If an object moves in a circle and the centripetal force “turns off” what happens? How are centripetal and centrifugal forces related?
How do you propagate errors through an equation that has three terms?
How do you compare two numbers when each has an error margin?
1) Derive the equation you will use to calculate the centripetal force from the mass of the
bob, m, the number of revolutions counted, N, and the frequency of revolutions, f. Check
with your instructor to verify you have the correct equation before continuing. Include
this derivation in the data analysis section of your report.
2) Adjust where the cross-bar is clamped to the rotating shaft so the un-stretched spring
pulls the bob about 2 – 3 cm away from vertical. Disconnect the spring and record the
radius of rotation from the index.
3) Reconnect the spring and rotate the shaft fast enough so the bob continuously maintains
this constant radius of rotation. Continually exert a small torque on the top of the rotating
shaft to keep the bob’s radius as constant as possible.
4) Examine the bob as it flies past the chosen index marker. Estimate the variation in the
radius of the bob’s circular path. If the bob swings back and forth through a small
distance of b centimeters, then the error in the radius is ± b/2. Record this error (in
meters) on your data sheet. Calculate the percent error in the radius.
Strings
Counter
Weight Rotating
Shaft
Spring
Pulley
Index
Rotating
“bob”
F = mv 2
r
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