Mechanical Arms

Parts Needed:

Zigzag Arm​

  • 2 models

  • 10 craft sticks

  • 13 push pins 

  • 1 pompom puffball

Digger

  • 2 models

  • 1 baseboard 4x4

  • 1 1x2, 1.5" long

  • 5 tongue depressors

  • 5 pushpins

  • 1 binder clip, small

  • 1 salsa cup

  • 1 nail, long and thin

Extra Tools: 
  • Jig for each project

  • Drill

  • Drill platform

  • Small nail bits

  • Extra hot glue

  • Beans, rice or sand to dig

  • Plates or pans to put the     beans in

Project Description:

Simple machines help us do work. There are three basic simple machines: the pulley, the inclined plane, and the lever. Each one takes an input force and multiplies it, or decreases it while increasing its distance. The equation is W = f x d, that is work equals force times distance. Work in equals work out - that the conservation of energy law - and so you don't get something for nothing.  

Usually we think of levers increasing force, such as the examples of a long pole lifting a heavy object, a jack handle, scissors or a long-handled wrench. They can just as well increase distance though, as in the example of the zigzag arm and most levers on machines and in our body. Think of your forearm. The elbow is the pivot point, called the fulcrum. The muscle to pull up the forearm pulls very near the elbow so it moves a small distance. But your hand, holding something such as a ball, moves a much longer distance. Again, you don't get something for nothing, so your arm muscle is providing a much larger force than the one that is given to the ball.

Increasing the size of the sticks or arms in these projects would increase their reach, but the ratio of movement would stay the same. So, for the lever arm, if your fingers move two inches pulling the two end levers together but the pompom goes forward 20 inches, that is a one to ten ratio. With bigger sticks, the ratio would stay the same, but the distances would increase. 

Concepts:

1. ​Many machines use levers to help do work. If you look at bulldozers, backhoes, cranes, and other construction equipment, there are levers everywhere. 

2. Levers can increase or decrease a force, and increase or decrease the distance it moves. However, when one of these quantities increases, the other always decreases.

3. Multiple levers working together can multiply the effect of any single one of them.

A Bit More Info:

Levers have three main parts: the Fulcrum, or pivot point; the Resistance Arm; and the Force Arm. The Resistance Arm is where the lever does work - it can lift, slide, dig, scoop, carry, or pull something. The Force Arm is where the lever gets its force - the part where you, or a motor, apply force to cause it to act. Some common levers are a see-saw (although most kids these days have never seen one - the seesaw has been kicked out of the playground), a wheelbarrow, and a broom.

A seesaw has the fulcrum in the center. You can always describe a pivot point as the place where something is attached, but it can still move. Demonstrate with your elbow - it is where your forearm is attached to your body, but can still swing through the air. Where is the pivot for your whole arm? The force arm of the seesaw is the side moving down. It is moving down because gravity (the Force) is pulling on the weight of the kid on that side. The other side, going up, is the Resistance arm, because it is lifting the other kid's weight. Then the sides switch.

A wheelbarrow has the fulcrum at the wheel. The handles can move up and down, but stay connected to the ground at the wheel. The force arm is where you lift the handles. The resistance arm is the bucket, where you put whatever you're carrying. 

Grab a broom and see where you put your hands. They always have to be a small distance apart - just try sweeping with your hands touching! Your lower hand is the pivot point. The bottom half of the broom is the resistance arm - friction against the ground provides the resistance. The top half of the broom is the fore arm - where your other hand is pushing. 

Try to find the pivots in the Digger, or the Lever Bopper. Also, find the force arms (each project has two, because they are complicated machines built out of many levers). You can also count the levers in each machine.

When you change the lengths of the force arm or the resistance arm in any lever, you change the effect of using the lever. The force you apply is multiplied by the length of the force arm, and the size of the motion you get is multiplied by the resistance arm. Using these two properties, humans have been able to build truly amazing machines by combining levers of different lengths. 

Questions:

  1. How much does the Zigzag Arm multiply your motion? That is, when you move your fingers together one inch, how far forward does the pompom move?

  2. What would happen if you doubled the size of the sticks on these two projects?

  3. What do you think would break first on the Digger if you tried to move too heavy of an object?

  4. Brooms and seesaws are examples of levers. Can you think of a few more levers that you use everyday?

© 2020 by Victoria Matelli, Calvin Norwood, Jade Murray

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