Physics P456 revision

Physics P456 revision

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Physics P4
A: Forces in all directions
Forces are needed to change the motion of an object, and are the pushes and pulls
experienced by an object when it interacts with another.
Forces can be represented by an arrow.
Forces arise from interactions between two objects.
Forces always come in pairs, called interaction pairs, that are:
1. Equal in size
2. Opposite in direction
3. Have two objects that act on different objects
Contact forces arise from when two objects touch one another.
Some forces act from a distance, for example gravity.
B: How things start moving
Rockets use the force from burning hot gases to move.
Jet engines work similarly to rockets, but with air instead of burning hot gases.
Cars need to grip the surface to exert a force on it, thus making an equal force forwards.

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When you walk, your feet exert a backward force on the surface to move you forward,
so it is more difficult to walk on icy surfaces.…read more

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C: Friction
Friction is the force exerted on an object due to the interaction between it and
another object that is sliding over it that is caused by the roughness of both surfaces.
Friction adjusts its size in response to the situation up to a limit.
The limit depends on the objects and surfaces involved.…read more

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E: Adding forces
If there is a force on an object, but the object is not moving, there must be another force
cancelling it out.
If forces balance each other, they are said to add to make zero.
Resultant force is the sum of all the forces acting on an object, considering their
directions.…read more

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Truvelo speed cameras use detector cables in the road at a particular distance apart,
which sense when a car is driving over them and uses computers to calculate the vehicles
Police radar guns bounce microwaves off the approaching cars, and use the change in
frequency between the reflected and original waves to calculate the cars instantaneous
G: Picturing motion
Distance-time graphs show how far an object is from its starting point at every
moment of the journey.…read more

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Tachographs are installed in lorries to monitor the drivers speed and time spent
driving before having a break.
Tachograph traces are speed-time graphs.
Velocity-time graphs are similar to speed-time graphs, but also show the
direction an object is travelling.
A negative velocity means the object is travelling in the opposite direction.…read more

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Crumple zones increase the time for which the force of the collision acts, reducing the
force exerted on the car.
Seat belts and air bags work similarly to crumple zones, increasing the time taken for the
change in momentum.
It is not possible to save yourself using your arms or legs during a vehicle collision,
because they happen to quickly for our reactions to work, and the force needed to be
exerted to change your speed is larger than its possible for us to exert.…read more

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Gravitational potential energy is the energy an object depending on its
height above the ground, and is defined by the equation:
o GPE (J) = weight (N) x vertical height difference (m)
Kinetic energy is the amount of energy an object has due to its motion, and is
defined by the equations:
o KE (J) = work done (J)
o KE (J) = ½ x mass (kg) x (velocity)2 (m/s)2
Conservation of energy is the principle that the total amount of energy at the
beginning…read more

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Physics P5
A: Static electricity
Static electricity is an electric charge that has built up on an object.
Plastic can become charged by rubbing it against an object.
Two charges the same repel.
Two different forces attract.
Electric charge is the overall charge that occurs when electrons are added or lost.
There are two types of charge; positive and negative.
Charge is not made when two objects are rubbed together, but moved around.…read more

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Electric fields are regions around an electric charge where the effects of the
charge can be felt, causing a force.
Electric current is the flow of electric charges around a circuit.…read more


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