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Summary GCSE Physics (AQA) revision notes for forces, energy and motion topic
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Terminal velocity Newtons third lawWhen an object moves through a fluid it Forces come in equal and opposite pairs.
continually collides with the particles of
They must be the same type of force ((non)contact)
the fluid. The overall effect of the tiny
collisions is a large force acting opposite Example: apple falling to the ground
to the way it is moving. This is called
drag or air resistance. In this situation there are 2 newton third law pairs of forces
The drag force increases the faster it 1. The apple exerts force on particles in the air and the
goes because as the object accelerates it particles exert an equal and opposite force on the apple
speeds up and collides with more (drag) – both contact
particles in the same time 2. The apple also exerts a force on the earth (weight) and the
earth exerts an equal and opposite force on the apple –
If you go fast enough the drag force both non contact
becomes as big as the driving force
which means the resultant force is 0N Momentum
and the speed remains constant. This is
How hard it is to stop something
terminal velocity
Momentum (kgm/s) = mass (kg) x velocity (m/s)
The principle of conservation of momentum
Weight stays the same
Resultant force decreases The total momentum before an event is equal to the total
because drag increases due
to acceleration increasing
momentum afterwards, provided no external forces act
When an external force does act
Then the momentum of an object does change. The larger the
force, the shorter the time for the momentum to change. This is
important in safety features and sport
Eg in a car crash a person’s momentum needs to change as
gradually as possible: the longer the impact time, the less force on
the person
Crumple zones in cars allow the impact time to increase and the
Drag force decreases because he is
decelerating until it equals the force felt by the person to decrease
weight force and a second, slower
terminal velocity is reached
Mass and weight
Mass is the amount of ‘stuff’ in an object, the particles that it is
made from and is measured in kg
Weight is the force on an object due to gravity and is measured in
N/kg 1kg = 10N
Weight (N) = mass (kg) x gravitational field strength (N/kg)
Newton’s second law
Ticker tape
Frequency = 50Hz = 50
Force (N) = mass (kg) x acceleration (m/s 2)
dots per second so 2
dots are 0.02s apart Force is directly proportional to acceleration
2mm 0 ⋅2 cm 0.002m Mass is inversely proportional to acceleration
initial= = = =0 ⋅1
0.02 0 ⋅02❑ 0.02 2.75−0 ⋅ 1 2
Δv= =4 ⋅416 m =∕ the
Force s rate of change of momentum =
0.6 Δρ
55 mm 5 ⋅5 cm 0.055 m
f inal= = = =2.75 t
0.02 0.02 0.02
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