Lab 8A: Center of Mass

Course: PHY 101

Instructor: Gerhard Gross

Name:______

Lab 8A: Center of Mass

PART A: People and their centers of mass

Stand with your heels and back against a wall and try to bend over and touch your toes. Explain why this doesn’t work. Draw a sketch.

Now measure the minimum distance of your heels from the wall for which you can touch your toes.

Compare this distance with others in the room. Find the distance for at least 4 others. Make sure that you get data from both men and women. Make a table of the data in the space below.

On average, did the men or women in your sample need to stand farther from the wall? What does this imply about our centers of mass?

PART B: Weight a Moment

Purpose: To look at how the concept of torque relates to our balance and then to apply the principles of torque to a simple lever.

Equipment: meter stick, set of slotted masses, meter stick holder & stand, mass hangers.

Carefully balance a meter stick horizontally on the meter stick stand. Suspend a 200g mass at 10cm from the fulcrum (pivot point). Suspend a 100g mass on the opposite side of the fulcrum at the point that balances the meter stick. Record the masses and distances in the table below.

Can a heavier mass be balanced by a lighter one? Explain how.

Make more trials and record them in the table below. For example move the 200g mass 5cm further away and then re-balance the meter stick with the lighter mass. You can also change the masses. Use a variety of masses at different distances on both sides.

Use any method to determine a pattern in the data of the table above. You may want to try graphing the large mass vs its distance from the fulcrum and the small mass vs its distance from the fulcrum. You can try forming ratios or products to discover the pattern.

When your group is convinced it has discovered the pattern, describe this pattern in a word statement and also convert this word statement into a mathematical equation. Explain what each symbol represents.

Suppose you work in the circus. You have a very light & very strong plank and you want to attempt to balance a 6300 kg elephant on one end of the plank while you stand on the other end (your mass is 50 kg in this problem). If the elephant stand 2 m away from the pivot point (sometimes called fulcrum), use your equation to find the distance from the fulcrum that you must stand in order to balance the elephant.

If the hangers and clamps are used to suspend the masses, the mass of the hangers and clamps must be taken into account. Why?

SUMMING UP: Suppose you are playing see-saw with your cat who weighs much less than you (and is less than impressed!). What can you do to balance the see-saw? Give 2 ways to achieve this.

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