Lab – Center of Mass Groups: There can be up to three people (including you) in your group. Each member must turn in their OWN lab report/handout. Attached to the Lab Report/Handout must be an “action shot” (or picture) of that person doing the lab. It is ok if it is an entire group shot. Purpose: During the lab, you will explore the center of mass of irregularly shaped objects and analyze the procedure for locating the center of mass. Materials: Nails or pencils, cardboard, scissors, coat hanger, paper, tape Part 1: Estimating the center of mass If an object is constructed of a uniform material, then the center of mass will reside at the geometric center. Although it is east to locate the geometric center of a circle or sphere, it is more difficult to locate the geometric center of an irregularly shaped three-dimensional object.

1. Carefully redraw each of the three images on your own sheet of paper. 2. Place an X on the spot that you think is the center of mass for objects B - H. 3. Using a series of Xs (on picture I), draw the trajectory of the center of mass of the sky-

rocket from the point after it explodes to the point at which it hits the ground.

Part 2: Center of mass of irregularly shaped objects

1. From a cardboard box, cut an irregular shape such as shown in figure J. Be creative, use your own irregular shape.

2. Use a nail or pencil to carefully punch three holes in the perimeter of the cardboard. Enlarge the holes so that board can swing freely.

3. Place a nail or pencil in one of the holes and hang a weighted string from it. If you don’t have a small mass to hang from the string, tie a small object to the end of the string. With a mass at the end of the string, the string should hang vertically straight down. If the string gets caught on your irregularly shaped object, swing the string until it hangs straight down.

4. Swing the cardboard and when it comes to rest, draw a line on the cardboard. 5. Repeat step 4 for the other two holes. 6. The center of mass of the object is where the lines intersect. Try to balance your object

from that location.

7. Tape a pencil to your irregularly shaped object (tape it to one side, not in the center). Now, you have an irregular shape and also an irregular mass. Find the new center of mass.

8. Tape a piece of paper to the hanger as shown in figure K. Using the same steps as for the irregular shape, find the center of mass for the hanger.

9. Using the cut out of California, find the center of mass of the state. 10. Draw the four objects (irregular shape, irregular shape + pencil, hanger, CA) carefully

on your paper and record where the center of mass is for each.

Questions:

1. The method works well for finding the center of mass of the irregularly shaped cardboard and hanger. Do you think this method works well for finding the center of mass of California? Why or why not?

2. If you are in a space station and move from one side to the other, do you change the position of the center of mass about which the space station rotates? Explain.

3. Why does a hardboiled egg spin nicely while a raw egg will wobble when it is spun? (Use center of mass in your explanation.)

4. A male and a female both stand with their backs against a wall, heels touching the wall. Then both lean over, legs straight, to touch their toes. The female student can usually do this, while the male student cannot. (Don’t believe it? Give it a try!) Why is this? (Hint: It is not because the female student is more flexible.)

5. A horizontal non-uniform bar 3.0 m long is supported at its ends by vertical cords attached to spring balances. If the balances read 4.6 N and 3.2 N, what is the weight of the bar and where is its center of mass?