P4 - Explaining Motion

Revision cards following the OCR 21st Century Science Board. Module P4 - Explaining Motion. 

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  • Created by: Hallie23
  • Created on: 29-03-13 11:41

Distance-Time Graphs

  • Gradient = Speed
  • Flat sections are where it's stationary.
  • The steeper the gradient, the faster it's going.
  • 'Downhill sections mean it's coming back towards its starting point.
  • Curves represent acceleration or deceleration.
  • A steeping curve means it's accelerating (increasing gradient).
  • A levelling-off curve means it's decelerating (decreasing gradient).
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Displacement-Time Graphs

  • When distances are referred to as either positive or negative, it means that an object can be going in one direction or in the opposite direction.
  • The displacement of something is its distance in a given direction, from its starting point, at any particular moment in time.
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  • The speed of an object is just how fast it's going - the direction isn't important.
  • Velocity is sometimes a more useful measure of motion, because it describes both the speed and direction.
  • Instantaneous velocity is its speed and direction at a given moment in time.
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Acceleration and Deceleration

  • Acceleration is the change in velocity(or speed) in a certain amount of time.
  • Deceleration is negative acceleration.
  • Acceleration (m/s squared) = change in velocity (m/s) / Time taken (s).
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Velocity-Time Graphs

  • Gradient = acceleration.
  • Flat sections represent moving in a straight line at a constant speed.
  • The steeper the gradient, the greater the acceleration or deceleration.
  • Uphill sections are acceleration in a straight line.
  • Downhill sections are deceleration in a straight line.
  • The area under any section of the graph is equal to the displacement travelled in that time interval.
  • A curve means changing acceleration.
  • Negative velocity means that the object is travelling in the opposite direction.
  • Speed-time graphs are similar to velocity-time graphs but ignore direction.
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  • Force is measured in Newtons.
  • A force is a push or a pull.
  • Forces come with a force in the opposite direction too.
  • When an object exerts a force on another object, it always experiences a force in return. These are called 'partner forces' or 'interaction pairs'.
  • An object resting on a surface experiences a reaction force
  • Moving object usually experience friction.
  • Arrows show the size and direction of forces.
  • Resultant forces decide the direction of the motion of the object.
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Forces and Momentum

  • If something accelerates or decelerates, the forces are unbalanced.
  • Momentum = Mass x Velocity.
  • The heavier the object is, and the faster it's moving, the harder it is to stop.
  • The greater the mass of an object, or the greater its velocity, the more momentum the object has.
  • Momentum (kg m/s) = Mass (kg) x Velocity (m/s).
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Change in momentum

  • The faster the change in momentum, the greater the chance of injury.
  • The change in momentum depends of the force.
  • Change in momentum (kg m/s) = Resulatant force (N) x Time for which the force acts (s).
  • The bigger the force, the longer it acts for, the bigger the change in momentum.
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Car Safety

  • Force = change in momentum / time
  • The grater the time for a chnage in momentum, the smaller the force.
  • The smaller the force, the less likely there will be an injury.
  • Safety features in a car increase collision time to reduce the forces on the passengers.
  • Crumple zones - crumple on impact, increasing the time taken for the car to stop.
  • Air bags - slow you down more gradually.
  • Seat belts - stretch slightly, increasing the time taken for the wearer to stop. This reduces the forces acting on the chest. 
  • Cycle and motorcycle helmets - provide padding that inceases the time taken for your head to come to a stop if it hits something hard.
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  • Work done means energy transferred.
  • When a force moves an object it does work and energy is transferred to the object.
  • Whenever something moves, something else is providing some sort of effort to move it and the 'thing' putting the effort in needs a supply of energy (fuel, food, electricity etc).
  • It does work by moving the object and transfers the energy it receives (as fuel) into other forms.
  • Whether energy is transferred usefully or is wasted, you cans till say the work is done.
  • Amount of energy transferred (J) = Work done (J)
  • If energy is transferred, the object doing the work loses energy.
  • Work done by a force (J) = Force (N) x Distance moved in direction of force (m)
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Kinetic Energy

  • Kinetic energy is energy of movement.
  • The Kinetic energy of something depends on its mass and speed - the greater the mass, the faster it's going, the bigger its kinetic energy.
  • Kinetic Energy (J) = 0.5 x mass (kg) x velocity squared ([m/s]squared)
  • To increase somethings kinetic energy, you need to increase it's velocity.
  • If you do work on an object but it doesn't accelerate, then you havn't increased its kinetic energy.
  • Energy is always conserved which means you can't create or destroy energy. Energy just gets transformed from one kind of energy to another. 
  • The increase in an objects Kinetic energy is normally a bit less than the amount of work done on it because some energy is wasted as heat - unless there is no friction or air resistance acting on an object.
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Gravitational Potential Energy

  • Gravitational potential energy is height energy.
  • Gravitational potential energy is the energy stored in an object when you raise it to a height against the force of gravity.
  • If you lift and object, its gravitational potential energy increases as it's raised.
  • As an object falls, it's gravitational potential energy decreases.
  • You increase gravitational potential energy by doing work.
  • The increase in gravitational potential energy is equal to the work done by the lifting force in order to raise its height. 
  • Change in G.P.E. (J) = Weight (N) x Vertical height difference (m)
  • When something falls, its gravitational potential energy is converted into kinetic energy.
  • Kinetic energy gained is gravitational potential energy lost.
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