PHYS 220 (PURDUE) QUESTIONS AND ANSWERS
A pendulum bob swings back and forth along a circular path. Does the tension in the
string do any work on the bob? Does gravity do work on the bob? - Answers- Only
Gravity does work
Imagine that you are comparing three different ways of having a ball move down
through the same height.
In which case does the ball reach the bottom with the highest speed? - Answers-
Dropping, Sliding on ramp, and Swinging down are all the same
You are sitting on a freely rotating bar-stool with your arms stretched out and a heavy
glass mug in each hand. Your friend gives you a twist and you start rotating around a
vertical axis though the center of the stool. You can assume that the bearing the stool
turns on is frictionless, and that there is no net external torque present once you have
started spinning. What happens to the angular momentum as you pull in your arms? -
Answers- stays the same: no external forces gives constant angular momentum
What happens to your angular velocity as you pull in your
arms? - Answers- Increases: "Angular velocity increases because inertia decreases"
"Pulling in your arms decreases your moment of inertia. Therefore, you will have an
increased angular velocity"
What happens to your kinetic energy as you pull in your arms? - Answers- Increases:
The mass is closer so you have less inertia and you speed up. If you speed up then,
your kinetic energy must increase as well.
A student sits on a barstool holding a bike wheel. The wheel is initially spinning CCW in
the horizontal plane (as viewed from above) L= 25 kg m2/s. She now turns the bike
wheel over. What happens? - Answers- She starts to spin CCW:Start with angular
momentum L pointing up from wheel. When wheel is flipped, no more angular
momentum from it pointing up, so need to spin person/stool to conserve L!
A rod is lying on a table and has two equal but opposite forces acting on it. What is the
net force on the rod? - Answers- Zero
A rod is lying on a table and has two equal but opposite forces acting on it.Will the rod
move? - Answers- Yes it will rotate
A person raises one leg to an angle of 30 degrees. An ankle weight (89 N) is attached a
distance of 0.84 m from her hip. What is the torque due to this weight?If she raises her
leg higher, the torque due to the weight will - Answers- Decrease
Two batons have equal mass and length.
Which will be "easier" to spin about the center? - Answers- When mass is in the center
A pendulum bob swings back and forth along a circular path. Does the tension in the
string do any work on the bob? Does gravity do work on the bob? - Answers- Only
Gravity does work
Imagine that you are comparing three different ways of having a ball move down
through the same height.
In which case does the ball reach the bottom with the highest speed? - Answers-
Dropping, Sliding on ramp, and Swinging down are all the same
You are sitting on a freely rotating bar-stool with your arms stretched out and a heavy
glass mug in each hand. Your friend gives you a twist and you start rotating around a
vertical axis though the center of the stool. You can assume that the bearing the stool
turns on is frictionless, and that there is no net external torque present once you have
started spinning. What happens to the angular momentum as you pull in your arms? -
Answers- stays the same: no external forces gives constant angular momentum
What happens to your angular velocity as you pull in your
arms? - Answers- Increases: "Angular velocity increases because inertia decreases"
"Pulling in your arms decreases your moment of inertia. Therefore, you will have an
increased angular velocity"
What happens to your kinetic energy as you pull in your arms? - Answers- Increases:
The mass is closer so you have less inertia and you speed up. If you speed up then,
your kinetic energy must increase as well.
A student sits on a barstool holding a bike wheel. The wheel is initially spinning CCW in
the horizontal plane (as viewed from above) L= 25 kg m2/s. She now turns the bike
wheel over. What happens? - Answers- She starts to spin CCW:Start with angular
momentum L pointing up from wheel. When wheel is flipped, no more angular
momentum from it pointing up, so need to spin person/stool to conserve L!
A rod is lying on a table and has two equal but opposite forces acting on it. What is the
net force on the rod? - Answers- Zero
A rod is lying on a table and has two equal but opposite forces acting on it.Will the rod
move? - Answers- Yes it will rotate
A person raises one leg to an angle of 30 degrees. An ankle weight (89 N) is attached a
distance of 0.84 m from her hip. What is the torque due to this weight?If she raises her
leg higher, the torque due to the weight will - Answers- Decrease
Two batons have equal mass and length.
Which will be "easier" to spin about the center? - Answers- When mass is in the center
