Center of Gravity Experiment

Due No Due Date
Points 100
Submitting a text entry box or a file upload

Background: The weight of an object is concentrated at the center of gravity. The term center of gravity is used interchangeably with center of mass. For a symmetrical object the center of mass is located at the geometric center of the object. If the object is not symmetrical we can determine the center of mass using the method below.

Materials:

Cardboard
Weight (washer, bolt, or fishing weight)
Hole punch
Nail
Pencil

Procedure:

Cut the cardboard into a strange shape. Do not use a circle, square, rectangle, or any other common geometric shape.
Punch a hole near the edge of the cut-out cardboard piece and hang it from a nail.
Place the weight, a washer or bolt, on a thread and tie it off.
Hang the thread and weight from the nail in front of the cardboard.
Use a pencil to draw a plumb line down the cardboard where the thread touches. This marks your center line from that hanging point.
Repeat step 2 and 5 in two other places on the cardboard.
Remove the cardboard from the nail and balance on the tip of the pencil. You should know where to balance because it will be where all of the pencil plumb lines intersect, finding the center for you.

Conclusion:

Would the object balance if you placed the center of gravity on the tip of your finger?

What would happen to the center of gravity of the shape if you were to spin it freely?

Other Fun Center of Gravity Activities

1. Balance a ruler with a hammer. Take a rubber band or string and make a loose loop around the hammer and ruler, as shown in the picture. Make sure the end of the hammer is touching the ruler, and then position the ruler at the edge of a table, as shown. (You might have to re-position the string/rubber band a few times to get it just right. Why does this trick work? Analyze where the center of mass might be. Where is the balance point? What is the heaviest part of a hammer?

2. Balance two forces with a toothpick. A common magic trick using the properties of center of gravity is shown below. The two forks are balanced on the edge of the glass by a toothpick. Where is the center of mass for the fork, cork, and toothpick system?

3. Stand with your back and feet against a wall. Have someone place a quarter on the floor at your feet. Try to pick it up. Most people can't do it.

Why? When leaning over to pick up object we use our keep our center of gravity between our feet by moving our buttocks back as we lean over. When you are standing against a wall you can't make these adjustments.

4. Stand against a wall sideways with your arm and leg touching the wall, with nothing to hold onto. Try to lift your other leg straight out away from the wall. You won't be able to do it.

Why? You are using the outside leg to balance. Lifting it would cause the center of gravity to be outside your feet.

5. The “girls always win” chair lifting challenge. Place dining chair against a wall. Bend over the chair so that your head touches the wall and your upper body is parallel to the floor. Lift the chair to your chest and then try to stand up.

How does it work? Men and women have their center of mass in different places. Men tend to have broad shoulders and narrow waists, giving them a higher center of mass. Adding the weight of the chair causes the center of mass to be over the chair. To stay balanced men have to use the wall. When he tries to stand, he falls.

Girls and women have their center of mass closer to their hips. Even with the added weight of the chair, the overall center of mass is over the feet that the body is able to right itself.

Once you complete the experiment, write a short explanation answering the questions above. What did you learn? Was this surprising to you? Why? When you complete the assignment, save your work in the appropriate place on the network. Then, upload your completed work to this assignment link by clicking the Submits Assignment button in the upper right hand corner of this window. Proofread your work carefully before you submit.