Force on an area

The idea

Pressure is the force acting per unit area: Pressure = Force ÷ Area, taking only the force that acts perpendicular to the surface.

Two bags

Megha and Pawan carry bags that weigh the same. Pawan's bag has narrow straps. Megha's bag has broad straps. Pawan's shoulders hurt.

Whose bag presses less on the shoulders?

• Megha's bag, with broad straps
• Pawan's bag, with narrow straps
• Both press the same

Change the area

Pick a force and an area. Make a pressure of 50 N/m².

Force per unit area

With narrow straps the weight acts on a smaller area of the shoulder. With broad straps it is spread over a larger area. So we define pressure as the force on each unit of area.

Pressure = Force ÷ Area. Here we take only the force that acts straight onto (perpendicular to) the surface.

• Force | Area | Pressure
• 100 N | 2 m² | 50 N/m²
• 200 N | 2 m² | 100 N/m²
• 100 N | 4 m² | 25 N/m²

Notes

Pressure = Force ÷ Area. For the same force, a larger area gives less pressure.

Check yourself

Pressure is the force acting on …

A force of 100 N acts on a cardboard of area 2 m². What is the pressure in N/m²?

Answer: 50 N/m²

100 N ÷ 2 m² = 50 N/m².

Broad straps make a bag feel comfortable because they …

People put a round piece of cloth under a load they carry on the head. This …

• each unit of time. Pressure is about area, not time.
• each unit of area — correct. Yes!
• each unit of mass. Pressure is the force on each unit of area.
• make the bag lighter. The bag weighs the same. The area is bigger, so the pressure is smaller.
• increase the pressure. Broad straps spread the weight over a larger area, which lowers the pressure.
• reduce the pressure on the shoulders — correct. Yes!
• increases the area, so the pressure is less — correct. Yes!
• decreases the area, so the pressure is less. The cloth makes the area larger, not smaller.
• makes the load heavier. The weight does not change. The area becomes larger.