Transcript Lesson 5 Pressure - MrSimonPorter

TEST!
• Next Monday
• Topic 2 “Matter and Forces”
• Use the “Can you…..?” sheet to
help you
• We’ll do some practice questions
on Friday.
Do now!
Can you
continue the
pressure
questions you
started last
lesson?
This lesson – 2.4 Pressure
• Relate (without calculation) pressure to
force and area.
• Recall and use the equation P = F/A
Oh no!
YouTube - falling through ice
YouTube - Jason Falls Through Ice
Spread the force over a greater
area
Camels
Snow shoes
Yoost
Caterpillar tracks
House foundations
My sofa!
I had a dream
Death?
Sharp = small surface area
Drawing pins
Pressure
N
Pressure = Force
Area
N/m2 or Pa
m2
Pressure
F
N
P
x
A
Pressure = Force
Area
N/m2 or Pa
Can you
copy this
please?
m2
An example
A woman of weight 600N has a total shoe area of 150
cm2 and a man of weight 750 N has a total shoe area of
360 cm2. What is the pressure beneath their feet?
An example
A woman of weight 600N has a total shoe area of 150
cm2 and a man of weight 750 N has a total shoe area of
360 cm2. What is the pressure beneath their feet?
Angelina pressure = force/area = 600/150 = 4 N/cm2
Brad pressure = force/area = 750/360 = 2.1 N/cm2
Bunny Suicides involving pressure
Bunny Suicides involving pressure
Bunny Suicides involving pressure
Bunny Suicides involving pressure
Calculating your own pressure
Can you do the
calculations carefully
and neatly on the
graph paper to show
your pressure (don’t
forget you have two
feet!) and stick it in
your book
Pressure under a car
Pressure under a car
•
•
•
•
Mass = 2000kg
Weight = mg = 2000x10 = 20000N
Area = ?
Pressure = ?
Let’s try some questions!
Gas pressure
• Phet
• http://phet.colorado.edu/sims/idealgas/gas-properties_en.jnlp
Temperature and pressure
• With a fixed volume and mass of gas, the
pressure increases with temperature
Can you copy
this please?
Atmospheric pressure
Atmospheric pressure
Atmospheric pressure
Atmospheric pressure
Atmospheric pressure
• The atmospheric pressure caused by air
particles colliding with objects is about
110000 Pascals (this is called ‘1
atmosphere’)
• It is this pressure that pushes drink up a
straw or causes suction cups to work.
Can you copy
this please?
U571
• YouTube - Ocean Odyssey - Ocean
Pressure
• YouTube - Styrofoam cup vs. deep sea
• YouTube - U-571 Trailer
Pressure and depth
Pressure acts in
all directions
and pressure
increases with
depth.
That’s it!
• Let’s remind ourselves of what we have
done in this unit.
Gravity
Gravity is a force between ALL objects!
Gravity
Gravity
The size of the force depends on the mass
of the objects. The bigger they are, the
bigger the force!
Small attractive force
Bigger attractive force
Gravity
The size of the force also depends on the
distance between the objects.
Gravity
The force of gravity on something is called
its weight. Because it is a force it is
measured in Newtons.
Weight
Gravity
On the earth, Mr Porter’s weight is around
800 N.
I love physics!
800 N
Gravity
On the moon, his weight is around 130 N.
Why?
130 N
Mass
Mass is a measure of the amount of
material an object is made of. It is
measured in kilograms.
Mass
Mr Porter has a mass of around 77 kg.
This means he is made of 77 kg of blood,
bones, hair and poo!
77kg
Mass
On the moon, Mr Porter hasn’t changed
(he’s still Mr Porter!). That means he still is
made of 77 kg of blood, bones, hair and
poo!
77kg
Calculating weight
To calculate the weight of an object you
multiply the object’s mass by the
gravitational field strength wherever you are.
Weight (N) = mass (kg) x gravitational field strength
(N/kg)
Forces
• Force is measured in Newtons
Friction opposes motion!
Newton’s 1st Law
Remember?
If there is no
resultant force
acting on an
object, it will move
with constant
velocity. (Note the
constant velocity
could be zero).
Newton’s 1st law
If there are no forces, or the
forces are balanced – constant
velocity
Pushing force
friction
NO RESULTANT FORCE
Newton’s 2nd law
There is a mathematical relationship
between the resultant force and
acceleration.
Resultant force (N) = mass (kg) x acceleration (m/s2)
F = ma
An example
What will be Mr Porter’s
acceleration?
Mass of Mr Porter and bike = 100
kg
Pushing force (100 N)
Friction (60 N)
An example
Resultant force = 100 – 60 = 40 N
FR = ma
40 = 100a
Mass of Mr Porter and bike = 100 kg
a = 0.4 m/s2
Pushing force (100 N)
Friction (60 N)
kg/m3
kg
Density (g/cm3) =
mass(g)
volume(cm3)
m3
Or using the formula triangle;
m
DxV
•http://www.youtube.com/watch?v=TRkCz3zG7w0&feature=related
You have
to LEARN
this!
Density of regular shapes
volume = length x width x height
height
width
length
density = mass/volume
mass using a scale
Density of liquids
Volume
Mass of liquid
Mass of liquid
and cylinder
Mass of cylinder
Density = mass/volume
Density of irregular shapes (1)
Difference in
level gives the
volume of the
shape
Density = mass/volume
mass
Density of irregular shapes (2)
mass
Displacement can
volume of object
Density = mass/volume
Adding forces
For example;
Resultant force
6N
4N
2N
Hooke’s law (F = kx)
Limit of proportionality
Force
(N)
The extension of a spring is proportional to
the force applied (until the limit of
proportionality is reached). The gradient
of the graph is equal to k, the spring
constant.
Extension (x) (cm)
Hooke's Law and springs - Physics – YouTube
http://www.youtube.com/watch?v=pVdGUTRI49E
What does k mean in F=kx?
• k is called the spring constant and is a
measure of the stiffness of the spring or
material
• It has units of N/m (newtons per metre)
• The higher the k the stiffer the spring
• Materials with a high k need a large force to
for a given extension
A material is said to obey
Hookes Law if its extension is
directly proportional to the
applied force
More info on Hooke at
http://www.roberthooke.org.uk/
Steel, glass and wood!
Force
Even though they don’t
stretch much, they obey
Hooke’s law for the first part
of the graph
Extention
Rubber
Force
Extension
Pressure
N
Pressure = Force
Area
N/m2 or Pa
m2
Learning profile/Test
• Use the learning profile to help you study!
• Test on Tuesday 29th November
• www.mrsimonporter.wikispaces.com
Let’s try some practice questions