P4: Explaining Motion

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How can we use energy changes to describe motion?

  • When you push an object and make it move,you are doing work.
  • Work done by a force (J)  = Force (N) x Distance moved by force (m) 
  • You transfer energy to the object, your chemical energy store decreases and the energy of the object increases.
  • Amount of energy transferred(J) = Work done(J) 
  • The moving object has kinetic energy.
  • Kinetic energy(J) = 1/2 x Mass(Kg) x (Velocity)2 (m/s)2
  • The faster the object moves and the greater its mass, the more kinetic energy it has.
  • If you push it with greater force, you do more work and transfer more energy, the object moves faster and its kinetic energy increases.
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Gravitational Potential Energy

  • When you lift an object up, you are doing Work and the Gravitational potential energy of the object increases.
  • Change in GPE (J) = Weight (N) x Vertical height difference (m).
  • When you let go of the object, its kinetic energy increases and its GPE decreases.
  • GPE lost(J) = KInetic energy gained(J)
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  • If an object gets faster, it is accelerating. The acceleration of an object is its change in speed, or change in velocity, in a given time period.
  • Equation to caculate acceleration,
  • Acceleration (m/s) = Change in velocity (m/s) / Time taken (s)
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Road Safety

  • If two cars collide,their momentum changes until it becomes 0.
  • The more time the change takes, the smaller the resultant force.
  • This idea is used in road safety for:
  • Crumple zones- squash slowly in a collision = longer time and smaller resultant force.
  • Seat belts -stretch in a collision = longer change in momentum and lesser force.
  • Helmets- change shape when hit = head stops moving slowly and force is less.
  • Air bags - increase time for the change of momentum.
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How are forces and motion connected?

  • All moving objects have Momentum.
  • Momentum (kg m/s) = Mass (kg) x Velocity (m/s)
  • When a resultant force acts on an object, the momentum of the object changes in the direction of the force:
  • Change of momentum (kg m/s) = Resultant force (N)  x Time for which it acts (s).
  • If the resultant force = 0, the Momentum doesn't change:
  • If it is stationary it remains still.
  • If it was already moving, it continues at a steady speed in a straight line.
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Speed-time and Velocity-time graphs

  • Speed-time = how speed varies with time.
  • Velocity-time = velocity of an object at every instance of its journey.
  • The gradient of a section of a velocity-time graph = to the object's acceleration.
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Why do objects keep moving?

Driving and counter forces

  • When you push a skateboard forwards, you are exerting the driving force on it. The counter force, in the opposite direction, is due to air resistance and friction.
  • If the driving force is greater than the counter force, the skateboard speeds up.
  • If the driving force is equal to the counter force, the skateboard moves at a constant speed in a straight line.
  • If the driving force is less than the counter force, the skateboard slows down.

Speed and velocity

Equation to calculate average speed:

Speed(m/s) = Distance(m) / Time(s)

  • If a horse runs 20m in 10s: average speed = 20m/10s = 2m/s

The instantaneous velocity of an object is its instantaneous speed in a certain direction.


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How do we describe motion?

Distance-time and Displacement-time graphs

  • Distance-time graphs describe movement.
  • Speed can be calculated using one = the gradient between 2 points on the line.
  • Steeper gradient = Greater speed.
  • Displacement- the straight line distance from its starting point, with an indication of direction.
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What are Forces?

Interaction Pairs

  • Forces arise from an interacton between two objects.
  • Each force in an Interaction pair acts on a different object.
  • The forces are equal in size and opposite in direction.

Resultant force

  • The resultant force on an object is the sum of the individual forces that act on it, taking their direction into account.

Reaction of surfaces

  • When you push down on the floor with your feet, the floor pushes up on your feet with an equal force. This force is the reaction of the surface.


  • When you push something forwards, the opposing force of friction if equal will stop the object from moving.
  • The harder you push, the greater the size of the friction force becomes.
  • Eventually the friction force reaches its limit and the object can now move.
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How do objects start moving?

Using friction

  • When a car engine starts, the wheels turn; exerting a backwards force on the road surface.
  • The other force in the interaction pair, the forward force, is the same size. This gets the car moving.

Rockets and Jet engines

  • A rocket pushes out hot gases as its fuel burns; the rocket pushes down on those gases. The escaping gases exert an equal and opposite force on the rocket, and push it upwards.
  • A jet engine draws in air at the front and pushes it out at the back. An equal and opposite force pushes the engine forwards.
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