CENTRIPETAL FORCE
(30 minutes)
(Alward File: "c:\textbook\centrip2.Doc" 10-12-96)
Exp. # 10
Name: _____________________ Partners: _______________________Section No. _______
Equipment:
spring)
centripetal force Apparatus (with cross-bar, counter-weight, black hanging weight,
meter stick stopwatch 20-N spring force scale
pan balances
Note to instructor: after the last lab section, the apparatus must be dismantled. Remove the cross-bar,
counter weight, and hanging mass, but leave the spring attached to the hook.
Purpose:
The purpose of this exercise is to verify that the centripetal force acting on a body of mass m moving in a
circular path of radius r is given by Fc = mv2/r.
Procedure
1. Use the triple-beam balance to find the mass of the black object; record this mass in the table.
2. If the crossbar and counterweight are not already attached, place them on the apparatus now; hang the
black weight from the string at the end of the cross bar. Adjust the position of the counterweight until
the platform remains stable when the weight is rotated.
1
3. Move the vertical pointer to its farthest position from the assembly’s rotational axis. This pointer
will serve as a reference point during manual rotation of the mass. Measure the distance between the
center of the axle to the reference pointer. This distance will be the radius of the circular path around
which the mass will be traveling. Record this radius r in the table.
Pull the mass out until it is directly over the reference pointer and adjust the arm with the counterweight
until the strings attached to the black mass are vertical.
4. Practice rotating the mass by twisting the axle; if the apparatus wobbles excessively, adjust the
position of the counter-balancing weight. Attempt to keep the pointed bottom of the mass directly above
the reference pointer each time the mass passes over the pointer. This is not an exact procedure; it will
be sufficient to be accurate to within about 0.5 centimeter.
5. Once the mass is rotating steadily (you will have to continue to twist the axle to keep it going), start
the stopwatch and begin counting revolutions. Record the amount of time t it takes to complete 100
revolutions.
6. The average velocity of the mass equals the total distance traveled (d = 100 circumferences), divided
by the total time. Calculate the distance d and the velocity v = d/t and record these values in the table.
7. The centripetal force acting on the mass while it is rotating is the spring force. We can measure what
that spring force must have been during the rotation by finding out how much force is necessary to
stretch the spring by the same amount it was stretched during the rotation.
Attach the cylindrical plastic force measuring apparatus to the side of the black mass and pull it outward
until the bottom of the mass is directly above the reference pointer. The reading on the scale is equal in
magnitude to the opposite force exerted on the mass by the spring, which is the same as the spring force
that was being exerted during the rotation. Record this centripetal force Fc (the spring force) in the
table.
8. Calculate the product ma = mv2/r and record it in the table; note: m is the mass of the cone-shaped
black mass; its value has already been measured and recorded in the table. This ma term should be close
in value to the centripetal force Fc acting on the mass. Calculate the percentage difference between ma
and Fc by taking the absolute value of the difference, dividing by Fc and multiplying by 100.
9. Push the reference pointer as close as possible to the axle, readjust the counter-weight arm until the
strings are vertical, and repeat the procedure above for the new radius of rotation.
2
Mass of Black Cone-Shaped Object:
m=
grams =
TRIAL 1
Radius of Circle
kilograms
TRIAL 2
r=
m
Radius of Circle
r=
m
Distance d = 100 (2r) =
m
Distance d = 100 (2r) =
m
Time t =
s
Time t =
s
m/s
Velocity
Velocity
v = d/t =
ma = mv2/r =
Fc from measuring device
Percentage Difference
ma = mv2/r =
N
=
v = d/t
m/s
N
N
Fc from measuring device =
N
%
Percentage Difference
%
3