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

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