Pressure and Moments
Lesson 1: Force and Pressure
Mr Kueres
Lesson Objectives
1. To know what a force does and about
balanced and unbalanced forces –
especially air resistance and friction
2. To know that pressure depends on the
size of a force and the area over which it
is exerted
Mr Kueres
What is a Force?
What Can a Force Do?
A force can change the:
Size
Shape
Speed
And Direction of an object
Mr Kueres
Balanced Forces
If the forces on an object are balanced, they will have
no effect on the motion of an object.
I.e. The person (opposite) will remain still
Or a car moving at 30mph with balanced forces acting on
it, will continue to travel at 30mph
Mr Kueres
Unbalanced Forces
If the forces on an object are unbalanced, they will
have some effect on the object’s motion (either
change the direction or cause the object to speed
up or slow down.
Will the car below speed up or slow down?
Drag
(Resistive
forces)
Thrust
(driving
force)
Mr Kueres
Pressure
Pressure tells us about the effect of a force
compared with the area over which it acts.
Mr Kueres
Things to Do
Answer the questions on the worksheet ‘Big Pressure, Small
Pressure’ – 10 minutes
(H) Draw a table, as shown below, illustrating examples of large
area/low pressure and small area/high pressure
(F) Draw examples of things that will exert a high pressure because
of its small surface area and a low pressure because of its large
surface area.
Large Area/Low
Pressure
Small Area/High
Pressure
Mr Kueres
Homework
Complete Worksheet ‘But Why?’
DUE IN: Next lesson
If you have any questions about the
homework, come and see me before it is
due in.
Mr Kueres
Pressure and Moments
Lesson 2: Calculating
Pressure
Mr Kueres
Lesson Objectives
1. To recall and use the formula for
pressure
2. To know the units for pressure, force and
area
3. To be able to estimate area reliably
Mr Kueres
Pressure
Pressure is dependent on f
and
a
.
If the f
applied gets larger, the p
will
be h
.
If the a
becomes larger, the pressure will
be s
.
Mr Kueres
Pressure
Pressure is dependent on force and
area.
If the force applied gets larger, the
pressure will be higher.
If the area becomes larger, the pressure will
be smaller.
Mr Kueres
Formula for Pressure
Pressure =
Force
Area
P= F
A
Force is measured in N
, N.
Area is measured in s
m , m2.
Pressure is measured in Newtons per
square metre, N/m2 or P
, Pa.
Mr Kueres
Formula for Pressure
Pressure =
Force
Area
P= F
A
Force is measured in Newtons, N.
Area is measured in square metres,
m2.
Pressure is measured in Newtons per
square metre, N/m2 or Pascals, Pa.
Mr Kueres
Finding the Pressure we Exert
on the Floor
APPARATUS
cm Squared Paper
Bathroom Scales
METHOD
Draw around your foot on the piece of squared paper.
Count the squares and estimate the area of your foot
(More than half a square counts as 1, less than half a
square doesn’t count).
Use the bathroom scales to find your body WEIGHT.
Remember: weight is a force and measured in
NEWTONS) (1kg ~ 10N).
Calculate the pressure exerted by each foot, then both
feet together.
Mr Kueres
Calculation for One Foot
Pressure exerted by one foot
=
Half body weight
Area of foot
=
=
Mr Kueres
N/cm2
Calculation for Both Feet
Pressure exerted by one foot
=
Body weight
2 x Area of foot
=
=
Mr Kueres
N/cm2
Evaluation
Give one reason why you think your
experiment was accurate and one reason
why you think your experiment was not
very reliable
Mr Kueres
Things to Do
(H) Answer the questions on the Pressure
Practice Worksheet in your exercise
books.
(F) Complete the worksheet 9La/3: Pressure
Points
Mr Kueres
Pressure and Moments
Lesson 3: Compression of
Liquids and Gases
Mr Kueres
Lesson Objectives
1. To review the formula for pressure
2. To know whether liquids and gases can
be compressed
3. To know that liquids transmit pressure
4. To know that pressure increases with
fluid depth
Mr Kueres
Pressure Formula
As the force increases, pressure increases.
If the surface area increases, pressure will
decrease.
Pressure =
P= F
A
Mr Kueres
Force
Area
Pressure
= force/area. There is a direct relationship
between pressure and force – increasing the force
the pressure (provided that the area stays
the same). There is an
relationship
between force and area – increasing the area decreases
the pressure (provided that
stays the
same). If force is measured in
, N,
and area is measured in
metres,
m2, then the unit of pressure is the
.
The atmosphere exerts pressure. Outside the Earth’s
, there is no air and no
atmospheric pressure. Water also exerts pressure.
Water exerts more pressure at greater
.
Mr Kueres
Pressure
Pressure = force/area. There is a direct relationship
between pressure and force – increasing the force
increases the pressure (provided that the area stays the
same). There is an inverse relationship between
pressure and area – increasing the area decreases the
pressure (provided that force stays the same). If force is
measured in Newtons, N, and area is measured in
square metres, m2, then the unit of pressure is the
Pascal.
The atmosphere exerts pressure. Outside the Earth’s
atmosphere, there is no air and no atmospheric pressure.
Water also exerts pressure. Water exerts more pressure
at greater depth.
Mr Kueres
Compression in Liquids and Gases
In a gas, the
are a long way apart and
it’s not too hard to push
them closer
.
In a
, it’s
very hard to
particles much closer
together.
So, the particle model can
explain the different
of
gases and liquids under
.
Mr Kueres
G
a
s
Ga
s
Compression in Liquids and Gases
In a gas, the particles are a
long way apart and it’s not
too hard to push them
closer together.
In a liquid, it’s very hard to
push particles much
closer together.
So, the particle model can
explain the different
behaviour of gases and
liquids under
pressure.
Mr Kueres
G
a
s
Ga
s
Introduction to Hydraulics
Many robots use hydraulics or
liquid pressure. There are two
cylinders – a master cylinder
and a slave cylinder –
connected by a sturdy pipe.
The master cylinder contains
the effort piston and the slave
cylinder contains the load
piston. The slave does
everything that its master tells
it to do, but it does it with more
force than the master could
manage. Hydraulics are also
used in jacks to lift up cars.
Mr Kueres
The Hydraulic Jack
Effort
Small piston
provides the effort
but moves along
way
Hydraulic fluid Increased
is pushed
pressure
through
results in load
being lifted
X
Non-Return Valve
Mr Kueres
Large
piston
lifts the
load but
doesn’t
move as
far
Hydraulics
If the load piston has an area 5 times that of
the effort piston, the load lifted is 5 times
the effort, BUT moves only 1/5 the
distance.
Mr Kueres
Questions
1. Why do we use oil and not water in
hydraulic machines?
2. What is the non-return valve for?
3. What would happen if there was an air
bubble in the pipe work between two
cylinders?
Mr Kueres
Motor Vehicle Brakes
If liquids are useful because they are
incompressible and air in hydraulic fluid
prevents it working effectively, why do
some vehicles have brakes operated by
compressed air?
Mr Kueres
Hydraulics Review
A
system can convert a small force into
a bigger force. The system is filled with liquid, which is
.
The force acting on the piston in the master cylinder acts
over a small
. The
is
transmitted through the liquid, so the same pressure acts
on the piston in the slave cylinder, but it acts over a
bigger area. The same pressure acting over a bigger
area produces a bigger
.
A gas is of no use in a hydraulic system because it is
. Pressure acting on a gas pushes the
closer together. In a
the particles are already close together and it’s very
hard to make them closer.
Mr Kueres
Hydraulics Review
A hydraulic system can convert a small force into a bigger
force. The system is filled with liquid, which is
incompressible.
The force acting on the piston in the master cylinder acts
over a small area. The pressure is transmitted through
the liquid, so the same pressure acts on the piston in the
slave cylinder, but it acts over a bigger area. The same
pressure acting over a bigger area produces a bigger
force.
A gas is of no use in a hydraulic system because it is
compressible. Pressure acting on a gas pushes the
particles closer together. In a liquid the particles are
already close together and it’s very hard to make them
closer.
Mr Kueres
Deep Sea Diving
Why does a diver need to breathe compressed air?
The pressure of the water
above him makes it hard to
inhale with out the gas
helping.
The pressure is due to the
mass of water above him.
Pressure = Weight of water
Area of diver
The pressure exerted
depends on:
1. the depth
2. the weight of the fluid
(hence, its density)
Mr Kueres
Depth of water in metres
100
20
200
30
300
40
400
50
500
Mr Kueres
Water pressure in kilopascals
10
Submarine Pressure
Depth of water in metres Water pressure in
kilopascals
10
100
20
200
30
300
40
400
50
500
Mr Kueres
Things to Do
Answer the Questions on the Submarine
Pressure worksheet
Answer the Questions on the Water
Pressure worksheet
Mr Kueres
Submarine Pressure
The bigger the depth of water, the b
the water pressure is. For
example, if the depth doubles then water
pressure also d
. And if the
depth trebles then w
p
trebles. In fact, whenever the depth
changes, the water pressure always
changes by the same p
.
Mr Kueres
Submarine Pressure
The bigger the depth of water, the bigger the
water pressure is. For example, if the
depth doubles then water pressure also
doubles . And if the depth trebles then
water pressure trebles. In fact, whenever
the depth changes, the water pressure
always changes by the same proportion.
Mr Kueres
Pressure & Moments
Lesson 4: Moments
Mr Kueres
Lesson Objectives
1.
2.
3.
4.
5.
To know that a force can produce a turning
effect
To know that the turning effect is called a
moment
To know that the moment of a force depends
on the size of the force and the distance of the
force from the pivot
To know that the pivot is also called a Fulcrum
To be able to calculate the moment of a force
Mr Kueres
Turning Effects
Write a list of examples of where a force
produces a turning effect.
Mr Kueres
Demonstration
Try opening a door in each of these three
places:
1. A point furthest away from the hinges
2. A point roughly in the centre of the door
3. A point close to the hinges
At which point is it the most difficult to open
the door?
Mr Kueres
Demonstration
Hold a metre ruler horizontally at arms length. If we apply a
10N weight near your hand, then gradually move the
weight along the ruler, what happens?
We find that it becomes increasingly difficult to hold the
ruler horizontal.
This is because the mass exerts a turning effect on the
ruler.
The turning effect is called a MOMENT and depends
on the weight and the distance from the pivot
Mr Kueres
Moments
The turning effect caused by a force is
called a moment and depends on the
force (weight) applied and the distance
from the pivot.
Mr Kueres
Moments
Moment = Force Exerted x Distance from the Pivot
Moment = F x d
UNITS
Force is measured in N
, N.
Distance is measured in m
, m (usually
c
, cm, for the distances we use
in class).
Moments are measured in N
m
,
Nm (or Newton centimetres if the distance is measured
in that unit)
Mr Kueres
Moments
Moment = Force Exerted x Distance from the Pivot
Moment = F x d
UNITS
Force is measured in Newtons, N.
Distance is measured in metres, m (usually
centimetres, cm, for the distances we use in class).
Moments are measured in Newton metres, Nm (or
Newton centimetres if the distance is measured in that
unit).
Mr Kueres
Things to Do
Complete 9Ld/3: Revision Questions
Complete the Moments Example Sheet 1
Mr Kueres
Balancing Act
We can use the idea of moments to see why
an object balances (or overbalances)
G
Centre of
gravity acts
between the
wheels – the
bus is
balanced Mr Kueres
G
Centre of
gravity acts
outside of
the wheels
Things to Do
Complete the Moments Example Sheet 2
Mr Kueres
Pressure and Moments
Lesson 5: Balancing Moments
Mr Kueres
Lesson 5 - Objectives
1. To know how to find whether moments
are balanced or unbalanced
2. To be able to balance simple levers
3. To know that if a lever is balanced, the
anticlockwise and clockwise moments
are equal
Mr Kueres
Moments - Review
Moment = Force Exerted x Distance from the Pivot
Moment = F x d
UNITS
Force is measured in Newtons, N.
Distance is measured in metres, m (usually
centimetres, cm, for the distances we use in class).
Moments are measured in Newton metres, Nm (or
Newton centimetres if the distance is measured in that
unit).
Mr Kueres
Balancing Levers
How can we alter levers e.g. seesaws, so that they
balance?
In which direction would you move the masses to make
the lever above balance?
Could you balance the lever by moving the pivot? If so
how?
Mr Kueres
Balancing Levers
We can balance the lever by:
1. Moving the light weight away from the pivot.
Lever moves
clockwise
Mr Kueres
Balancing Levers
We can balance the lever by:
2. Moving the heavy weight towards the pivot.
Lever moves
clockwise
Mr Kueres
Balancing Levers
We can balance the lever by:
3. Moving the pivot towards the heavy weight.
Lever moves
clockwise
Mr Kueres
Practical – Balancing Levers
Use the ‘Levers’ worksheet to perform the
investigation
Record your results in an appropriate table
Mr Kueres
Conclusion
In order for a lever to balance, the
c
moment must e
clockwise m
.
Mr Kueres
the anti-
Conclusion
In order for a lever to balance, the
clockwise moment must equal the
anti-clockwise moment.
Mr Kueres
Total anti-clockwise moment = Total clockwise moment
Anti-Clockwise Moment
=
Clockwise Moment
Force x Distance from pivot
=
Force x Distance from Pivot
Mr Kueres