How do scissors cut through materials?

1. Tear a sheet of paper and the edge comes out ragged. Scissors leave a clean line exactly where you want it. What is their trick?
2. Each half is a lever: a handle at one end, a blade at the other. They cross at a pin called the pivot. Push the handles together and the blades swing together.
3. Look from the end. One blade pushes down on one side of the line and the other pushes up on the other side. The paper is forced to slide apart. That is called shearing.
4. The two edges meet at just one small spot. As the blades close, that spot runs from the pivot toward the tips, so the cut travels neatly along the paper.
5. Thick card jams at the tips. Move it close to the pivot and it cuts. Near the pivot the blades move less, but they push much harder.
6. Two levers, one pivot, two sharp edges. Together they turn a gentle squeeze into a clean cut exactly where you aim it.

Tap each part of the scissors to see how it helps to make a clean cut.

• Handles. You squeeze the handles. The loops let you pull the blades open again, ready for the next snip.
• Pivot. The pivot is a screw or rivet that joins the two halves. Both levers turn around it, and it presses the blades tightly together.
• Blades. The blades are the other ends of the levers. They are made of hard steel so their edges stay sharp.
• Cutting edge. Each blade is ground to a thin edge. A thin edge pushes on a very narrow line of paper, so even a small push is enough to split it.
• Cutting point. The two edges cross at one spot. That spot is the only place where cutting happens, and it slides along as you squeeze.
• Tips. The tips move the farthest and push the least. They are best for small, careful cuts in thin paper.

Choose a material, slide it along the blades and squeeze. Find out where thick card will cut and where it jams.

Experiment: Where along the blades will thick cardboard cut?

1. Choose thick cardboard and slide it out to the tips. Squeeze.
2. Open the scissors, slide the strip close to the pivot and squeeze again.
3. Try paper at the tips. Why does that work?

What we found: Thick cardboard cuts only close to the pivot. There the blades move a short way but push very hard. Out at the tips they move a long way but push gently, which is enough for thin paper.

Turn on JavaScript to try it yourself.

Cut all three materials. Choose scissors strong enough for each job. For 3 stars, use the fewest snips you can.

A pair of scissors is two simple machines working together. Each half is a lever: a bar that turns around a fixed point. The fixed point is the screw in the middle, called the pivot. When you push the handles together, each lever turns on the pivot and the blades at the other end swing together too.

The blades do not chop like an axe. One blade pushes the paper down on one side of a line while the other pushes it up on the other side. The paper between them is forced to slide apart. This sliding-apart is called shearing. Because the blade edges are ground thin, all the push lands on a very narrow line, so the paper splits cleanly instead of folding.

The two edges touch at only one spot at a time. As you squeeze, that spot runs along the blades from the pivot to the tips, a bit like a zipper closing. So your whole squeeze is always working on one tiny bit of paper.

Where you cut matters. Close to the pivot the blades hardly move, but they push very hard, which is what thick card needs. Out at the tips they move a long way and push gently, which is perfect for small, careful cuts. A crocodile's mouth is only a rough comparison: jaws bite straight down on things, but scissor blades slide past each other.

Key words

Lever

A bar that turns around a fixed point to move or squeeze something.

Pivot

The fixed point a lever turns around. It is also called the fulcrum.

Shear

Pushing two parts of something in opposite directions so that they slide apart.

Mechanical advantage

How many times a tool multiplies your push.

Bevel

The slanted surface ground on a blade to make its thin edge.

Check your thinking

1. How do scissor blades cut paper?

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   They push it in opposite directions until it slides apart. One blade pushes down beside the line and the other pushes up, so the paper shears apart.

2. Thick card jams at the tips of your scissors. What should you do?

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   Slide the card closer to the pivot. Near the pivot the blades push much harder, because the distance from the pivot is short.

3. You squeeze with 20 N on handles 8 cm long. What is the cutting force 4 cm from the pivot?

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   40 N. Force × distance is the same on both sides of the pivot: 20 × 8 = F × 4, so F = 40 N.

4. Why are scissor blades ground to a thin edge?

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   So the push lands on a very narrow line of paper. The same push on a narrower line presses harder on each bit of paper, so it splits.

5. Which scissors would cut a tough material best?

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   Long handles and short blades. Long handles and short blades multiply your squeeze the most.

6. The cutting point moves from 3 cm to 9 cm from the pivot while you squeeze equally hard. What happens to the cutting force?

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   It falls to one third. The force is inversely proportional to the distance from the pivot. Three times the distance gives one third of the force.