Lab: Centripetal Force
Purpose:
The purpose of this lab is to investigate the relationship between the velocity of an object
in uniform circular motion and the centripetal force on the object.
Variables:
 Radius of circular motion (r): The horizontal distance measured from the top of the tube to
the stopper when the stopper is swinging in a circle. Radius must be measured in meters.
 Period (T): The time in second for the satellite (rubber stopper or ball) to do one complete
circle.
 Centripetal Force: The force causing the satellite to travel around in a circle. In this case,
the satellite is attached to a string which has weights weighing it down on the other end.
Without the weights, there would be nothing to counteract the satellite. Therefore, the
force on gravity on the weights must be supplying the centripetal force for the satellite.
Apparatus:
Pre-Lab Questions:
1. Define centripetal force.
2. What is providing the centripetal force in this lab? How would you calculate this force?
3. How will you calculate the linear velocity of the stopper?
Procedure:
Speed vs. Centripetal Force (Constant Radius)
1. Place one weight on the bottom clip of the apparatus (or tie it on). Record this total mass.
2. Pick a target radius. Measure out the target radius from the satellite mass to the top of
the glass tube. Then attached a paper clip to the string a little below the bottom of the
glass tube.
3. Practice whirling the stopper until you can keep the paper clip a short distance below the
tube. IMPORTANT! If the clip touches the bottom of the glass tube, the weights are no
longer supplying the centripetal force. If the clip raises or falls appreciable as the stopper
whirls, the radius of the circle is changing. Practice!
4. Use a stopwatch to measure the time taken for a reasonable number of revolutions (20 – 30
perhaps). Record your data.
5. Change the number of weights on the bottom clip.
6. Repeat steps 3 – 5 for at least 5 different weights.
Data Table:
Using Excel, create a data table to adequately hold the needed data.
Calculations:
Show an example calculation for the period, the velocity, and the centripetal force.
Graphing:
Using Excel, create a graph. Your graph should graph the centripetal force on the x-axis
against the velocity on the y-axis. A POWER trendline needs to be fitted to the data to produce
the needed equations.
Percent Error:
The correct equation is:
Fc  msatellite
v2
 Fc  v2  v  Fc  v  Fc1/ 2
r
The excel graph should give an exponent for the y-value (velocity). Use the Excel exponent
as the experimental value and 0.5 as the actual value to find the percent error.
Conclusion:
 Restate the purpose of this lab.
 Summarize your procedure.
 State your findings (include your equation with the appropriate variables, not x and y) and
percent error.
 Identify three sources of error while doing the lab.
 How would this error effect your data (would it make your data too large, too small, ect)
 How could this lab be made better to fix your sources of error?
Post Lab Questions: (use real centripetal force equations from in class)
1. Why is an unbalanced force required to produce circular motion?
2. A rubber stopper of mass 0.014 kg is swung at the end of a string 0.95 m long with a period
of 0.75 sec. Compute the centripetal force exerted on the stopper using the true equation.
What to turn in both to class & turnitin.com:
Typed paper with the following: (25 pts)
 Title (1pt)
 Purpose (1pt)
 Pre-lab Questions (3pts)
 Data table (includes raw data) (3 pts)
 Sample Calculations (3 pts)
 Graph (3 pts)
 Percent error calculation (1 pt)
 Conclusion (8 pts)
 Post Lab Questions (using correct centripetal force equation from notes) (4 pts)