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Counterweight: What is a Counterweight, and How Does it Make Lifting a Load Easier?

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Author: Janice VanCleave

Problem

What is a counterweight, and how does it make lifting a load easier?

Materials

  • 2 sheets of typing paper
  • paper hole-punch
  • string
  • marking pen
  • broom
  • 2 chairs
  • 2 unopened cans of soda

Procedure

  1. Make a paper cradle by following these steps:
    • Fold one sheet of paper in half lengthwise twice.
    • Use the hole-punch to cut one hole at each end of the folded paper.
    • Hold the ends of the paper together so that the holes are aligned.
    • Insert one end of a 1-yard (1-m) piece of string through the two holes and tie.
  2. Repeat the steps above to make a second paper cradle for the other end of the string using the sheet of paper. Tie the folded paper to the free end of the string.
  3. Use a marker to label the cradles A and B.
  4. Place a broom across the backs of two chairs positioned about 1 yard (1 m) apart.
  5. Place one soda can in cradle A, and lay the cradle on the floor directly below the center of the broom handle.
  6. Counterweight

  7. Loop the string over the handle of the broom, allowing cradle B to hang on the opposite side of the handle.
  8. With your hand, hold the string directly above cradle B and pull downward until the can rises about 2 inches (5 cm) above the floor.
  9. Observe the effort required the raise the can.
  10. Place the other can in cradle B.
  11. Place cradle A with its can on the floor again, and pull on the string above cradle B as before to lift it about 2 inches (5 cm).
  12. Observe the effort required to raise the can.

Results

It took less effort to raise the first can when a second can was placed on the opposite side of the broom.

Why?

The can placed in cradle B acted as a CounterWeight (a weight that balances the weight of a load). Since both cans weigh the same, they pull down with the same force; thus, one of the cans supports the weight of the other can. Only a small effort is needed to overcome the friction (resistance to motion) between the string and the broom handle in order to move the cans. Machines often use counterweights to raise a load. By balancing the weight of the load with a counterweight, the machine has only to move the load and not to support it.

Counterweight

Let's Explore

How can the friction between the string and the broom handle be reduced? Repeat the experiment, making these changes one at a time and recording the results after each test.

  • Increase the size of the area the string loops around by cutting both ends out of a coffee can and slipping the broom through the can.
  • Remove the can and place oil on the broom handle where the string moves over it. (Be sure to wipe the broom handle clean when you are finished.)
  • Use a smooth ribbon instead of the string.

Show Time!

A counterweight is used to balance the weight of an elevator so that the elevator motor only has to apply enough force to raise the weight of the passengers inside. Build a model of an elevator by cutting the front out of a tall box. Ask an adult to make holes in the sides of the box large enough for a ¾ -inch (2-cm) dowel rod to fit. Cut the front out of a small box for the elevator compartment. Place clay figures inside the elevator to represent passengers. Weigh the elevator and its contents. Place a paper cup on the scale, and add coins to the cup until it equals the weight of the elevator and contents. Attach a string to the top of the elevator, loop it over the dowel rod, and attach the end to the paper cup. The elevator can be raised up and down easily with a slight pull on the string.

Counterweight

Check It Out!

Elisha Otis invented the first elevator. Prior to this invention, buildings were not very tall because people had to climb stairs to reach the top. Find out more about the design of elevators and how modem elevators differ from Elisha Otis's original design.

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