Importance of pressure

Pressure is important in a range of everyday situations.

  • Different styles of shoe can cause different pressures due to their area. Flat shoes spread the force over a large area, reducing the pressure. Snow shoes have a much larger area than feet to spread the force over a larger area and reduce the pressure on the snow - this stops people sinking into the snow.
  • High heeled shoes or boots transfer the force through a much smaller area, causing a much greater pressure. It will hurt more if a person steps on someone’s foot in high heels than if they are wearing flat shoes.
  • When using a sharp knife, the small area of the blade creates a large pressure, making cutting easier.
  • Having caterpillar tracks on vehicles means their weight acts over a large area. This reduces the pressure they exert and are less likely to sink in to wet ground.
Question

A metal box is 60 cm long, 20 cm wide and 50 cm tall.

It has a mass of 30 kg. Find the maximum pressure which the box can exert on the top of a table.

The maximum pressure will occur when the face with minimum area is in contact with the table

The face with minimum area has dimensions 20 cm by 50 cm.

Area = 20 cm x 50 cm.

A = 1 000 cm2.

Force on the bench = weight of the box = mg.

Force = 30 kg x 10 N/kg.

Force F = 300 N.

p = \frac{\text{F}}{\text{A}}

p = \frac{\text{300 N}}{\text{1000 cm}^{2}}

p = 0.3 N/cm2

The maximum pressure exerted by the box is 0.3 N/cm2.

Question

A girl standing on one foot exerts a pressure of 6 N/cm2 on the ground. If the area of her foot is 75 cm2, calculate the downward force.

F = pA.

p = 6 N/cm2

A = 75 cm2

P = 6 N/cm2 x 75 cm2

P = 450 N.

The downward force exerted by the girl is 450 N

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