Pendulum Activity to Demonstrate Momentum

Newton's first law of motion defines inertia. Objects at rest stay at rest while objects in motion stay in motion with the same speed and direction unless an outside force acts on them.

The word pendulum comes from the Latin, 'pendulus' which means 'hanging'. A pendulum is something hanging from a fixed point. Usually, a pendulum consists of a wire or string suspended at a fixed point with a weight at the bottom of the string. This weight is called the 'bob'.When pulled back, it swings forward due to the force of gravity. It swings back to its original position due to the force of inertia.

• Gravity pulls the 'bob' towards the center of the earth;
• Inertia keeps a moving pendulum moving or a resting pendulum at rest.

• As he watched a lamp swinging in the cathedral of Pisa, Italy where he was a university student, Gallileo became interested in pendulums.
• In 1602 while he experimented with pendulums, Gallileo discovered that the period of a pendulum is not affected by its amplitude.
• Gallileo conceived the idea that a clock could be made using a pendulum but he never designed or built one.
• Christiaan Huygens, a Dutch scientist, created the first pendulum clock in 1665.
• It was reported that his clock was accurate losing only one minute per day and he later improved his design to where it was losing only ten seconds per day.
• Because of his pendulum clock, an era of precision time keeping was born allowing scientists to determine:

1. the effect of force on objects over time,
2. the distance of fall over time, matter.
3. the change of speed over time,
4. the radial movement of planets over time, and
5. the progress of chemical reactions over time.

• As well, navigators had a solution to the age old dilemma of how to determine longitude. As longitude can be determined by change in time, the development of an accurate pendulum clock which could be carried on voyages was the solution navigators had been seeking for determining longitude accurately.

Because there are several different variable to manipulate easily, pendulums make excellent objects of scientific study. Pendulums are a simple means of exploring physics for all age groups and because of those simple variables also make great tools for applying and gaining experience with the scientific method.

• cycle (period): one swing of the pendulum including both the trip out and the return swing
• amplitude (arc or pull-back distance): the angle at which the bob is released
• frequency: the number of cycles that occur in set period of time

These terms are important when discussing experiments done with a pendulum.

Materials:

• One string 4 feet long
• Two strings 1 feet long
• Two matching mugs with handles
• Two chairs
• Scissors

Creating the Pendulum:

1. Set two chairs back to back about 4 feet apart.
2. Tie one end of the 4 foot long string to the back of one chair.
3. Tie the other end of the same string to the back of the other chair.
4. Make sure the chairs are positioned so there is little or no slack in the string.
5. Tie one of the 1 foot long pieces of string to the the string suspended between the two chairs about 18 inches from the chair back.
6. Tie the other 1 foot length of string to the string suspended between the two chairs about 18 inches from the other chair back.
7. The two hanging strings should be about 1 foot apart.
8. Tie a mug handle to the end of each hanging string.

The Experiment

1. Pull back on one mug and let go, letting it swing back and forth until it stops.

Questions to Answer

1. Does the mug stop?
2. What does the other mug do?

Variations to Try in the Penulum Experiment

1. Replace the mugs with matching objects of different weights.
2. Replace the mugs with two matching plastic mugs.
3. Try hanging two metal forks from the string followed by two plastic forks.

This first mug that was released is kept swinging back and forth by momentum. The mug will continue to swing until it is slowed by friction from air molecules and friction from the string knot rubbing on the handle.

When the mug that was pulled back is allowed to swing for a few seconds, the other mug starts to swing even though it was not touched physically by your hand. Kinetic energy from the pulled back mug travelled along the string to the second mug, giving it the energy to swing. Although the momentum of the first swinging mug slowed down due to friction, some of the kinetic energy was still transfered to the string which carried that kinetic energy to the second mug. You might notice the first mug moving faster again after the second mug picks up speed. The second mug also transfers kinetic energy along the string back to the second mug. Eventually, due to friction with air molecules and friction of the string knot rubbing against the handle both mugs will stop swinging.

Matthews, Michael R., Michael P. Clough, and Craig Ogilvie. The Story Behind the Science. Pendulum Motion: The Value of Idealization in Science.