edexcel P2 topic 4

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  • Created on: 06-12-12 19:34
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TOPIC 4 ­ momentum, energy, work and power
Stopping distance is the time it takes a car to stop and it depends on
two factors:
1. Thinking distance ­ distance travelled in between the driver
noticing the hazard and applying the brakes. Factors that
affects thinking distance are:
Greater speed
Tiredness
Drugs and alcohol
Distraction
They all increase the thinking distance.
2. Braking distance ­ the distance taken to stop once the brakes
are applied. Factors that affect braking distance are:
Greater speed
Poor road conditions (icy, wet roads affect friction)
Car conditions (tyres, brakes, mass of vehicle)
They all increase the braking distance.
The stopping distance is much further for faster cars therefore the
driver should:
Keep their distance from the car in front
Keep to a speed limit
Stopping distance = thinking distance + braking distance
Friction
There is an interaction pair of friction forces: A backward and
forward force.
E.g. the friction between foot and floor exerts a backward force on
the floor. The other force of the interaction pair is the floor pushing

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Page 2

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The result is that you move forward but the floor
stays still.
Common experiment is to show change in friction with different
surfaces.
The greater the forces needed to pull the block, the higher the
friction.
The smaller the friction between block and surface, the smaller the
force needed.
Momentum
A moving object has momentum therefore the object has the
tendency to keep moving in the same direction. It is difficult to
change the direction if the object has a lot of momentum.…read more

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Momentum of bullet = mass × velocity
= 0.03 kg × 1,000 m/s = 30 kg m/s
Rearrange the equation: velocity = momentum ÷ mass
Velocity of recoil on gun = 30 kg m/s ÷ 1.5 kg = 20 m/s
Car safety
In a car crash, the great forces caused by the change in momentum
(when a force acts on an object) can cause injury.…read more

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Whenever work is done energy is transferred from one place to
another. Everyday examples include walking up the stairs or lifting
heavy objects.
Both energy and work are measured in joules.
Work done (joules, J) = energy transferred
(joules, J)
Equation:
Work done (joules, J) = force (newton, N) x distance (metre, m)
Power is a measure of how quickly work is being done therefore
how quickly energy is being transferred.…read more

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When a car brakes, the kinetic energy is converted to heat energy
at the brakes and tyres:
Work done by brakes = initial kinetic energy (or loss)
F x d = ½ x m x v²
If the speed of the car is doubled, then the kinetic energy and
braking distance is increases by a factor of four.
Gravitational potential energy
Gravitational potential energy is the energy stored (potential) in an
object when it is above the earth's surface.…read more

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Energy is never created nor destroyed but only transferred from
one form to another.
Energy is only useful when it's transferred from one form to
another.
The pendulum: gravitational potential energy is converted to kinetic
energy and back.
At the start the bob has no KE as it's not
moving however its GPE is at a maximum as its
high.…read more

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