Denise Nadworny
Teslamania 2014
 Based on AP C Free Response Question
 Adapted from Kim Geddes’ AP C Rotational
Motion Lab (http://geddesphysics.weebly.com/)
 A falling mass causes a pulley to rotate. The
rotational acceleration can be determined using
the acceleration of the falling mass. This can be
used to determine the moment of inertia of the
pulley.
 Place a ring stand on the edge of the table.
 Attach a long pole (ex – spare ring stand pole) to
the ring stand.
 Hang a pulley from the farthest end of the pole.
 Tie a string around the pulley and a 50 g mass. Tape
an index card to the mass. (Thin cardboard from
back of a note pad worked best)
 Place a motion detector on the floor beneath the
hanging mass.
 See setup video on website
 Students need to measure mass of mass
with index card and radius of pulley.
 Set up the CBL calculator to collect
distance and time data
 Data Collection – Time Graph
 Time Frame = 0.02 s
 Number of samples = 45
 Wind up the pulley.
 Press enter to collect data.
 Wait for the clicks before releasing the
pulley.
 Confirm the graph looks alright before
importing to LoggerPro
 See setup video on website
 Connect the CBL unit to the computer
 Import data into LoggerPRo
 Raw Data
 Delete extraneous data
 Add point protectors,
column labels, title
 Analyze with quadratic
fit
 Note – This is from a different
trial than the ‘raw data’
example graph
 Create a straightened
graph by squaring time
data
 Analyze with a linear fit
 Slope =
(0.5)acceleration
 Which quantities should be graphed in order to best determine the acceleration of
the block? Explain your reasoning.
Distance versus Time Squared
Slope = ½ acceleration
d  v i t  12 at 2
y  mx
 Plot the quantities determined in (1), following all graphing rules.
 Determine the acceleration of the block.
Slope = ½ acceleration
a = 2(slope)
 Derive an expression for the tension in the string. Calculate this value.
 F  ma
Ft  F g  ma
Ft  mg  ma
Students need to plug in the acceleration as negative
because it was falling, g is positive because the negative
was already included in Ft - Fg
 Derive an expression for the angular acceleration of the pulley.
a  r
a

r
 Derive an expression for the moment of inertia of the pulley using the torque of a
rotating body.
  I
I   Ft r
  Ft r
Ft r 2
I


a
Ft r
 Use the expression determine in step 6 to calculate the moment of inertia for the
pulley.
 Calculate the moment of inertia of the pulley using the solid disk formula.
I  12 MR 2
 Compare your two results with a percent difference.
 Explain why the differences in the two values likely occurred.
 Friction in the pulley – it does not rotate freely
 Assume constant radius (and that radius of pulley is radius of string)
 The radius decreases as the string unwinds
 The radius of pulley was measured from far edges – string stays inside
 “Blips” in the motion detector – individual needs to remove extraneous data
points and may have removed too many or too few.
 Teslamania Tab:
 Student copy of lab
 Motion detector and LoggerPro instruction sheet
 This powerpoint
 Disclaimer – I make no guarantee that this is actually what AP 1 wants for rotational motion. 