Sharp and blunt

The idea

For the same force, the smaller the area over which it acts, the greater the pressure; the larger the area, the smaller the pressure.

A nail

You hammer a nail into wood. The same hammer blow acts on the nail either way.

Which end of the nail should touch the wood?

• The pointed end
• The flat head
• Either, it makes no difference

Two ends of a nail

Open both tabs. See what the same force does.

Small area, big pressure

The force is the same for both ends of the nail. Only the area is different. The smaller the area, the greater the pressure.

For the same force, the smaller the area, the greater the pressure. The larger the area, the smaller the pressure.

• Situation | Area | Pressure
• Nail, pointed end | Small | High
• Sharp edge of a knife | Small | High
• Broad strap | Large | Low

Notes

For the same force, a smaller area gives a greater pressure.

Check yourself

It is easier to cut an apple with the sharp edge of a knife because the edge has …

The same force acts on two areas. The smaller area gives …

A force of 100 N acts on 1 m². Then the same force acts on 4 m². What is the pressure on the 4 m² area in N/m²?

Answer: 25 N/m²

100 N ÷ 4 m² = 25 N/m². The larger area gives less pressure.

Why is a nail pointed at one end?

• a small area, so the pressure is high — correct. Yes!
• a large area, so the pressure is high. A sharp edge has a small area, not a large one.
• a small area, so the pressure is low. A small area gives a high pressure, not a low one.
• the smaller pressure. Smaller area means greater pressure.
• the greater pressure — correct. Yes!
• the same pressure. Pressure = force ÷ area, so a smaller area gives a greater pressure.
• The tip is heavier. The tip is not heavier. Its small area gives a high pressure.
• The tip gives a smaller force. The force is the same. The small area makes the pressure high.
• The small area at the tip gives a high pressure — correct. Yes!