cell D21 along with its uncertainty in cell F21.

13.  Repeat this procedure for three more trials (giving a total of four trials in all), each time moving the pointer and the bob out by an additional distance of 1 cm. Take care to move the counterweight out further from the axle to maintain rotational stability.