Name________________________________________________________ Date__________________________
Investigation 4
Rotational Motion
1.
The picture shows an LP record (you all remember LP’s?) as
viewed from above. The record is turning in a clockwise
direction at 33 1/3 rpm. Two bugs are located at positions A
and B on the record.
a.
Which bug is moving at the higher speed?
A
BUG B
b.
5
B
Which bug has the higher angular (rotational) speed?
BOTH HAVE THE SAME ANGULAR SPEED
4
c.
When bug B is at the position shown in the picture, in
what direction is bug B traveling (1, 2, 3, 4, or 5)?
1
3
2
PATH 2
d.
When bug B is at the position shown in the picture, suppose bug B falls off the edge of the
rotating LP. While falling off the record, other than gravity, are there any other forces acting on
the bug? If there are name them.
NO OTHER FORCES
e.
On what path will the bug most likely travel after falling off?
PATH 2
2.
A
B
C
A long, uniform rod is rotated about
axes through A, B, and C. The rod is
rotated around each axis. Rank the axes from the largest moment of inertia produced to the smallest
moment of inertia produced.
A, C, B
When trying to evaluate the relative moments of inertia, what did you look for when finding the
largest moment of inertia?
HOW FAR THE MASS IS LOCATED FROM THE AXIS OF ROTATION – THE GREATER THE
DISTANCE, THE LARGER THE ROTATIONAL INERTIA
3.
A wrench is being used to loosen the nut on a bolt. A 100 N force is applied at different positions on
the wrench and in different directions.
a.
Which force (or forces) produces the greatest torque?
b.
Which force (or forces) produces the least torque?
A
E – LONBGEST LEVER ARM
F – SHORTEST LEVER ARM
E
C
B
F
D
4.
Draw an X to show where the center of mass of the following objects is located.
a. a uniform rod
b. a uniform rectangular plate
c. a donut
X
X
If each of the above objects is slightly tipped to the side, does their center of mass move up, move
down, or stay at one level?
a.
DOWN
b.
UP
c.
STAYS LEVEL
Indicate whether each of the objects is in stable equilibrium, unstable equilibrium, or neutral
equilibrium.
a.
UNSTABLE EQUILIBRIUM b.
STABLE EQUILIBRIUM
c.
NEUTRAL EQUILIBRIUM
5.
The picture shows a car is traveling at a constant speed of 50 km/hr
along a circular road that is curving to the left.
a.
Percy, ,who is sitting in the passenger seat with a seatbelt, feels a
horizontal force acting on him in addition to the gravitational force
and the force the seat exerts on him. Name this horizontal force (i.e.,
the force of what acting on Percy), and draw the direction of the
force on the picture.
SEATBELT
ON PERCY
SEATBELT EXERTS A FORCE ON PERCY – DIRECTION IS TOWARD THE CENTER OF
THE CIRCLE
(This force is a centripetal force.)
b.
Name the centrifugal force.
THE FORCE PERCY EXERTS ON THE SEATBELT
c.
Is there also a force acting on the car causing it to travel around the curved path? If so, then
what is that force and in what direction does it point?
FRICTION FORCE OF THE ROAD ON THE CAR POINTING INWARD TOWARD THE
CENTER OF THE CIRCULAR PATH
direction of rotation
3.
A 600 N physics student is sitting on a bathroom scale
that is resting on the seat of a chair on a Ferris wheel.
(Santa Monica pier has a very nice one.)
a.
What is reading on the scale when the Ferris
wheel is not moving?
B
A
600 N
Suppose the Ferris wheel is now moving at a constant
rotational speed. In this case, we know that the net
force acting on the student is the centripetal force,
with its direction being toward the center of the circle.
b.
Draw vectors to represent the forces acting on the
student when she is at the top and at the bottom
of the circular path, and show the direction of the
net force.
top:
SEAT ON STUDENT
SEAT ON STUDENT
bottom:
NET FORCE
NET FORCE
WEIGHT = 600 N
WEIGHT = 600 N
What is the reading on the scale when she is at the top of the circular path and then at the
bottom of the circular path? (600 N, greater than 600 N, or less than 600 N) The diagram above
showing the vectors will help.
top:
c.
< 600 N
bottom: > 600 N
The readings on the scale when the student is at positions A and B will both be 120 lbs. Do you
know why? (rhetorical question)
NO NET FORCE IN THE VERTICAL DIRECTIONS
5.
a.
When an ice skater, who is spinning, moves her arms and legs in closer to the axis of rotation of
her body, what happens to her moment of inertia?
HER MOMENT OF INERTIA DECREASES – MASS CLOSER TO THE AXIS OF ROTATION
b.
What happens to her angular velocity (rate of spin)?
`ANGULAR VELOCITY INCREASES
c.
Why did the above occur?
ANGULAR MOMENTUM IS CONSERVED – SO WHEN THE MOMENT OF INERTIA
DECREASES, THE ANGULAR VELOCITY INCREASES AND VICE VERSA
6.
If all the ice at the poles of the earth were to melt, then the water trapped in the ice would distribute
itself evenly over the earth. Would the length of the day get longer or shorter? Why?
THE LENGTH OF THE DAY INCREASES. AGAIN ANGULAR MOMENTUM IS CONSERVED AND
THE WATER DISTRIBUTING ITSELF AROUND THE EARTH INCREASES THE EARTH’S
MOMENT OF INERTIA. THIS INCREASE IN MOMENT ON INERTIA IS ACCOMPANIED BY A
DECREASE IN ANGULAR VELOCITY.