Chapter 7 - Pressure

1.

Explain why a sharp knife cuts better than a blunt one.

  • A sharp knife has a smaller area of contact with the object to be cut.
  • For the same force exerted, a smaller area gives greater pressure as indicated by Pressure = Force / Area

2.

*Explain why the height of the mercury in a barometer remains unchanged when the diameter of the inverted tube is changed.

  • Since Pressure = (height of liquid column) x (density of liquid) x (gravitational field strength), then the height = pressure / [(density of liquid) x (gravitational field strength)]. It means that the height of the mercury in the barometer depends only on the pressure and gravitational field strength.

3.

*Explain why the atmospheric pressure decreases with height

  • The pressure at any given altitude in the atmosphere is due to the weight of the air above.
  • As altitude increases, there will be less air above. The density of air also decreases.
  • Since Pressure = (height of liquid column) x (density of liquid) x (gravitational field strength), a decrease in height and density of the air column (or any other fluid) will lead to a decrease in pressure.