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Forces
Newton’s Laws:
Law of Inertia states that an object will not move until
a force is exerted
Law of constant acceleration- if a force keeps acting
on the moving objects then it will accelerate
Conservation of Movement- For every action there is
an equal and opposite reaction
What Is A Force?
• A force is a push or a pull
• You have to push or pull doors to open
them
• Which do you do when you jump up?
• Your legs push off the ground
• Which do you do when you climb a rope?
• You pull yourself up
What Units Are Forces Measured
In?
• The are measured in Newtons (N)
• These units are named after
Sir Isaac Newton
• Weight is measured in Newtons
• There are 10 N in every 1 Kg
• If my mass is 80 Kg how much do I weigh?
• 80 x 10 = 800 N
What Can Forces Do?
1. They can change the speed of an object
2. They can change the direction an object is
moving
3. They can change an objects shape
• Which of these happens when you stamp on a
coke can?
• Change it’s shape
• Which of these happens when you flick an
elastic band?
• You change it’s speed and it’s shape
Measuring Forces
• Forces are measured using Newton
meters (sometimes called Force meters)
• These have springs inside them
• This means that they can break if too
much weight is put on them
Mass Or Weight
• Mass is how much matter we are made of
• Your mass is the same everywhere you go
• Weight is a mass being pulled by gravity
towards the Earth
• You are weightless is space
• You would be six times lighter on the moon
• You would be 10 times heavier on Jupiter
• In all of these places you would have the same
mass
Mass or Weight
• What is the astronauts mass
on Earth?
• 120Kg
• His mass on the moon?
• 120 Kg
• His weight on Earth?
• 1,200 N
• His weight on the moon?
• 200 N
• Why does he weigh less on
the moon?
• It is six times smaller than
the Earth, it has less gravity
Gravity
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This is when objects are pulled towards large objects
What does the Earth’s gravity do to us?
It pulls us towards the Earth
Why does the Earth go around the Sun?
The Sun’s gravity keeps us in orbit
Which would have more gravity; the Earth or the Sun?
The Sun because it is much bigger
The escape velocity of a rocket leaving the Earth is 11.2
km/sec (40,000 km/hour)
Galileo
• Legend has it that Galileo
took a canon ball and a
wooden ball the same
size to the top of this
tower
• He dropped them at the
same time
• Which hit the ground first?
• They both hit the ground
at the same time!
Leaning Tower of Pisa
Friction
• This happens when two surfaces rub against
each other
• It creates heat
• It slows things down
• How do bike brakes stop a bike?
• The brake pad presses against the wheel until
friction slows it till it’s stopped. The tyre of the
wheel then rubs against the ground slowing
the bike down, until friction stops it
Friction
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Why is friction useful to:
Us when we are running?
Friction allows us to stop
The brakes in a car?
It allows the car to slow down or stop
Why is friction a nuisance if:
You fall on gravel?
You get burns or grazes
If you don’t oil a bike chain?
The chain will stop moving
Reducing Friction
• Engine parts and bike chains are oiled
• This is called lubrication
• This prevents wear and
tear
• It saves money because
the engine does not have
to work as hard
Reducing Friction
• Ball bearings
can be used
(e.g. in gear
boxes)
• Swimmers attempting
long swims cover
themselves in grease
Upthrust
• This is the force that pushes things up
when they are in water
• It is a weak force
• Polystyrene is full of air holes, this
means it is light and can be pushed to the
surface of a swimming pool
• Why do steel ships float?
• They are hollow (full of air) and have a
large surface area for upthrust to push up
Thrust and Air Resistance
• Thrust if a forward force
• Air Resistance (drag) is when air
particles slow things down
• When would air resistance be a
nuisance?
• It slows down cars, aeroplanes, animals
running etc.
• When would it be useful?
• Parachutes
This Ferrari GG50 is streamlined to make it go
faster
Streamlining Reduces Drag
This bottlenose dolphin is streamlined to
reduce drag in water
Force Diagrams
• Arrows show the direction of the force
• Their thickness illustrates the size of the
force
• These two forces are balanced, the book is not
moving
Force Diagrams
400N
500N
• Name the force that
slows the car down?
• Friction (and drag)
• What should happen
to the car?
• It will slow down
Force Diagrams
•What will happen to this truck?
•It will stay still
•What will happen to this truck?
•It will move forward
Gravity
1
Gravity
Upthrust
They are equal
0
Gravity
5
Gravity
Upthrust
A is bigger than b
4