October 21 - Center of Gravity, Buoyancy

The students learned about center of gravity this week. We also began an introduction to buoyancy.

I gave the students three kinesthetic tasks to begin the lesson. See if you can do these at home!

1) The Impossible Leap

Bend over and hold your toes with your hands while keeping your knees slightly bent. Now try to jump forward while remaining in that position. Don't unbend your knees or take your hands away from your toes. Can you jump?

2) The Super Glue Chair
Sit in a straight-backed, armless chair with your feet planted flat on the floor and your arms folded across your chest. Now try to stand up while keeping your feet flat and your back straight. Can you do it?

3) Pick Up Trick

Place a pencil (or other object) on the floor about 20 inches from the wall. Stand with your back and heels flat against the wall. Keep your feet together. Try to pick up the object without moving your feet or bending your knees. Can you do it?

These experiments all demonstrate center of gravity. Center of gravity can be described as the point at which the entire weight of a person or object is concentrated or held. In symmetrical objects, the center of gravity is the geometric center of that object. In non-symmetrical objects, such as the human body, the center of gravity changes with every movement we make. In the experiments above, the students' centers of gravity changed each time. Here are explanations of what happened in each task:
The impossible leap. If you were to re-do this but jumped backward you would have no problem. When a person jumps, their center of gravity shifts in the direction they want to jump. To prevent falling over, the person must move their base of support in that same direction. The students were unable to jump forward because they would need to use their toes (base of support) to do so.
The super glue chair. When a person sits down, their center of gravity is at the base of their spine. If you try to stand up while keeping your back straight, your center of gravity is unable to move to a position above your feet (where you need support to stand).
Pick up trick. When standing straight against the wall a person's center of gravity is over their feet. Normally, when a person bends over, their center of gravity moves forward. In order to maintain their balance in this trick, the students would have had to move their feet. Since they weren't able to do this, they weren't able to pick up the pencils.

After discussing the center of gravity movement activities, the students completed a lab ("Shake Up") to show how shape affects speed. They had three objects - a roll of masking tape, a marble, and two jar lids that were taped together. The students predicted which they thought would roll to the end of a tilted table first. The students then let the objects go and observed what happened.

The marble reached the end of the ramp first. This is due to the marble's small size. Because of this, the marble's center of gravity is closest to its overall weight. The slowest object was the roll of tape since it was the largest and its center of gravity is furthest from its overall weight. We decided that this would be more obvious if we had a larger roll of tape. :)

We moved on to begin a lesson on buoyancy that we will continue next week. I placed 1 cup of water into a glass measuring jug and asked the students what they thought would happen to the water level if I added an ice cube. They correctly guessed that the water level would rise. I then asked them how many ice cubes I would need to add to raise the water level by 1/4 cup, 1/2 cup, and 1 cup. The students made predictions for each and we tried each one out.
The raising of the water level when something is added to the water is called displacement. The ice cube displaced or pushed away some of the water causing the level in the jug to rise. We then spoke a little about Archimedes and Archimedes' Principle (displacement and the idea that the weight of the water displaced by an object is equal to the buoyant force in the water).
We also talked a little about buoyant force. Buoyancy is a pushing force while gravity is a pulling force. When we put the ice cube in the jug of water, we noticed that only part of the ice cube stayed above water. This is because gravity is trying to pull the ice cube down to the bottom of the jug while buoyancy is pushing it up to the top of the water/jug. 

The experiments "The Impossible Leap," "The Super Glue Chair," and "Pick Up Trick" are all from this website: http://www.escapadedirect.com/plwigr.html
The "Shake Up" lab is from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.