# P2a

Summary of everything in unit P2a:

Motion and forces

Static electricity

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## Distance-time graphs

• A distance-time graph shows the distance an object moves in period of time.
• To work out the speed of the object from you graph you look at the gradient and do:

speed = change in distance/change in time

• The steeper the gradient, the faster the object is moving.
• If the graph is curved, this shows speed changing gradually.
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## Velocity, acceleration and direction

• The velocity of an object is its speed in a given direction.
• A change in velocity (speeding up or slowing down) is called acceleration or deceleration.
• The calculation for working out acceleration is:

acceleration = change in velocity/ time taken for change

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## Velocity-time graphs

• A velocity-time graph shows how the velocity of an object changes with time.
• The gradient of a velocity-time graph represents acceleration.
• The area under the line shows distance traveled.
• Terminal velocity:
• An object falling through the air eventually reaches a top speed, its terminal velocity.
• At this speed, the force of weight downwards balances the force of air upwards.
• At this point the resultant force is 0.
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## Forces

• For every force, there is an equal and opposite force.
• Balanced forces have the same value but act in opposite directions.
• An object will remain stationary or move at a constant speed unless an external force acts upon it.
• To calculate force, the equation is:

Force = Mass x Acceleration

• The smaller the mass, the bigger the acceleration.
• The bigger the force, the bigger the acceleration.
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## Stopping Distance

• If a driver has to stop suddenly, the stopping distance depends on the distance the car travels during the drivers reaction time (thinking distance) and then while the brakes are applied (braking distance).
• The calculation for stopping distance is:

stopping distance = thinking distance x braking distance

• The calculation for thinking distance is:

thinking distance = speed x reaction time

• The greater the speed, the greater the stopping distance.
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## Work, Energy and Force

• The work done is the energy required to move an object.
• The equation for this is:

work done (J) = force (N) x distance moved (m)

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## Transferring and Transforming Energy

• Kinetic energy is the energy of movement.
• The kinetic energy of an object increases when its mass/speed increases.
• The kinetic energy of an object decreases when its mass/speed decreases.
• If an object hits another object and stops, the kinetic energy is transferred, but if the object does not quite stop, then only some of the energy is transferred.
• In an experiment using a car rolling down a ramp, the car would have potential energy at the top of the ramp which would then be transformed into kinetic energy as it started to move.
• The equation for calculation kinetic energy is:

kinetic energy = 1/2mv  (mass, velocity)

• The equation for calculating potential energy is:

potential energy = mgh  (mass, gravity, height)

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## Momentum

• Objects have momentum when they move.
• The calculation for working out momentum is:

momentum = mass x velocity

• Momentum is not the same as kinetic energy because momentum is a vector quantity meaning it has a direction as well as a size but kinetic energy is a scalar quantity meaning it only has a size.
• A force can change an objects momentum by changing its velocity:

change in momentum = mass x change in velocity

• A force can also change an objects momentum by changing its direction.
• A change in momentum depends on the size of the force and the time over which it acts. The force is given by:

force = change in momentum/time

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## Static electricity

• When two materials are rubbed together, negatively charged electrons are transferred from one to the other making both materials electrically charged.
• If an object gains electrons, it has a negative charge and if it loses electrons it has a positive charge.
• Their charges are equal and opposite static electrical charges.
• Like charges repel, unlike charges attract.
• Insulators (eg. plastic, glass) hold electrical charge.
• Electrical conductors (eg. metals, water) allow charge to flow as an electrical current in a closed circuit.
• Static electricity can be dangerous if sparks cause flammable materials to ignite of explode.Earthing prevents a build up of static charge.
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