Physics: 9. Force, Work and Power
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Syllabus
OP4 Appreciate the concept of force, recall that the newton is the unit of force, and describe forces and
their effects
OP5
Investigate examples of friction and the effect of lubrication
OP6
Investigate the relationship between the extension of a spring and the applied force
OP7
Recall that weight is the force of gravity and that weight can vary with location; recall that mass in
kilograms multiplied by 10 is approximately equal in magnitude to weight in newtons on the surface
of the earth.
OP15 Define and give the units for work, energy and power, state the relationship between work and
power, and perform simple calculations based on this relationship
Student Notes
Force
A force is something that can cause an object to change velocity or accelerate
The newton (symbol N) is the unit of force.
Forces can be measured using a spring-balance (the proper name is a newton-meter).
Friction
Friction is a force that opposes motion between two objects in contact.
Friction can be a nuisance (e.g. it slows things down when you don’t want it to when driving a car or riding
a bike).
Friction can also be useful (e.g. brakes on a bike, and to enable us to walk).
Experiment: To demonstrate friction and how a lubricant can be used to reduced friction.
Attach a spring balance to a wooden block and use it to pull the block over the bench. Note the reading on
the spring-balance.
Now put some oil or grease under the block and repeat.
Notice that the force required to pull the block is much less.
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Hooke’s Law
The extension of a stretched string is directly proportional to the force that is stretching it
This means that if the force on the string doubles so will the extension; if the force triples
then the extension will also triple, etc.
Experiment: To investigate the relationship between the extension of a spring and the force
that is stretching it.
1. Use a newton-meter to stretch a spring and note the extension and the force used.
2. Repeat for lot of different forces.
3. Plot a graph of force used against extension.
4. Result: Your graph should be a straight line.
5. Conclusion: The graph is a straight line through the origin which shows that the extension
is directly proportional to the applied force.
This is what your graph should look like:
{Strictly speaking stating Hooke’s Law is not on the
syllabus but it did get asked by mistake one year so it’s
best to learn it just in case it happens again. You do
however have to know the experiment.}
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Weight and Mass
Mass is a measure of the amount of matter (“stuff”) in something.
The weight of an object is a measure of the gravitational pull on it.
Weight is a force (due to gravity)
The mass of an object doesn’t change from one place to the next because it still has the name amount of
stuff inside (the same number of atoms), but the weight of the object will be much less on a planet smaller
than Earth (or on the moon) while it will weigh much more (it will be much ‘heavier’) on a bigger planet. In
fact if you could go to one of the bigger planets you would not even be able to stand up because you weigh
so much!
Maths Problems
Weight (in newtons) = Mass (in kilograms) × 10
e.g. a student of mass 60 kg has a weight of 600 N.
Note: There are 1000 grams in a kilogram (kg).
A little more on Mass and Weight
There is a very important and often misunderstood difference between mass and weight.
Mass is the amount of matter present in the object. It doesn’t change, i.e. the amount of ‘stuff’ in your phone
doesn’t change, regardless of where you bring it (even if you bring it to the moon it will still have the same
number of atoms in it.)
Another way to look at it is to compare weight, which is caused by gravity, to magnetism.
Bringing a magnet from one place to another won’t change the amount of stuff (mass) in it, but if you bring
the south end of a magnet closer to the north pole of another magnet it will experience a greater attraction to
the north pole. It will appear to be heavier. The force of attraction between two magnets depends partly on
the strength of the two magnets. Similarly the force of gravitational attraction between two objects depends
partly on the mass of the two objects. ‘Weight’ is our shorthand way of talking about gravitational attraction.
So the gravitational attraction between you and the Earth is greater than the force of attraction between you
and the moon.
Weight is the force with which the object is attracted to the Earth (or other planet) by gravity.
Weight (in newtons) = Mass (in kilograms) × 10
The number 10 is an indication of the size of the force of gravity. It would be too much of a coincidence for it to be
exactly 10. It turns out to be about 9.8, and because the force of gravity is less on the moon than on the Earth, this
number is smaller for the moon (it’s about 1.6).
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Energy, Work and Power
Energy
The unit of energy is also the joule.
Energy is the ability to do work
Work
The unit of work is the joule (J).
Work done = Force × distance
Power
Power is the rate at which work is done
The unit of power is the watt (W)
𝐏𝐨𝐰𝐞𝐫 =
𝐖𝐨𝐫𝐤
𝐓𝐢𝐦𝐞
Physics: 10. Centre of Gravity
Please remember to photocopy 4 pages onto one sheet by going A3→A4 and using back to back on the photocopier
OP8
Syllabus
Find the centre of gravity of a thin lamina
Investigate the role of centre of gravity in design for stability and equilibrium
Student Notes
The Centre of Gravity of an object is the point through all the weight of the object appears to act.
Experiment: To find the centre of gravity of a sheet of cardboard.
1. Set up as shown.
2. Hang the cardboard from a nail so that it
rotate freely.
3. Hang a weight from this point using cord so
that the weight hangs straight down.
4. Draw a line to represent the position of the
cord.
5. Rotate the sheet of cardboard and repeat.
6. The intersection of the two lines represents
the centre of gravity of the object.
7. Now repeat one more time to verify that this
line also goes through the same point.
Stability
An object can be made more stable by:
(i) giving it a low centre of gravity
(ii) giving it a wider base
An object will topple if a vertical line through its centre of gravity falls outside its base.
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Physics: 11. Moments
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Syllabus
OP9 Investigate the law of the lever; recall two everyday applications of levers
Student Notes
A lever is a rigid object which is free to turn about a fixed point called a fulcrum.
Everyday applications of levers
Seesaw, scissors, door handle, wheel-barrow, spanner, etc.
(Can you identify the fulcrum in each case?)
The Moment of a force = the force × distance (between the force and the fulcrum).
Example
A force of 75 N is used to turn the spanner in the diagram. If the distance between the force and the nut is 10
cm calculate the moment of the force.
Answer
Moment = Force x distance
= 75N × 0.1 m
=7.5 N m
What would be the effect of using a longer spanner?
The Law of the Lever:
When a lever is balanced, the clockwise moment acting on it is equal to the anti-clockwise moment
acting on it.
Experiment: To Verify the Law of the Lever
Procedure:
1. Hang a metre stick from the 50 cm mark as shown.
2. Attach two weights and adjust their positions until the metre stick balances.
3. Note the position and force of each object.
4. Show mathematically that the force multiplied the distance clockwise equals the force multiplied by the
distance anti-clockwise.
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Physics: 5. Pressure
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Syllabus
OP10 State the relationship between pressure, force and area; perform simple calculations using this
relationship
OP11 Investigate the relationship between pressure and depth for a liquid
OP12 Show that air has mass and occupies space
OP13 Associate change in the pressure exerted by the atmosphere with change in altitude
OP14 Examine weather charts to observe variations in atmospheric pressure and relate these to weather
conditions
Student Notes
𝐏𝐫𝐞𝐬𝐬𝐮𝐫𝐞 =
𝐅𝐨𝐫𝐜𝐞
𝐀𝐫𝐞𝐚
Pressure is caused by a force that pushes against an object.
The pressure increases when the force increases or when the area (that the force is acting on) decreases.
The unit of pressure is the pascal (Pa).
You can also use the N/m2 or N/cm2.
Example 1:
The wind exerts a horizontal force of 1000 N on a wall of area 20 m2. Calculate the pressure at the wall.
Answer: P = F ÷ A = 1000 ÷ 20 = 50 pascals.
Example 2:
A lady weighs 800 N and is standing in a pair of stiletto heels, each of area 2 cm2.
An elephant weighs 27000 N and the total area of its feet is 1800cm2.
Which exerts more pressure on the ground – the lady in stilettos or the elephant?
Answer
Total pressure exerted by the lady’s (two) feet = 800 ÷ 4 = 200 N/cm2
Total pressure exerted by the elephant’s (four) feet = 36000/1800 = 20 N/cm2
The lady exerts ten times more pressure on the ground than the elephant.
Pressure in Liquids: the pressure in a fluid increases with depth
Demonstration
Set up as shown.
The water coming out of the bottom hole travels the farthest, because it is
under the greatest pressure.
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Air has mass
Demonstrations
1. Can you explain what’s going on in this diagram?
2. Weigh an empty plastic bottle (with the top on) and note the
mass.
Pump air into it and weigh it again. Note the new mass.
Result: there was an increase in mass as a result of pumping air
in, therefore air has mass.
Air occupies space
Demonstrations
1. Can you explain what’s going on in this diagram?
2. Glue some cotton wool to the bottom of a glass (inside).
Turn the glass upside down, submerge it completely into a large bowl of water
and then remove it again.
Notice that the cotton wool is still dry.
Conclusion: the water could not enter the glass because there was air already in
there.
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Atmospheric Pressure
The atmosphere exerts pressure
The atmosphere is made up of giggling atoms and exerts a huge amount of pressure
on all objects, including the earth
Demonstration
Use a large metal drum and pump the air out using a vacuum pump.
The drum will collapse (implode) because there is now a greater pressure outside
acting inwards than there is inside acting outwards.
Atmospheric pressure varies with height
Atmospheric pressure decreases as the height above sea level increases (because
there is less air overhead).
A barometer measures atmospheric pressure
Atmospheric Pressure and the weather
Areas of high atmospheric pressure are represented on a
weather chart by the letter H and will normally have dry,
clear, settled weather.
Areas of low atmospheric pressure are represented on a
weather chart by the letter L and will normally have cloudy,
windy, wet weather.
Why is low atmospheric pressure associated with bad
weather?
Low atmospheric pressure means that it is easy for water to
evaporate into the air (because the ‘blanket’ of atmospheric
pressure pressing down on the water is lighter)
This results in more clouds and therefore more rain.
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Physics: 16. Velocity & Acceleration
Please remember to photocopy 4 pages onto one sheet by going A3→A4 and using back to back on the
photocopier
Questions to make you think
1. Imagine you are travelling in a car. You have a glass bottle in your hand. In which direction should you
throw it to minimise the danger of its breaking on hitting the ground?
2. Two friends are playing with a ball on board a ship moving at a steady speed. One is standing nearer the
aft and the other nearer the bows. Which one of them finds it easier to throw the ball to the other?
(Ignore wave and wind effects).
Syllabus
OP1 Perform simple calculations based on speed, velocity, and acceleration
Student Notes
Speed
Speed = distance ÷ by time
The unit of speed is the metre per second (m/s).
𝐒𝐩𝐞𝐞𝐝 =
𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞 𝐭𝐫𝐚𝐯𝐞𝐥𝐥𝐞𝐝
𝐭𝐢𝐦𝐞 𝐭𝐚𝐤𝐞𝐧
Velocity
Velocity is much the same as speed, except when we talk about velocity we usually include a direction, e.g.
the velocity of a car is 10 m/s due East.
Velocity is the same as speed in a given direction
The slope of a distance-time graph corresponds to the speed of the object
𝐬𝐥𝐨𝐩𝐞 𝐨𝐟 𝐚 𝐠𝐫𝐚𝐩𝐡 =
𝐲𝟐 − 𝐲𝟏
𝐱𝟐 − 𝐱𝟏
A mirror was left on the Moon by NASA Astronauts. It reflects lasers shone from Earth & tells us how far
the Moon is. How can we use this to work out the distance between the Earth and the moon?
See the clip about this on The Big Bang comedy series (search “mirror on the moon” on YouTube).
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Acceleration
Acceleration = velocity ÷ by time
The unit of acceleration is the metre per second squared (m/s2).
𝐀𝐜𝐜𝐞𝐥𝐞𝐫𝐚𝐭𝐢𝐨𝐧 =
𝐜𝐡𝐚𝐧𝐠𝐞 𝐢𝐧 𝐯𝐞𝐥𝐨𝐜𝐢𝐭𝐲
𝐭𝐢𝐦𝐞 𝐭𝐚𝐤𝐞𝐧
Consider an object which is speeding up as it is moving. Its velocity after one, two, three and four seconds is
given in the following table:
Time
Velocity
1 second 10 m/s
2 seconds 20 m/s
3 seconds 30 m/s
We can see that with every second that passes the velocity increases by 10 m/s.
Alternative ways of writing this are
 the velocity increases by 10 m/s per second,
 the velocity increases by or 10 (m/s)/s
 the velocity increases by 10 m/s2
Instead of saying that ‘the velocity increases by 10 m/s per second’, we simply say that ‘the acceleration is
10 m/s2’.
Problem: A sports car can go from 0 to 300 m/s in 10 seconds. Calculate the acceleration
Answer:
300 − 0
Acceleration =
= 10 m/s2
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The slope of a speed-time graph corresponds to the acceleration of the object
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