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Balance, Leverage, & Torque

Main concepts:

• We can Balance things off center, if we place the fulcrum at the center of Gravity
• Center of Gravity is the center of the distribution of weight.
• Leverage allows you to lift a very heavy object
• Torque is a rotational force around an axle, and is equal to a force times the length of a moment arm.
• The reason why Leverage works is that a longer moment arm provide higher Torque.

Key Words:

balance, leverage, fulcrum, center of gravity, torque, stability

What to do

Associated Art Project

Students built mobiles during the week before this science demo. As part of building the mobiles, they had to balance the weight of objects hanging from the strings. Mobiles can have many tiers and be as complicated as the student can handle.

Balance

Start by explaining that Force is a pushing or pulling. In this demo we will put some weight on each end of the board. Those weights are pulling the board down. Weight is a Force.

Then show the Balance Board, and explain what the fulcrum is. Balance it with one weight on each end and show the students. Then add a second weight on one end and shift the board off center until you get it to balance. The point where it balances on the fulcrum is the center of the weight distribution. We commonly call this the “Center of Gravity”.

Have the students break into groups of 2. Give them a book, a yard stick, and 4 large Hex Nuts. On a carpeted floor, have each group stand the book up like an upside down “V” to act as a fulcrum. First balance the yard stick across the spine of the book with no Hex nuts on it. Then add one hex nut to one end and shift the yard stick off center so that it balances. Continue adding hex nuts to that same end one at a time, each time shifting the yard off center to make it balance, until you’ve added all 4 hex nuts to one end. Let the students work in pairs for approximately 10 minutes.

Leverage

Show the Big Balance Board and fulcrum. Ask for volunteers. Get the heaviest child and the lightest weight child in the class. Weigh them on the scale. Have the lightest child sit on one of the TeeterTotter (with a wooden box underneath). Shift the fulcrum far away from that child, so the opposite end only has approx ¼ of the length left. Now you have the light child sitting on the long end, and the heavy child standing up at the short end. Ask the heavier child to pull down on the board and lift up
the light child.

Caution: You must explain to the children that if they push down, and then let go suddenly, that the
board will pop up in their face. So once you push down, let up slowly. The instructor should stand
close by with a hand over the board to prevent any accidents.

The idea here is that the heavier child should not be able to lift the lighter child. Next move the wooden box to the other end and have the light weight child (still on long end) lift the heavier child sitting on the short end. Ask the students. Why can the light child lift the heavy, but the heavy cannot lift up the lighter child ? Explain that the secret is that I have moved the fulcrum. Demonstrate that if we move the fulcrum towards the other end, then the situation changes. The heavy child can lift the lighter child.

Torque

• Give the student a weight (3 to 10 Lb range). Have them old the weight near their chest. Then hold it half way out from their chest. Then hold it at full arm’s length away from their chest. Ask whether the weight feels heavy or light or the same at each position.
  
• Explain on the drawing board that Torque is like a rotational force (a force around axle).
  Torque = Force x Distance, where the distance is the length of a Moment Arm. When the light weight girl sits on the balance board, but she has more length of the board on her side, the greater length of her moment arm gives her an advantage so that she can produce a higher torque than the heavier child at the other (short) end of the board.
• Show the Wrench and Cheater bar. If you tighten the bolt as hard as you can with he wrench, but then try to remove it with the cheater bar, it is easy to remove. But if you tighten it with the cheater bar, and then try to remove it with the wrench only, then you cannot loosen the bolt. The reason is that the cheater bar gives extra length, so it causes a higher torque, so the bolt gets so tight into the hole, that you cannot possibly loosen it with the short wrench.
• Show the Door Stop. If you wedge a door stop under the door at a location far from the hinge, then it works just fine. But if you move the same door stop closer to the hinge, then the door stop cannot keep the door open. This is another example of Torque. The door has a spring that is trying to close the door. That spring causes some Torque about the hinge. The door stop that you put under the door provides a Torque to overcome the torque of the hinge. When you move your door stop closer to the hinge the moment arm becomes smaller until the door stop cannot provide enough torque to overcome the torque of the spring. Then the door will close.

Stable Balancing

Discuss Examples of balancing: On your bike, Airplane,

Explain stability and give some examples.
A ball in a bowl (vs. a ball on top of a hill).

Show the Tinker Toy man that balances on the end of a vertical Dowel. The secret to why he is so stable, is that all the weight is in his feet.

Tie a string down at both ends so that it reaches horizontally across the room. One end should be lower than the other so that you have a significant incline. Show the students the balancing Man. You can build one out of Tinker Toys or Kenex if you have a wheel. The secret to balancing is to put most of his weight in his feet which hang down below the string. The balancing Man should easly roll down the string to the other end. Let the students design their own balancing men. Encourage them to experiment with the design. Longer arms, longer feet.

Equipment Needed

• Small balance board, Fulcrum, & weights
• Yard sticks, Books, Hex Nuts
• Forks & a Glass
• 12 foot (2x 8) board, Large Fulcrum, & Boxes to sit on
• Marker Board, Markers, Stand, & Clamps
• Weight to hold at a distance
• Torque Fixture, Bolt, Wrench, & Cheater Bar
• Door & Door stop
• Model Airplane
• Bowl & Ball
• Tinker Toys (and or Knex) & Nylon String, Eye hook(s).

Balance, Leverage, & Torque Summary

Balance

• Explain Force
• Small Balance Board (Add weights and balance off center)
• Explain Fulcrum
• Yard sticks with hex nuts

Leverage

• Big Board
• Get 2 volunteers and Weigh them.
• Heavy Student cannot lift the Light weight student.

Torque

• Hold Weights at a distance
• Explain Torque on the Drawing board
• Wrench and Cheater Bar
• Door Stop

Stable Balancing

• List some examples of Balancing (Bike, Unicycle, Airplane, Teeter Totter).
• Explain stable vs. Unstable
• Ball in a bowl / upside down w ball on top.
• Balancing Forks on a Glass
• Tinker Toy Man.
• Balancing Man on a string