The purpose of this lab is to investigate uniformly circular motion by investigating the forces that lead to circular motion.

Part A: Circular motion caused by a string

Calculate the number of revolutions per minute N required to keep the hanging mass at an angle of 45 degrees from the vertical as it rotates around. 

(HINT: first find v as a function of the quantities in the problem such as R, m, g, q, etc. Next recall that the period of the motion T (not to be confused with the tension T in the string) equals the distance travelled divided by the velocity, so T = (2pR)/v. This gives you the time in seconds for one revolution, but we want the number of revolutions in one minute:  N = 60/T.)






N = 

Now MEASURE the number of revolutions in one minute for your apparatus if the mass is kept at 45 degrees.

Nexpt =

Now compare the theoretical and experimental values.

% Difference = 



Part B: Circular Motion with the spring attached

If the spring keeps the hanging mass exactly vertical as it spins around at a known number of revolutions per minute, derive an expression for the spring force Fs that is required for this motion. (First draw a freebody diagram for the situation.)







(Answer: Fs = 4p2Rm/T2 )

Measure the time for one revolution by timing 20 revolutions and dividing by 20. Record your mass value and the radius of the circle.

20T =              T m =   R =

Calculate the theoretical spring force:

Fs =


Measure the length of the spring when the mass is vertical.

x = 

Use another technique to measure the force required to stretch the spring to that length.

F =

Now compare the theoretical and experimental values.

% Difference =