PHYS 1405 - Conceptual
Physics
Rotational Motion
Materials
Masking
Tape
Large
ball of clay
Introduction
In this
experiment we will investigate several aspects of rotational motion including
angular velocity, angular acceleration, torque, and moment of inertia. Our apparatus will consist of a turntable
rotating on a low friction bearing.
Part I Angular Velocity
1. Given a
piece of tape and a stop watch, describe a procedure for determining the angular
velocity of the turntable as it spins.
2. Employ the
procedure and measure the angular velocity of the turntable when you spin it at
three different speeds, slow medium and fast.
Create a data table in the space
below and record your results in the table.
Part II Angular Acceleration
In this part of the lab, we will let the apparatus rotate freely, then slow it for a while using a light pressure due to our finger and then let it rotate freely again
3. During which part of the motion will there be
an angular acceleration?
4. Describe a
procedure for determining the angular acceleration for this procedure using the
same materials as in part I. Check your
procedure with your instructor.
5. Start the
turn table spinning and then slowly slow it with a light pressure from your
finger. Record your data and your
determination of the angular acceleration in a table in the space below.
In this
part we will examine the conditions necessary to produce an angular acceleration. With the turn table initially at rest, use
your index finger to apply a force along the tangent at the edge of the turn
table.
6. Describe
the resulting motion of the turn table.
7. Was there
an angular acceleration? Explain.
Trying to
keep the force about the same, apply the force along the tangent but at four
successively smaller radii. Each time
start the wheel from rest.
8. Describe
how the resulting motion of the turn table changes as you reduce the
radius. In particular describe the
affect on the angular acceleration.
Starting
the turn table at rest each time, push on the edge of the turntable along the
tangent with successively harder forces.
Do this three of four times.
9. Describe
how the angular acceleration changes when you increase the tangential force
along the edge of the turn table.
Now apply
a force along a line directed towards the center of the turn table, i.e.
radially.
10. Does
directing the force radially produce the same angular acceleration as directing
it tangentially? Explain.
The
quantity which produces an angular acceleration is called torque. In questions 7 – 10 we have investigate three
things which might torque.
11. From your
observations, describe how torque might depend on force.
12. From your
observations, describe how torque might depend on the radius at which the force
is appled.
13. From your
observations, describe how torque might depend on the direction at which the
force is applied.
We
usually combine these observations into a single formula for torque given by
Torque = Force x Lever Arm, where force is the
tangential force that is applied and the lever arm is the distance from the
axis of rotation at which the force is applied.
Part IV Moment of Inertia
In this part we will
examine the factors that affect the moment of inertia.
Spin the turn table so
that it is rotating about 1/2 turn per second.
Take a large ball of clay and drop it as close to the center of the trun
table as you can.
14. Describe the effect on the rotation of the
turn table.
Repeat but drop the clay
further from the axis this time.
15. Describe the effect on the rotation of the
turn table.
Repeat two more times
dropping the clay further from the axis, with the last drop being about at the
edge of the turntable.
16. Describe the effect on the rotation of the
turn table.
Now again spin the turn
table at about 1/2 rotation per second and drop along the edge clay balls of
increasing mass.
17. Describe the effect on the rotation of the
turn table.
Summarize your
observations:
Dropping the clay ball
further from the axis produced a ____________ change in the rotation. Dropping larger clay balls at the edge
produced a __________ change in the rotation.
Now place the clay ball at
the center of the turntable and push gently along the tangent at the edge of
the disk.
Move the clay ball to a
radius about ½ way from the center. Push
on the edge with about the same force as before.
18. Compare the angular acceleration produced in
the two cases.
Move the clay ball to edge
of the turntable. Push on the edge with
about the same force as before.
19. Compare the angular acceleration produced in
all three cases.
In linear motion we saw
that it was mass that resisted acceleration.
In rotational motion, it is more complicated. It is not just mass, but also where it is
located that resists motion. In fact
what determines the resistance of an object to a angular acceleration is
proportional to mass x radius2.
The quantity is called the moment of inertia. (Your