Eight Activities about rotational inertia and Torque

It's a new Twist on physics before the final!

Name______

Score______/40 pts

During this lab go with your group of 2-3 people from station to station. Mrs. Marentette will put a timer on the board so that you know when to move to the next station.

Station 1:

Finding Your Body's Center of Mass

For a symmetrical object such as a ball or donut, the center of mass is located in the center of the object. For asymmetrical objects, such as a baseball bar or a hammer, the center of mass is located closer to the heavier end. Some objects have their center of mass outside the object itself, think of a shape like a boomerang. People have their center of mass near their belly button, where your umbilical cord was attached. This is no coincidence – unborn often rotate about their center of mass while in the womb.

Your center of mass is a point that moves when you move. When you raise your hands above your head, your center of mass is a little higher than when your hands are by your sides. In this activity, you will locate your center of mass when your hands are by your sides.

Procedure-

  1. Stand on the center of the board (marked with a center line). Notice that the two scales give very nearly the same readings.
  2. Now get off the board and place one foot on either side of the board.
  3. Step forward about 20 centimeters towards one end of the board.
  4. Gently sit down on the board, then lie down on the board. Hold your arms to your side and relax, try not to move.
  5. Have your lab partner read the scale results and let you know which is giving a lower reading or if they are the same. Adjust your position on the board until the two readings become equal. Hint: move towards the scale with the smaller reading.
  6. Once the two readings are equal find the middle line drawn on the board. Your center of mass is directly above that middle line.

Questions-

  1. Did you have to move towards the foot end or towards the head end of the board in order to match the two scale readings?
  1. Where approximately is your center of mass? At your waist, at your knees, at your elbows?

Station 2

Effect of Rotational Inertia on a Spinning Stool

This activity involves sitting on a stool that is attached to a rotating turntable and using your rotational inertia to change the speed at which you spin. This activity recreates the motion of ice skaters spinning then bringing their arms in against their body to increase the speed of their spin.

Procedure-

  1. Sit on the stool, try to center yourself on the stool so that you will spin smoothly and stay on the stool.
  2. Your partner will give you the hand weights, one in each hand. Hold your arms straight out to each side.Hold on to the weights during this activity. If you let them go while spinning rapidly, they will become projectiles that can hurt people badly or cause serious damage.
  3. Pick your feet up off the ground. Your partner will provide a torque by pushing you gently to get you spinning.
  4. While spinning slowly, bring your arms in towards your chest.What happens to your speed when you reduce your rotational inertia?
  1. Extend your arms again, slowing your spin and put your feet down to stop.

Station 3

Rolling Disks and the Importance of Rotational Inertia

This activity involves rolling matched pairs of disks down a ramp. The basic concept of this activity involves how much rotational inertia each disk has. The more mass a disk has the larger the inertia and the farther the mass is from the center of rotation the larger the inertia is.

Set of Disks- There are two disks with the same diameter, one hoop and a solid disk; there are two disks with the same diameter but each having a different weight; there are two disks of different diameter but very nearly the same weight; there is one disk that has a pole holding two sliding weights which allow you to change the rotational inertia of the disk.

Procedure-

  1. Have races with two disks at a time.
  2. In the table below, enter which of the two disks won the race down the ramp. Conduct at least 3 races for each pair of disks

Race / Solid Disk vsHoop / Heavy vs Light weight / Large vs Small Diameter
1
2
3
4

3. Do the disks with higher or lower rotational inertia win the race?

Station 4

Rolling Up Hill?

This activity involves observing the seemingly impossible, that a mass is rolling up hill.

Equipment- a wooden track, wide at one end and narrow at the other; a rolling mass that is thick in the middle and narrow at both ends; a wooden base which elevates the track off the table; an angled wooden block used to elevate the wide end of the track above the narrow end.

Procedure-

  1. Start with the wooden track level, resting on the wooden stand .
  2. Place the rolling mass with the tapered ends at the narrow end of the track and release it. What does it do?
  1. Use a ruler to measure from the table to the center of the rolling mass when it is at the narrow end and when it is at the wide end of the track. Enter the results in the table below for flat track.
  2. Place the small wood block under the wide end of the track (see figure below). This should lift the wide end up higher than the narrow end. Place the rolling mass at the narrow end of the track and release it. What does it do?
  1. Use a ruler to measure the height from the table to the center of the rolling mass when it is at the narrow end and when it is at the wide end of the track. Enter the results in the table below for track on wood block.

Height of center of mass when placed at narrowend of the track / Height of center of mass when placed at the wide end of the track
Flat Track
Track on wood block
  1. How can the rolling mass climb the raised track? Do things really roll up hill?

Station 5

Rotational Inertia of Spinning Batons-

This activity will let you feel the effects of attempting to twist your wrist while holding one of two colorful batons at their center. The batons have identical mass, the difference is that one has the masses close to the center while the other has the masses located out in the tips.

Procedure-

  1. Hold the red baton in the middle while holding it vertically.
  2. Slowly rotate the baton from vertical to horizontal and back.
  3. Switch to the blue baton and repeat steps 1 and 2.
  4. Hold both batons, one in each hand and evaluate the difference in weight between the two batons.

Questions-

  1. Does one baton feel heavier than the other?
  1. Is one baton easier to rotate back and forth than the other? Which one is easier and why?
  1. Where are the masses located inside the baton and which one has the larger rotational inertia?

Station 6

How Not To Spill Water from and Upside down Bucket

This activity involves holding a bucket of water and spinning the bucket in a vertical circle, does the water stay in the bucket?

Procedure-

  1. Start with a bucket with some water in it. It is a good idea to do this activity outside.
  2. Swing the bucket of water in a vertical circle fast enough that the water won’t fall out of the bucket at the top of the arc.
  3. Good luck stopping the motion without spilling the water. Whatever you do, do not let go of the bucket! It will make a mess, people might get hurt and your arm and shoulder might hurt.

Question

  1. Why does the water stay in the bucket?

Station 7

Demonstrating Torque- Hard To Handle

This activity will let you feel the effect of increasing torque that is applied to the handle you are holding.

Procedure-

  1. Hold the torque demonstrator with both hands grasping the thick wooden handle, point the narrow pole away from you and parallel to with floor and the metal hooks must face the floor.
  2. The lab partner will attach a 200 gram mass to the hook closest to the handle.
  3. How difficult is it to keep the long wooden pole parallel with the floor?
  4. Move the 200 g mass to middle hook,describe the ease of holding the pole parallel with floor.
  1. Move the mass out to farthest hook and describe the ease of holding the pole parallel to floor.
  1. Change the mass to 500 g and repeat steps 3 through 5.
  2. Now orient the long pole so that it points straight up. Even with the 500 g mass, describe the difficulty in maintaining the pole pointing up.

Question-

  1. Why does the task get more difficult when the mass is moved away from the holder?
  1. Why does the task get more difficult when larger masses are used?
  1. Why does the task get simple when the pole is pointing vertically instead of horizontally?

Station 8

Pulling a String Without Producing a Torque

This activity involves pulling on a string attached to a spool with large diameter disks and a slender central post. The torque imparted upon the spool by pulling on the string varies with the angle of the pull relative to the ground.

Discussion-

The force of the string pulling on the spool produces a torque which spins the spool. The pull of the string is the force, the distance from that force to the contact point is on the table is the length of the lever arm.

Procedure-

  1. Place the spool of string on a table with the string coming off the top of the spool's center axle(as shown in the top left picture).
  2. Pull gently on the string so that the spool rolls without skidding. Did the spool go with the pull or opposite your pull?
  1. Pull the spool with the string at the bottom of the center axle (as shown in the top middle picture). Did the spool go with the pull or opposite your pull?
  1. What happens if you pull the string at an angle (as shown in the top right picture)?