Pressure

Chapter 44

What is Pressure?

• Pressure is the Force per unit Area

• Pressure = Force(N)

Area(m2)

• The unit of pressure is N/m2 or the Pascal

Question

• A stone block of weight 22500N is to be used as a base for a sculpture.The block has dimensions 3m x 2m x 1.5m.

(a) What is the greatest possible pressure the

block could exert on the ground?

(b) What is the least possible pressure the

block could exert on the ground?

Answer a

• The pressure of the block will be greatest when the area that is resting is the smallest.

Pressure = Force = 22500(N)

Area 2 x 1.5(m2)

= 7500 N/m2 or 7.5kPa

Answer b

• The pressure of the block will be smallest when the area that is resting is the largest.

Pressure = Force = 22500(N) Area 3 x 2 (m2)

= 3750 N/m2 or 3.75kPa

Pressure in Liquids

Pressure increases with Depth

The pressure in a fluid acts equally in all directions.

Atmospheric Pressure

Atmospheric pressure is caused by the weight of the atmosphere.

To show atmospheric Pressure

• To Show atmospheric pressure

• Boil a small amount of water in the can. This drives out the air

• Remove the heat and seal the can.

• Cool the can under water. The can then collapses due to atmospheric pressure.

Measuring Pressure

A barometer is used to measure atmospheric pressure.

There are two types of barometer– A mercury barometer– An Aneroid barometer

Mercury Barometer

• Normal atmospheric pressure is 76mm of Mercury.

• This will vary with weather conditions and height above sea level.

Aneroid Barometer

Atmospheric Pressure and weather

• High pressure over an area gives sunny days, light winds and clear blue skies.

• Low pressure gives rain, strong winds and poor weather.

Measuring Altitude

• As a plane goes higher, the height of the atmosphere overhead gets less, and therefore the atmospheric pressure gets less.This can be used to read height above sea level.

• A barometer that is used to measure height is called an altimeter.

Using Pressure

• Liquids cannot be compressed and transfer pressure well.Hydraulic machines use liquid pressure like this

Textbook solutions• A1 Force, area.

• A2. Force, area.

• A3. N/m2, pascals.

• A4. Greater.

• A5. Depth.

• A6. Equal.

• A7. Higher.

• A8. Higher, greater.

• A9. Atmospheric pressure.

• A10. Barometer.

• A11. 76,mercury.

• A12. Atmospheric pressure.

• A13. Decreases.

• A14. 76,mercury,1013.

• A15. Barometer, height.

• A16. High.

• A17. Low.

Question B1

• B1.

• (a)Pressure is the force per unit area. Pascal(Pa).

• (b)Weight=20x10=200N;Pressure=200/4=50Pa.

• (c)The higher it is, the greater the water pressure.

Question B2 (a) See Experiment 43.4, p.293.

(b)Atmospheric pressure pushes down on the mercury in the bowl and this holds mercury in the tube up to a height of 76 cm. (c) Normal atmospheric pressure holds up a tube of mercury 76 cm high, higher atmospheric pressure would raise the level of mercury in the tube to above 76cm.(d) Lower atmospheric pressure would drop the level of mercury in the tube below 76cm. The mercury level in the tube would drop due to lower atmospheric pressure. Low atmospheric pressure allows the water to boil (become a gas) more easily.

Question B.3

(a)The water shoots out further through the lowest hole in the container.

(b)The water shoots out equally from the holes in all directions.