III. Procedure


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

  1. 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.
  2. Reattach the mass (C) to the crossarm string and let the mass hang freely.
  3. Position the indicator rod (D), by loosening the thumbscrews, so that it sits at the position closest to the vertical shaft (F).
  4. Adjust the crossarm (A) so that the mass (C) hangs freely exactly over the indicator rod (D).
  5. 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. 
  6. 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 Fspring.  
  7. Remove the weight hanger by detaching the hook from the mass (C).  
  8. 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.  
  9. 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.)   
  10. Repeat steps 2-9 for at least four more positions of the indicator rod, including the position farthest from the vertical shaft.  
  11. From this data, calculate the centripetal acceleration ac and plot a graph of Fspring versus ac. Does the plot approximate a straight line?  
  12. 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.