ZIMSEC O Level Combined Science Notes: The concept of pressure

  • Pressure is the force acting over an area
  • In such pressure is usually the force per unit area exerted by a fluid/gas
  • over a surface with which it is in contact
  • \text{Pressure=} \dfrac{Force}{Area}
  • If a big force is exerted over a small area it results high pressure
  • An example is the force at the tip of a women’s high heeled shoes
  • It is higher than the pressure exerted by flat shoes
  • The pressure of high heeled shoes is even higher than that of a man wearing flat shoes
  • Even when the man weighs much more than the women.
  • The pressure of a stiletto is higher than that of an elephant’s foot!
  • As already pointed out the formula for pressure is:
  • \text{Pressure=}\dfrac{Force}{Area}
  • In unit terms this can be written down as:
  • \text{Pa=}\dfrac{N}{m^2}
  • Pa-stands for pascals
  • This is the unit for pressure
  • N-is for newtons
  • m2-is the area in square metres
  • One Pascal is equal to 1 Newton of Pressure per square meter
  • Like most units the Pascal can have prefixes such as 1 kilo pascal (1 000N/m2) or even mega pascals

Example calculations

  1. A rectangular block with dimensions 100cm x 120 cm x 150cm and weighs 1.5 N.
    What pressure does it exert:

    1. On its 100cm x 120 cm face
    2. 100cm x 150cm face
    3. 120 cm x 150 cm face
  2. Solution:
      1. \text{Pa=} \dfrac{1.5}{1.2}=\text{1.25 Pa}
      2. \text{Pa=}\dfrac{1.5}{1.5}= \text{1Pa}
      3. \text{Pa=}\dfrac{1.5}{1.8}=\therefore \text{0.83 Pa}
  3. A nail is hammered into wood using a hammer of weight 10N. If the nail has an area of 1 mm2 at the tip what is the pressure exerted on the wood?
  4. Solution:\text{Pa=}\dfrac{10N}{0.000001}\text{10000000Pa}

NB Remember to convert the square cm and square mm into square meters

To access more topics go to the Combined Science Notes page