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Learning about two machines that are actually modified inclined planes.
How can a person weighing only 80 pounds split a massive oak log or lift a heavy car with just a few turns of a handle?
A wedge is a simple machine that is thick at one end and tapers to a thin, sharp edge at the other. You can think of a wedge as two inclined planes joined back-to-back. While a ramp (inclined plane) stays still while you move an object up it, a wedge is designed to move through an object. The most amazing thing about a wedge is that it changes the direction of force. When you apply a downward force on the flat end, the wedge converts it into a powerful outward force that pushes materials apart.
1. Place a knife edge on a piece of cheese. 2. Apply a downward force () by pushing on the handle. 3. The slanted sides of the blade push the cheese to the left and right (). 4. Because the blade is very thin, it concentrates all your force onto a tiny area, making it easy to split the cheese.
Quick Check
If you push down on a wedge, in which direction does the output force move?
Answer
The output force moves outward (sideways) to the sides of the wedge.
A screw might look complicated, but it is simply an inclined plane wrapped around a central cylinder. The spiral ridges on a screw are called threads. Imagine you have to get to the top of a tall building. You could climb a vertical ladder (hard!), or you could walk up a long, winding spiral staircase (easier!). A screw works the same way. By turning the screw, you move a long distance along the spiral threads to move a short distance into the wood. This trade-off between distance and force is what makes the screw so powerful.
1. Look at the threads on the neck of a peanut butter jar. 2. When you turn the lid, you are moving it along a circular inclined plane. 3. You might turn the lid 3 full circles (a long distance) just to move the lid down 1 centimeter. 4. This long distance allows you to create a very tight, high-pressure seal with very little effort.
Quick Check
What is the name for the spiral ridges found on a screw?
Answer
The ridges are called threads.
Imagine a road winding around a mountain like a screw thread. 1. A steep road goes straight up (short distance, high force/effort). 2. A 'switchback' road wraps around the mountain multiple times (long distance, low force/effort). 3. The switchback road is a screw in action! It allows a weak car engine to reach a high peak by spreading the climb over a much longer path.
Which of these is the best example of a wedge?
If a screw has threads that are very close together, what is true?
A screw converts rotation (turning) into linear (straight-line) motion.
Review Tomorrow
Tomorrow, look around your kitchen. Can you find three wedges and two screws? Explain to someone why the jar lid is easier to close than pushing a cork into a bottle.
Practice Activity
Take a rectangular piece of paper and draw a dark line diagonally from one corner to the opposite corner. Wrap the paper around a pencil. The diagonal line will form the 'threads' of a screw, proving it is just a wrapped inclined plane!