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Name: Michelle
Status: educator
Grade: 6-8
Country: USA
Date: April 28, 2011


Question:
Why does length of a string affect the period of a pendulum?

Replies:
The motion of a pendulum can be explained by equations describing rotation. The component of the gravitation force driving a free swinging pendulum depends on the angle it makes with the vertical. That angle is a function of the length of the pendulum. In addition, the resistance to the swinging is dependent on the rotational moment of inertia which also depends on the length of the pendulum. These two factors involving length do not cancel each other out. For a simple pendulum the moment of inertia is mass x Length squared. The result is

Period of Pendulum = 2Pi x (square root of L/g) where L is length of simple pendulum and g is the gravitation constant.

David S. Kupperman


Michelle, Consider two pendulums of different lengths, both moving at the same speed when at the bottom of the swing. Each will swing up to the same height, the same potential energy. The very short pendulum moves upward quickly, over a very short distance. If the pendulum is too short, it may even loop over the top. The very long pendulum will have to move over a long distance before rising up much. Because both pendulums would have the same speed at the bottom, the long pendulum would take more time to get to the maximum height. This corresponds to a longer period. Dr. Ken Mellendorf Physics Instructor Illinois Central College


Well, if you have two pendulums, both pulled back to the 30-degree position and let go, the one with the longer string has a longer distance sideways to go, before it reaches center.

Or suppose you pulled each pendulum back a fixed sideways distance before letting go. Then the one with the longer string would be travelling down a shallower slope, so it would not accelerate towards the center as fast.

Draw two circles on the blackboard, one big and one smaller. The point in the center is the attachment point of the string, and the circle is the path the weight must travel. Draw two vertical lines across each of the circles, offset to left and right of center by about half the radius of the smaller circle. It will be apparent to the eye that the slope at the intersection points is shallower for the larger circle, and steeper for the smaller circle. I think they will understand that the steeper slope makes for faster acceleration sideways.

It is quite a bit like a ball falling off a table or rolling down a ramp. If the table is 4 times as high, it takes 2 times as long for the ball to hit the floor. The rule with accelerating-motion such as gravity is: time goes as square root of the distance, or specifically: distance = 1/2 * acceleration * time^2

I am not sure how familiar your students are with square-roots, squares, and functions in general. But square-root seems to be the usual compromise response between being unchanging and being proportional.

I think if you had a two-foot pendulum and a 6-inch pendulum, (or 1-foot and 4-feet), then it would be very clear to the eye that the short one did two swings for every one swing of the long one.

Jim Swenson


The longer the distance between mass (pendulum) and pivot, the greater the "rotational inertia". In real words, it means the longer the pendulum the harder it is to move. It looks "heavier" than the same mass on a shorter pendulum. As a result it moves slower with the same gravity. After all we ca get a light car up to running speed faster than a heavy car or bus. Hope this helps without getting too technical. A good high school physics text will go into more detail and math should you want more detail. Take care and good luck from a former 8th grade science teacher! R. W. "Bob" Avakian Instructor Arts and Sciences/CRC Oklahoma State Univ. Inst. of Technology


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