P3 - Specification

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  • Increasing the speed, increases the distance travelled in the same time
  • Increasing the speed, reduces the amount of time needed to cover the same distance.

Distance = speed x time

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Distance/Time Graph


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Speed-Time Graphs


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Speed-Time Graphs 2

  • A greater acceleration is shown by a higher gradient
  • A positive gradient = acceleration
  • A negative gradient = deaceleration
  • Distance = area underneath the line

Acceleration is the change in speed per unit of time:

  • increase in speed results from a positive acceleration
  • decrease in speed results from a negative acceleration or deaceleration

Acceleration = change in speed/time taken

Acceleration can involve either a change in:

  • speed
  • direction
  • both speed and direction
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Equal Velocities

Two objects moving in opposite directions but at the same speed, their velocities will have identical magnitude but opposite signs.

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Forces and Stopping Distance

force = mass x acceleration

Factors that may increase thinking distance:

  • driver tiredness
  • influence of alcohol or other drugs
  • greater speed
  • distractions or lack of concentration

Factors that increase breaking distance:

  • road conditions
  • car conditions
  • greater speed
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Stopping Distance 2

Braking distance is increased if:

  • the mass of the vehicle is increased - a loaded vehicle has greater kinetic energy
  • the friction between tires and road is decreased (wet or greasy road surface)
  • the braking force is decreased
  • the vehicle is travelling faster - more kinetic energy

Road safety regulations affected by stopping distance:

  • driving too close to the car infront (inside thinking distance)
  • speed limits
  • road conditions
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Stopping Distance 3

Thinking distance - increases linearly (double speed = double the distance travelled whilst reacting)

Braking distance - increases in a squared relationship (double speed = quadruple the braking distance, triple speed = multiply the braking distance by 9)

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Work and Power

weight = gravitational field strength

work done = force x distance

power = work done/time

power = force x speed

Fuel consumption has:

  • environmental issues
  • different costs
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Kinetic Energy

KE = 0.5 x m x v2

A car with more kinetic energy will have a longer braking distance.

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Fuels for Vehicles

Battery Powered Cars


  • They do not pollute any greenhouse gases at the time of use
  • Fuel (electricity) is cheaper


  • cause pollution indirectly through the production of electricity in power stations
  • have to be recharged regularly

We may have to rely on biofuelled and solar powered vehicles in the future because resources of non-renewable fuels are running out.

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Fuels for Vehicles 2

Bio-fuelled and solar powered vehicles:

  • reduce pollution at point of use
  • produce pollution in their production
  • may lead to an overall reduction in carbon dioxide emissions

Car fuel consumption figues depend on:

  • energy required to increase the kinetic energy
  • energy required to work against friction
  • driving style and speeds
  • road conditions
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Car Safety

momentum = mass x velocity

The greater the mass of the object and/or greater the velocity, the more momentum the object has in the direction of motion.

force = change in momentum/time

Newton's second law of motion:

F = ma (Force = momentum x acceleration)

If the collision stopping time is increased, the rate of acceleration is decreased so the force is decreased.

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Car Safety 2

Spreading the change in momentum over a longer time reduces the liklihood of injury. This is because the force experienced during a collision depends upon the rate of changes of momentum. The quicker the change in momentum, the greater the force experienced.

Forces can be reduced when stopping (crumple zones, braking distances, escape lanes, crash barriers, seatbelts and airbags) by:

  • increasing the collision time
  • increasing the stopping or collision distance
  • decreasing acceleration
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Car Safety 3

Seatbelts, crumple zones and airbags are useful in a crash because they:

  • change shape
  • absorb energy
  • reduce injuries

ABS Brakes:

  • make it possible to keep control of the steering of a vehicle in hazardous situation (eg when braking heard or going into a skid)
  • work by the brakes automatically pumping on and off to avoid skidding
  • sometimes reduce braking distances
  • friction between the two surfaces is increased, so the braking distance is reduced and the car is able to stop more quickly.
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Falling Forces

Forces reach a terminal speed because:

  • higher speed = more drag
  • larger area = more drag
  • weight (falling object) or driving force (eg a car) = drag when travelling at terminal speed.

Acceleration due to gravity is the same for any object at a given point on the Earth's surface.

Gravitational field strength or acceleration due gravity:

  • is unaffected by atmospheric changes
  • varies slightly at different points on the Earth's surface
  • will be slightly different on top of a mountain or down a mind shaft
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GPE and Parachute Jumps

GPE = mgh

A body falling through the atmosphere at terminal speed:

  • kinetic energy does not increase
  • gravitational potential energy is transferred to increased internal or thermal energy of the surrounding air particles through the mechanism of friction.
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The energy of Games and Theme Rides

On a theme ride, KE is transferred into GPE.

  • Doubling the mass of the object = doubles KE
  • Doubling speed = quadruples KE

Remember KE = GPE

KE = 0.5 x mv2

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