Where Is Your Center Of Gravity?

SCIENCE CONCEPT:

The earth pulls down on each particle of an object with a gravitational force that we call weight. Although individual particles throughout an object all contribute weight in this way, the net effect is as if the total weight of the object were concentrated in a single point - the object's center of gravity.

STUDENT OBJECTIVE:

The student will explore their center of gravity using some simple tools and themselves as the 'center of the experiment.'

OVERVIEW:

In this activity, the student will lie down horizontally between two chairs on a board with bathroom scales under the board on the chairs, along with a pencil on top of each scale balancing the board. There will need to be a partner to help with the scale readings and your height calculation.

PREPARATION TIME:

15 minutes.

LESSON TIME:

30 minutes.

TEACHER PREP:

Gather the materials that are needed for the experiment.

WORDS TO KNOW:

center of gravity

geometric center

equilibrium

torque

translational motion

rotational motion

TEACHER TEXT:

In a regular geometric solid with the uniform density, the center of gravity is the object's geometric center. Finding the center of gravity of irregular objects can be tricky. There must be an upward force exerted at the board's two points of contact with the chairs, the pencils, otherwise the body and board would be falling freely toward the center of the earth. The readings on the scales give the magnitude of those upward forces at these points.

The system is motionless. Therefore, it is in equilibrium. Certain principles of physics regarding the state of equilibrium will allow you to calculate the distance from the right pencil to the person's center of gravity. The first condition of equilibrium is that upward and downward forces balance. The downward forces are the weights of the board and the person respectively. There must also be an upward force exerted at the board's two points of contact with the chairs and the pencils. Otherwise, the body and board would be falling freely toward the center of the earth.

There are two general types of motion caused by forces acting on an object. The first, called "translational motion", is the kind of straight motion which is an example of the system falling downward under its weight if it were not balanced by upward forces. The second kind of motion that could result from these same forces is called "rotational motion." Imagine that the left chair was removed. The left end of the board with the person on it would swing downward in an arc toward the floor. In other words, it would rotate in a counterclockwise direction.

The rotational effectiveness of a force is equal to the amount of force multiplied by the distance from the line of action of the force to the axis of rotation you are considering. This product is called torque, and its direction is either clockwise or counterclockwise. Think of the board and person as the hand on a clock. It would tend to turn the 'hand' counterclockwise. The center of gravity of the board is its geometrical center because it is a uniform rectangular solid. The position of the center of gravity of the person is what we are trying to determine and will be defined by X, the distance from A, the axis we have chosen.

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Last modified: Tue Mar 24 16:49:21 PST 1998