Pressure and Pivots
Pressure P = F / A
Fluid pressure
P=ρgh
The Manometer
Pivots
Moments
Pressure
Why do predators often have
SHARP teeth and claws?
Why does wearing skis stop you
SINKING into the snow?
CIE IGCSE Pressure
• Relate pressure to force and area, using appropriate
examples
• Recall and use the equation p = F/A
• Relate (without calculation) the pressure beneath a
liquid surface to depth and to density, using
appropriate examples
• Recall and use the equation p = hρg
• Describe the simple mercury barometer and
its use in measuring atmospheric pressure
• Use and describe the use of a manometer
Pressure – Stick notes in your book
• What pressure is the force per unit
area exerted by an object on a surface
with which it is in contact with.
• How to calculate it
• Pressure = Force / Area
• P=F/A
• Units N / m2
• Special unit the pascal Pa
• 1000 Pa = 1kPa kilopascal
Exercise 1: words to use surface area ,
weight , pressure, sink
1. Why do tractors have such large wheels?
The large wheels have a large surface area in contact with the
ground. Tractors have a large weight (force) and often have to
work on soft ground like mud and soil. The large area of the
tyres means that they put less pressure on the ground and they
are therefore less likely to sink.
2. Why do camels have such large feet?
Camels are often found in
deserts. The camels have a
large weight (force) The
large surface area of their
feet on the ground means
that they put less pressure
on the sand and are less
likely to sink into the sand.
Camels have large feet
Exercise 2: Pressure
Pressure tells us over what sort of area a _____
force is
acting. Pressure depends upon two factors: the force
that is being applied and the ____
area over which the
force is acting.
The larger the force, the greater the ________.
pressure The
______
smaller the force, the lower the pressure.
The smaller the area, the _______
greater the pressure. The
larger the area, the smaller the pressure.
greater
area
pressure
force
smaller
Pressure Formula
The pressure formula allows you to calculate the pressure,
force or area for a problem given two of the factors.
Pressure =
Force
Area
Pressure measured in Pascals (Pa).
Force measured in Newtons (N).
Area measured in metres squared (m2).
1 Pa = 1 N/m²
Example
A force of 10 N acts over an area of 2 m2. What pressure is
created by the force?
Pressure = Force ÷ Area
Pressure = 10 ÷ 2 Pa
Pressure = 5 Pa or 5 N/m2
Pressure formula and triangle
• Pressure = Force / Area
• P=F/A
• Units N / m2 special unit the pascal Pa
Exercise 3: Hammer and Nail
A hammer is used to drive a nail into a wooden floor. The
hammer is brought down with a force of 200 N. The area of the
top of the nail is 0.5 cm2. What pressure is put upon the top of
the nail by the hammer blow?
Pressure = Force ÷ Area
Pressure = 200 N ÷ 0.5 cm2
Pressure = 400 N/cm2
Exercise 4: Pressure Calculations 1
1. What are the units of pressure, force and area?
Pressure measured in Pascals.
Force measured in Newtons.
Area measured in metres squared.
2. A force of 500 N acts over an area of 10 m2. What pressure is
created by this force?
50 Pa.
3. A girl of weight 400 N has feet of area 100 cm2. What pressure
does she put on the ground?
4 N/cm2.
4. A car of weight 6 400 N has four wheels. Each wheel has an
area of 80 cm2. What pressure does the car put on the ground?
20 N/cm2.
Exercise 5: Pressure Calculations 2 –
Pressure worksheet
1. A force of 300 N creates a pressure of 4 N/m2. Over what area is
the force acting?
75 m2.
2. A pressure of 200 Pa is created over an area of 5 m2. What force
acts to create this pressure?
1 000 N.
3. A tractor has tyres of area 3 m2. It has a weight of 12 000 N. What
pressure does the tractor put on the ground?
4 000 Pa.
4. A truck has tyres of area 5 m2. It puts a pressure of 15 000 N/m2
on the road. What is the weight of the truck?
75 000 N.
Graph skills
3. Find the gradient
Graph skills
Large Force - Large surface area – lower pressure
Large force - weight and large surface area of balloon tyres - means less
pressure this works most of the time but sometimes the mud is just to
bad.
Meanwhile in Siberia, Russia Federation
..\..\..\Videos\Balloon Tyres Have Greater Surface Area.flv
Fluid Pressure
The pressure in SOLIDS always acts DOWNWARDS because
of the PULL of GRAVITY.
In FLUIDS (LIQUIDS and GASES) the pressure acts IN ALL
DIRECTIONS.
Pressure in fluids can be used to TRANSFER FORCES.
Also, in fluids, the PRESSURE INCREASES WITH DEPTH.
Less
pressure
Higher
pressure
DAMS are WIDER
AT THE BOTTOM
than at the top
because pressure
increases with depth
in water.
Copy this
Pressure in liquids
•
•
•
•
acts in all directions
it increases with depth
it depends on the density of the liquid
it is the same at any depth, it does not
depend on the shape or width of the
container
• Demo
Pressure in Liquid
To calculate the pressure at a depth h and a
density ρ
Volume of liquid = base area x depth = Ah
Mass of liquid = density x volume = ρ Ah
Weight of liquid = mass x g
= ρ g Ah
g = 10 N/kg
Force on base = ρ g Ah
This force is acting on area A
Pressure = force / area = ρ g Ah = ρ g h
A
Pressure in water equation
Pressure = density x gravitational field strength x depth
p= ρ g h Pa
ρ = density
kg/m3
g = gravitational field strength
g= 10
N/kg
h= depth – height of water – deep
m
Pressure in a liquid
If the density of water is 1000kg/m3,what is the
pressure due to the water at the bottom of a
swimming pool 2m deep?
p= ρ g h
ρ = 1000 g=10
h=2
Pressure in Liquid
This equation allows us to calculate the pressure at
any point below the surface of a liquid due to the
liquid alone.
To determine the total pressure at this point, we
must add on the pressure value at the water
surface which is due to the atmosphere (so is
called atmospheric pressure).
Atmospheric pressure varies slightly over the
earth's surface and changes due to weather
conditions –
The value used for calculations is
100 000 Pa = 1.0 x 105 Pa
Copy this
Total Pressure in liquids
= Water pressure + atmospheric pressure
p= ρ g h Pa + 100 000 Pa
Atmospheric pressure is
100 000 Pa
1 x 105 Pa
Atmospheric pressure varies slightly over the
earth's surface and changes due to weather
conditions
Diver question
Example
(a)Calculate the pressure due to
water (density = 1 000 kg m-3)
at a point 20 m below the water
surface.
(b) Calculate the actual
pressure a diver would
experience if she was at this
point.
Atmospheric pressure
= 100 000 Pa = 1.0 x 105 Pa
Questions
Please do question 1 – 2
Exam questions…..
Remember about atmospheric pressure
Mark scheme
Mark scheme
Air pressure demonstration
Copy this
Air Pressure
acts in all direction
becomes less as you rise up through it,
because there is less and less weight
above.
Kinetic Theory – in a gas, the molecules
are constantly striking and bouncing off
the wall of a container. The force of
these impacts causes pressure
Copy this
Air Pressure
acts in all direction
becomes less as you rise up through it,
because there is less and less weight
above.
Kinetic Theory – in a gas, the molecules
are constantly striking and bouncing off
the wall of a container. The force of
these impacts causes pressure
Air Pressure
Air pressure is usually 100000 N/m2.
The can contains air particles and is also
surrounded by air particles. They are
hitting the sides of the container on the
inside and the outside. The vacuum
pump removes the air particles from
inside the can. The pressure caused by
the particles on the outside collapses
the can.
Manometer
A mercury barometer can be made by
filling a long glass tube with mercury, and
then turning it upside down in a bowl of
mercury.
The mercury is held up by air pressure.
As the air pressure varies every day
(depending on the weather), the height of
the mercury varies. The height of the
mercury can be used to predict the
weather.
The height of the mercury is usually
760mm and this is called Standard
Atmospheric pressure (written 760mm
Hg).
Show Barometer - aneroid barometer
Manometer
Questions
Answers
1. Stick in book
2. What liquid does a
barometer contain?
3. What are
barometers used
for?
4. What holds up the
mercury?
5. What is Hg?
6. What is special
about 760mm?
2. Mercury
3. Measuring
atmospheric pressure –
helps to predict the
weather
4. Atmospheric
pressure
5 Chemical symbol for
mercury
6 It is atmospheric
pressure
Manometer
A manometer is a u-tube
containing a liquid, usually
water. It can be used to
measure the pressure of
the gas supply. The higher
the pressure the further
the water is pushed
around the u-tube and the
greater height h.
Demo manometer
Now do multiple choice
questions on manometers and
barometers
Rotating Forces
If an object is fixed at one end and a force is applied then the object
will rotate around the fixed end. We call this fixed end the PIVOT.
Which factors govern the TURNING EFFECT of a force?
The SIZE of the FORCE (F). The LARGER THE FORCE, the
GREATER THE TURNING EFFECT.
The DISTANCE (d) of the force from the pivot. The GREATER THE
DISTANCE of the FORCE FROM THE PIVOT, the GREATER THE
TURNING EFFECT.
We call the turning effect of a force the MOMENT of the force.
d
F
Turning
effect
Manometer
Question
2. What can the manometer
be used to measure and
how does it work?
Answer
2. It can be used to
measure the pressure of the
gas supply
3. What is a vacuum?
3. A vacuum is a volume of
space that is empty of
particles
Moments Formula
The moments formula allows you to calculate the moments,
force or distance for a problem given two of the factors.
Moment = Force x Perpendicular distance of force from pivot
Moments measured in Newton metres (Nm).
Force measured in Newtons (N).
Distance measured in metres (m).
Example:
A force of 8 N acts 3 m away from a pivot. What moment is
created by the force?
Moment = Force x distance
Moment = 8 x 3 Nm
Moment = 24 Nm
¼
½
¼
Exercise 6: Spanner and Nut
A spanner is used to undo a nut. The spanner has a force of
350 N applied to it at a distance of 0.2 m from the nut. What is
the moment of the force?
Moment = Force x distance
Moment = 350 N x 0.2 m
Moment = 70 Nm
Exercise 7: Moment Questions
Calculate the moments for the diagrams below and give the
direction of the moment.
a)
60 N
Moment = 900 Ncm
15 cm
b)
5N
Moment = 37.5 Ncm
7.5 cm
Moment = 125 Nm
c)
500 N
0.25m
Exercise 8: Moment Questions 2
1. What is the formula for calculating moments?
Moment = Force x perpendicular distance of force from pivot
2. What are the units for moments, force and distance?
Moments – Newton metres, Force – Newtons, Distance - metres
3. A force of 40 N is applied 0.4 m from a pivot. What is the moment
created by the force?
16 Nm
4. A moment of 15 Nm is created by a force of 60 N. What is the
perpendicular distance of the force from the pivot?
0.25 m
5. A moment of 200 Ncm is created by a force acting 4 cm from a
pivot. What is the size of the force?
50 N
Principle of Moments
If the turning forces on a system are BALANCED then there
must be the same CLOCKWISE MOMENTS as there are
ANTICLOCKWISE MOMENTS. This is known as the
PRINCIPLE OF MOMENTS.
F
F
CLOCKWISE MOMENTS = ANTICLOCKWISE MOMENTS
Exercise 9: Principle of Moments
1. What is the principle of moments?
Clockwise moments = anticlockwise moments
2. A girl of weight 300 N is sat 3.0 m away from the centre of a seesaw. Where must a boy of weight 450 N sit on the opposite side to
balance the see-saw?
2.0 m away from the centre of the seesaw
3. A metre ruler is supported by a string in the centre at the 50 cm
mark. A weight of 30 N is placed on the ruler at the 65 cm mark.
Where should a weight of 10 N be located to balance the ruler?
At the 5 cm mark
4. What will happen to a system if the turning forces on it are not
balanced?
The system will rotate