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Discovering how circular motion can reduce friction and multiply force.
Have you ever tried to open a door by grabbing just the tiny metal rod behind the knob? It is nearly impossible to turn! But with the wide knob attached, you can open it with a single finger. How does a simple circle turn you into a powerhouse?
A wheel and axle is a simple machine made of two circular objects of different sizes that are attached and rotate together. The larger circle is the wheel, and the smaller rod or cylinder inside it is the axle. For this machine to work, the two parts must be locked together. When you turn the wheel, the axle turns too! This setup helps us either multiply our force (to lift heavy things) or multiply distance (to move things faster).
Quick Check
If you turn a doorknob but the metal rod behind it doesn't move, is it still acting as a 'wheel and axle' simple machine?
Answer
No, the wheel and axle must be attached and rotate together to function as a simple machine.
Quick Check
If you want to make it even easier to turn a stubborn screw, should you use a screwdriver with a thicker handle or a thinner handle?
Answer
A thicker handle, because increasing the radius of the 'wheel' increases the Mechanical Advantage.
We usually turn the wheel to get more force at the axle (like a steering wheel). However, you can also do the opposite! If you apply force to the axle, the wheel turns much faster and covers more distance. Think of a bicycle: your chain turns the small axle, which makes the large tire spin a huge distance across the road. You trade force for speed and distance.
In a Ferris Wheel, a powerful motor turns the small axle at the center. 1. The motor applies a huge force to the small axle. 2. The axle turns the giant outer wheel where the seats are. 3. While the axle only moves a few feet per rotation, the riders on the outside travel hundreds of feet in the same amount of time!
Which part of a pencil sharpener (manual crank) acts as the 'wheel'?
If the wheel radius is and the axle radius is , what is the Mechanical Advantage?
A wheel and axle can be used to increase either force OR speed, but not both at the same time.
Review Tomorrow
Tomorrow morning, look at a faucet or a doorknob and try to visualize where the 'lever arm' is located inside the circle.
Practice Activity
Find three 'wheel and axle' machines in your kitchen (Hint: look at a can opener, a pizza cutter, or a faucet handle) and identify which part is the wheel and which is the axle.