Fast Food
Hoo Sze Yen
www.physicsrox.com
Physics SPM 2011
CHAPTER 3: FORCES AND PRESSURE
3.1 Pressure
Units of pressure Unit Note Pa SI unit N m-2 Equivalent to Pa N cm-2 cm Hg m water atm 1 atm = atmospheric pressure at sea level bar 1 bar = 1 atm
Pressure is the force which acts normal per unit area of contact.
P= F A
where P = pressure [Pa] F = force [N] A = area [m2]
For atmospheric pressure only
3.2
Pressure in Liquids
Pressure in liquids are not dependent on the size or shape of the container; only on its depth.
P = hρ g
where P = pressure [Pa] h = height of liquid [m] ρ = density of liquid [kg m-3] g = gravitational acceleration [m s-2]
3.3
Atmospheric and Gas Pressure
3.3.1 Atmospheric Pressure
• Atmospheric pressure is measured by barometers.
Mercury barometer
Chapter 3: Forces and Pressure
Aneroid barometer
Page 1 of 8
Hoo Sze Yen
www.physicsrox.com
Physics SPM 2011
Atmospheric pressure measured by a simple mercury barometer can be calculated using the formula P=hρg. Standard atmospheric pressure at sea level is usually 76 cm Hg or 10 m water. It can also be expressed as 1 bar.
Note: cm Hg is an accepted unit of atmospheric pressure. Other similar units involving different liquids are also accepted.
For example: Given that h = 76 cm and the density of mercury is 13 600 kg m-3: Answer: Atmospheric pressure = 76 cm Hg State the answer in Pascal: P = hρg = 0.76(13 600)(10) = 103 360 Pa
3.3.2 Gas Pressure
• Gas pressure can be measured using manometers and Bourdon gauges.
Manometer
Bourdon gauge
In manometers, pressure can be calculated based on pressure equilibrium. For example: Pressure at point A = Pressure at point C Gas pressure = Atmospheric pressure + hρg
Chapter 3: Forces and Pressure
Page 2 of 8
Hoo Sze Yen
www.physicsrox.com
Physics SPM 2011
3.4
Pascal’s Principle
Pascal’s Principle dictates that pressure which is exerted onto the surface of a fluid in a
www.physicsrox.com
Physics SPM 2011
CHAPTER 3: FORCES AND PRESSURE
3.1 Pressure
Units of pressure Unit Note Pa SI unit N m-2 Equivalent to Pa N cm-2 cm Hg m water atm 1 atm = atmospheric pressure at sea level bar 1 bar = 1 atm
Pressure is the force which acts normal per unit area of contact.
P= F A
where P = pressure [Pa] F = force [N] A = area [m2]
For atmospheric pressure only
3.2
Pressure in Liquids
Pressure in liquids are not dependent on the size or shape of the container; only on its depth.
P = hρ g
where P = pressure [Pa] h = height of liquid [m] ρ = density of liquid [kg m-3] g = gravitational acceleration [m s-2]
3.3
Atmospheric and Gas Pressure
3.3.1 Atmospheric Pressure
• Atmospheric pressure is measured by barometers.
Mercury barometer
Chapter 3: Forces and Pressure
Aneroid barometer
Page 1 of 8
Hoo Sze Yen
www.physicsrox.com
Physics SPM 2011
Atmospheric pressure measured by a simple mercury barometer can be calculated using the formula P=hρg. Standard atmospheric pressure at sea level is usually 76 cm Hg or 10 m water. It can also be expressed as 1 bar.
Note: cm Hg is an accepted unit of atmospheric pressure. Other similar units involving different liquids are also accepted.
For example: Given that h = 76 cm and the density of mercury is 13 600 kg m-3: Answer: Atmospheric pressure = 76 cm Hg State the answer in Pascal: P = hρg = 0.76(13 600)(10) = 103 360 Pa
3.3.2 Gas Pressure
• Gas pressure can be measured using manometers and Bourdon gauges.
Manometer
Bourdon gauge
In manometers, pressure can be calculated based on pressure equilibrium. For example: Pressure at point A = Pressure at point C Gas pressure = Atmospheric pressure + hρg
Chapter 3: Forces and Pressure
Page 2 of 8
Hoo Sze Yen
www.physicsrox.com
Physics SPM 2011
3.4
Pascal’s Principle
Pascal’s Principle dictates that pressure which is exerted onto the surface of a fluid in a