Making some judgment about the validity of the last statement in the Theory
section is your goal. You must make comparisons for values of F_{spring}
and a_{c} for five different values of the radius of rotation and these
radii should cover the entire range of values allowed by the apparatus.

- Remove the
mass (C) from the crossarm (A) and weigh it. Call up Excel and begin to
enter your data into a well-formatted spreadsheet.

- Reattach the
mass (C) to the crossarm string and let the mass hang freely.

- Position the
indicator rod (D), by loosening the thumbscrews, so that it sits at the
position closest to the vertical shaft (F).

- Adjust the
crossarm (A) so that the mass (C) hangs freely exactly over the indicator
rod (D).

- The
radius of rotation of the mass (C) is the distance from the center of the
indicator rod (D) to the center of the vertical shaft (F). Measure this
distance with the ruler.

- Attach
the spring (G) to the mass (C). Attach the weight hanger to the mass using
the short string and hook. Suspend the weight hanger over the pulley. Add
enough mass to the weight hanger so that the spring is stretched and the
mass (C) hangs directly over the top of the indicator rod. It is important
that the mass be vertical, as it should be if you did step 4. The total
weight, weight hanger plus any weights on the hanger, needed to stretch the
spring to this point is the spring force F
_{spring}.

- Remove
the weight hanger by detaching the hook from the mass (C).

- Rotate
the system by applying torque with your fingers on the knurled portion of
the vertical shaft (F). With a little practice, the rate of rotation can be
adjusted to keep the mass (C) passing directly over the indicator rod (D). A
piece of white paper located to provide a light background should help you
to see that the mass passes directly over the indicator rod.

- While
maintaining a steady rotational speed, use the stopwatch to measure the time
required for your choice of number of revolutions. It is important that the
mass passes as close as possible to a point directly over the indicator rod.
(Make sure you count correctly.)

- Repeat
steps 2-9 for
__at least four more positions__of the indicator rod, including the position farthest from the vertical shaft.

**From this data, calculate the centripetal acceleration a**_{c}and plot a graph of F_{spring}versus a_{c}. Does the plot approximate a straight line?

- Do
a least-squares fit of your data to find the slope and intercept of the
best-fit line and plot the best-fit line on your graph. Obtain a copy of
both the spreadsheet and the graph for your notebook.