P4 -- Explaining Motion

Revision Cards for P4 Module of OCR 21st Century Physics A Unit 2.

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  • Created by: Alex
  • Created on: 07-05-10 21:26


  • Speed = Distance/Time
  • EXAMPLE: A cat skulks 20 metres in 40 seconds. Find: a) its speed, b) how long it will take to skulk 75m.
  • a) speed = 20/40 = 0.5ms^-1
  • b) time = 75m/0.5ms^-1 = 150s
  • Pretty rare in real life for an object to stay at exactly the same speed for a long period of time.
  • Usually want to find the average speed if the speed varies constantly for a long period of time.
  • Speed cameras take an instantaneous speed of a car: Evenly spaced lines are printed on the road and the speed camera measures the time in which the car travels over the lines.
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  • On a distance time graph:
    • Gradient = speed
    • Flat sections = stationary
    • Steep gradient = faster speed
    • 'Downhill' = travelling in opposite direction
    • Curves = Acceleration/deceleration
    • Steepening curve = speeding up
    • Levelling off curve = slowing down
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  • Gradient = Δy/Δx
  • Always use standard units: m, kg, l etc.
  • Distances can be positive or negative.
  • 0 is always start point, +ve = one direction, --ve = other direction.
  • Speed is the gradient of a distance/time graph.
  • Speed = how fast something is going, it does not have a direction .
  • Velociy describes the speed and direction of an object.
  • Speed = scalar quantity (mass, temperature, time, length).
  • Velocity = vector quantity (force, acceleration, momentum).
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  • Velocity can be positive or negative.
  • Travel one direction at 20ms^-1, turn around and travel in the opposite direction at -20ms^-1.
  • If two objects heading in opposite directions, one can be said to have positive veolcity while the other have a negative velocity.
  • On a velocity/time graph:
    • Gradient = acceleration
    • Flat sections = steady speed
    • Steeper sections = greater acceleration/deceleration
    • 'Uphill' = acceleration
    • 'Downhill' = deceleration.
    • Area under any section (or all of) graph is equal to the distance travelled in that time interval.
    • Curve = changing accelerations
  • Tachographs plot speed/time when direction isn't important.
  • Tachographs are found in lorries to tell managers how long a driver has gone without a break or if they have been speeding.
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  • Forces occur when two objects interact
  • When an object exerts a force on another subject, it interacts with an opposing force: an 'interaction pair'.
    • If you push against a wall, the wall will push back just as hard.
    • As soon as you stop pushingthe wall, so does the wall.
    • If there was no opposing force, you and the wall would fall down.

    • If you exert a force of 10N, the wall's results force will be 10N.
  • NEWTON'S THIRD LAW!: If object A exerts a force on object B, then object B exerts anequal and opposite force on object A.

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  • Moving object usually experience friction.
  • When an object is moving relative to another, both objects experience a force in the direction that opposes the movement -- FRICTION!
    • Friction between solid surfaces which are gripping (static).
      • The Earth's tectonic plates trying to move but friction is so strong they stay put.
    • Friction between solid surfaces which are sliding past each other.
      • Eg. moving bits of a car.
      • Reduce friction by using lubricant.
    • Resistance or "drag" from fluids (liquids or gases).
      • An object has to force its way past the molecules of the fluid.
      • Big squarish objects carry more (air) resistance than a streamlines object.
  • Friction only occurs if an object is moving through a fluid.
    • Space has no fluids (as it's a vacuum) therefore there is no friction is space.
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  • Arrows show the size and direction of forces.
  • Length of arrow shows the size of force; direction shows the direction of force.
  • If opposite pairs (same direction AND size), the forces are balanced.
  • If an object is resting on a surface its weight is pushing down (gravity), causing an equal reaction forcepushing up from the surface; the two forces are the same size so the arrows are the same size.
  • If an object is moving with a steady speed the forces must be in balance. Just because something is moving doesn't mean there is an overall force acting on it -- unless it's changing speed or direction, the overall force is zero.
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  • The resultant force is the overall force acting on an object.
  • This is the force when you take into account all individual forces and direction.
  • Forces decides motion of the object (accelerate, decelerate or stay at a steady speed).
  • 'Accelerate' means to change velocity. As velocity has speed AND direction accelerating doesn't always mean changing speed -- it could be changing direction, even if you stay at a steady speed.
  • If there is a resultant force, its speed or direction (or both) changes.
  • Unless there is an overall force on something it won't accelerate.
  • Acceleration = unbalanced forces.
  • If a larger force is exerted forward than backwards (DRAG!), it will not accelerate.
  • If there is a bigger thrust arrow (forward) than the drag arrow (backward), there is a resultant force in the forward direction.
  • The bigger the resultant force, the greater the acceleration.
  • There are still forces in the other directions.
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  • Momentum = mass x velocity (kg.ms^-1 = kg x ms^-1).
  • Momentum is how hard it would be to stop an object moving.
  • Heavy, fast objects would have a larger momentum than light, slow objects.
  • Momentum is a vector quantity -- it has size and direction.
  • A resultant force of zero means that a stationary object will stay still, if an object was moving it would stay at the same speed in the same direction.
  • If the resultant force of an object changes, its momentum changes in the direction of the force.
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  • Change of momentum (kg.ms^-1) = Resultant force (N) X Time for which the force acts (s).
  • A resultant force causes a change of momentum.
  • The change it causes depends on the size of the force and the time in which it acts for.
  • EXAMPLE: A rock with mass 1kg is travelling through space at 15ms^-1. A comet hits the rock, giving it a resultant force of 2500N for 0.7 seconds. Calculate the rock's initial momentum, then calculate the change of momentum resulting from the impact of the comet.
    • Momentum = mass X velocity. 1kg X 15ms^-1 = 15kg.ms^-1.
    • Change of momentum = 2500N X 0.7s = 1750kg.ms^-1.
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  • If somebody's momentum changes very quickly, the forces on the body will be very large and more likely to cause injury,
  • Car safety reduces forces at change of momentum.
  • You cannot affect the change of momentum in a collision. The average force of an object can be lowered by slowing the object down over a longer time period.
  • Safety features increase collision time to reduce forces:
    • CRUMPLE ZONES crumple to increase time for the car to stop,
    • AIRBAGS slow the passenger down gradually.
    • SEAT BELTS stretch (slightly) increasing time to stop and reducing forces on chest.
    • HELMETS provide padding that increase the time for the head to stop if it hits something hard.
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  • "Work Done" is "Energy Transferred".
  • When a force moves an object, energy is transferred and work is done.
  • Whether the energy is transferred "usefully" or is "wasted", you can still say that work is done.
  • Change in energy (J) = Work done (J).
  • Work Done (J) = Force (N) X Distance (m).
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  • Kinetic energy = energy of movement,
  • Greater the mass and faster it is going, the bigger the kinetic energy.
  • Kinetic Energy (J) = 1/2 X mass (kg) X velocity^2 ((ms^-1)^2).
  • EXAMPLE: A car of mass 2450kg is travelling at 38ms^-1. Calculate the kinetic energy.
    • KE = mv^-2/2 = 2450kg X 38(ms^-1)^2/2 = 1768900J.
  • To increase kinetic energy, you have to increase its speed (you must apply a force).
  • If you apply a force, you are doing work, If object A is causing object B's velocity to increase by exerting a force on it, then it is doing work and increasing object B's kinetic energy.
  • If you do work on an object but it doesn't accelerate, you have not increased its kinetic energy.
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  • Increase in kinetic energy = work done.
  • Energy is always conserved (it cannot be created or destroyed), it gets transferred from one form of energy to another.
  • Some energy that gets transformed is wasted as heat due to friction and air resistance.
    • 30J of work hitting a stationary object will be a bit less because of the heat created by air resistance.
  • The increase in an object's kinetic energy is normally a bit less that the amount of work done on it.
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  • Gravitational potential energy is basically 'height energy'.
  • Gravitational potential energy is the energy stored in an object when you raise it to a height against the force of gravity (a way of storing kinetic energy).
  • The energy is only released when an object falls.
  • Change in gravitational potential energy (J) = weight (N) X change in height (m).
  • Falling objects convert gravitational potential energy into kinetic energy.
    • The further an object falls, the faster it falls.
  • Some gravitational potential energy is dissipated as heat due to air resistance.
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  • Kinetic energy gained = gravitational potential energy lost
    • On a rollercoaster, if you ignore air resistance and friction, the kinetic energy that the carriages gain will be the same as the gravitational potential energy that is lost.
    • EXAMPLE: The carriage on a rollercoaster has a weight of 5000N and the vertical height difference between point A and B is 20m. a) Ignoring friction and air resistance, how much kinetic energy is gained by the carriage moving from A to B. b) Assuming the rollercoaster was stationary at point A, calculate its speed at point B.
      • a) Gain in kinetic energy = loss in gravitational potential energy. Weight X change in height = 5000N X 20m = 100000J.
      • b) At B, it has 100000J of kinetic energy. As kinetic energy = 1/2 X mass (kg) X velocity^2 ((ms^-1)^2), 1/2 X mass (kg) X velocity^2 ((ms^-1)^2) = 100000J. velocity^2 = 100000 X 2/500kg = 400ms^-1. velocity = sqaure root of 400 = 20ms^-1.
    • Rollercoasters constantly transfer energy from gravitational potential energy to kinetic energy and back again.
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