Where's the Doorknob

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Physics, Engineering: Torque
Grade Range: Elementary School, Middle School
Format: Hands-on

This demonstration is great for science nights when the students are accompanied by their parents. Sometimes a parent will try the demonstration and be amazed by it as well!

Contents

Materials

  • Doorknob Board
  • Table Clamp

Safety Precautions

Please read the Physical Demonstration section of the Demonstration Safety page before performing this demonstration.

Demonstration

  1. Set up the doorknob board: Clamp the short end onto the table, with the long end going across the front of the tabletop.
  2. When students come over, have them try to lift the board by using a handle. Is it easier to lift the board when the handle is close to or far from the hinge? Why does the distance from the hinge make a difference?

Why This Works

This demonstration is about Torque, or a rotational force. Torque is affected by how much force you put into the motion and how far you are from the center of the motion. If you are farther from the center of motion, then you don't need to apply as much force to make it turn. Likewise, if you are really close to the center of motion, then you need to apply a lot of force to make it turn. This is why doorknobs are always on the side farthest from the hinge: if the doorknob was closer to the hinge, then you would have to apply more force to open the door. In the case of our doorboard, you might not be able to move the doorknob that is closest to the hinge very far, or even at all, because it requires too much force!

Torque is something that we utilize in a lot of ways. A wrench makes it easier to turn a bolt because the handle provides distance from it, which means we need less force to turn it. The pedals on a bicycle move a circular gear as you pedal, and the size of the gear determines how much force you need to apply. On a bicycle with adjustable gears, higher gears are smaller so that you apply more force and speed the bike up, and lower gears are bigger so that you apply less force and cannot go as fast.

Additional Information

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