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Slide 1

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Forces and Acceleration
Forces in Motion Topic 2…read more

Slide 2

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Mass, Weight and Gravity
· Gravity is the force of attraction between all masses.
Weight and mass are not the same!
Mass Weight
· The amount in an object. · Caused by the pull of
· For any given object this gravity.
will have the same value · The weight of an object
anywhere in the Universe. will be different anywhere
· Mass is not a force. else than on the Earth.
· It is measured in · Weight is a force.
kilograms (kg). · It is measured in newtons
(N).…read more

Slide 3

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The Three Laws of Motion
1. Balanced Forces Mean No Change in Velocity
If the forces on an object are balanced, it's either
stationary or moving at a constant speed.
2. When a train or car or bus or anything else is
moving at a constant velocity then the forces on
it must all be balanced.
3. To keep at a steady speed, there must be zero
resultant force.…read more

Slide 4

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The Three Laws of Motion
2. A Resultant Force Means Acceleration
If an object has a resultant force acting on it, it accelerates in the direction of
the force. The acceleration is proportional to the force and inversely
proportional to the mass.
· An unbalanced force will always produce acceleration (or deceleration).
· This "acceleration" can take five different forms: starting, stopping, speeding
up, slowing down and changing direction.
· On a force diagram the arrows will be unequal.
a) The bigger the force, the greater the acceleration or deceleration.
b) The bigger the mass, the smaller the acceleration.
c) To get a big mass to accelerate fast, a bigger force is needed.
Resultant force = mass x acceleration
N = kg x m/s…read more

Slide 5

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The Three Laws of Motion
3. Reaction Forces
For every action there is an equal opposite reaction.
· For example, if you push a shopping trolley, the trolley
will push back against you, just as hard. So if you stop
pushing so will the trolley.
· Skater A at 55g pushes on skater B at 65kg (the `action'
force), she feels an equal and opposite force from skater
B's hand (the `reaction' force). Both skaters feel the
same sized force, in opposite directions, and so
accelerate away from each other. Skater A will be
accelerated more than skater B as she has a smaller
mass.…read more

Slide 6

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Stopping Distances
Thinking Distance Breaking Distance
· How far you travel · How far you travel
whilst you react. whilst you break.
· Thinking distance is · Breaking distance is
affected by: affected by:
Drugs Road surface
Alcohol (oil/water/gravel)
Tiredness Condition of tyres
Caffeine Condition of breaks
Visibility (Eyesight) Travelling up/down a
Distractions (Phones) hill
SPEED Number of
passengers (i.e. how
Total Stopping Distance heavily loaded the
= vehicle is)
Thinking Distance
Breaking Distance…read more

Slide 7

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