Experiment  #                                              P205  General College Physics I


                                       Centripetal  Forces



Group Names:__________________________________________________


Object:       To measure the centripetal force acting on an object traveling in a circular path at constant angular velocity.


Apparatus: Centripetal force apparatus, stopwatch, set of weights, weight hanger and pulley.


Theory:      The linear speed of the rotating bob is given by

                                        V = 2pR/T

                        Where      R = radius of the circle

                                        T = Time taken for one rotation (Time period)

                        The centripetal force is given by

                                       Fc = Mv2/R

                        Where     M  = mass of the bob


Procedure:     The apparatus has a vertical spindle with a cross arm at the top.  From this arm is suspended a plumb bob, which will tend to move outward as the spindle rotates.  The outward motion is restrained by a spring.  The experiment consists of comparing the centripetal force, calculated from the observed speed and radius, with the static force required to stretch the spring to the same length.

1.                                         Before you make any measurements, the rotating system should be leveled.

2.                                         The rotating system should also be balanced.  To do this first establish the radius at which you want the bob to rotate, by setting the movable pointer (on the base) to this position.  Unhook the spring and remove the counter-weight from the cross arm.  Slide the cross arm until the bob hangs directly over the pointer.  Level the base with the 3 screws provided, so that the rotating arm does not tend to settle in any one position.  Now replace the counter-weight and set it so that the arm does not tend to rotate even when the base is tipped. Be sure all the screws and locknuts are tight!



3.                                          Attach one of the given springs (hard or soft).  Turn the spindle at such a rate that the bob passes directly over the pointer on each revolution.  Record the time t, taken to complete 25 revolutions.  Calculate and record time T, taken for one revolution. 

4.                                          Repeat #3 procedure at least one more time to get an average value of   T, after two trials.

5.                                        Find the equivalent static force by connecting one end of a string to the plumb bob and passing the other end of the string over the pulley provided and hang weights from it.  The weights should be adjusted until the bob is once again over the pointer.  Record this weight as the static force value in the table.

6.                                          For the same radius add 0.150 kg mass to the bob.  Repeat procedures #2 to #5.

7.                                          Change the radius and repeat procedures #2 to #6.

8.                                          Change the spring and repeat procedure s#2 to #7.





          TABLE I



Radius  m

Total mass of the bob  in kg

Time    t in     sec

Time  T in sec

Centripetal Force in N

Static Force in  N

%  diff.











































































1.    Calculate the centripetal force for each of the row entries in

        Table 1. 

2.    Compare this with the static force in each case.  Find the percentage difference between the centripetal force and the corresponding static force.


Results:      Report your results in a brief statement.

Sources of Error: Write down the sources of error in this experiment.