Page 6 - CLASS VII Lesson Notes Forces and Energy

SUBTOPIC: 3.5 PRESSURE BETWEEN SOLIDS

Key Words:
pressure, surface area, point, sharp, newtons per square metre

Understanding Pressure:
• Pressure is the pushing effect of a force spread over an area.
• A force acting on a small area creates high pressure.
• A force spread over a large area creates low pressure.

Equation for Pressure:

Pressure = 1*,'(
2,(%

Ø Force is measured in newtons (N)
Ø Area is measured in square metres (m²) or other units like cm² or mm²
Ø The unit of pressure is:
• N/m² (newtons per square metre)
• N/cm², if area is in cm²
• N/mm², if area is in mm²

Examples from Everyday Life:
• A knife cuts better when it’s sharp because it has less surface area, which
increases pressure.
• A camel has large feet to reduce pressure on the sand and avoid sinking.
• A bed of nails does not hurt because the person’s weight is spread over
hundreds of sharp points, so the pressure at each point is low.
• A pin or thumb tack has a very small tip to increase pressure and push easily into
surfaces.
• Scissors have sharp edges to increase pressure and make cutting easier.

Worked Example 1
An elephant weighs 50,000 N and its feet cover 0.4 m².
Pressure =50000/0.4
=125000 N/m²
Worked Example 2
A bicycle and rider together weigh 1000 N. Each tyre has an area of 5 cm².
Weight on each tyre = 500 N
pressure=5005=100 N/cm²

Important Notes:
• To increase pressure: increase force or reduce area.
• To reduce pressure: reduce force or increase area.
• The same force gives different pressure depending on the area over which it acts.

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