P4: Explaining Motion

Revision cards for Physics P4,P5,P6 modules

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  • The speed of a moving object can be calculated if the distance travelled and the time taken are known.
  • The faster an object moves, the steeper is the line representing it on a distance-time graph.
  • The velocity of an object is its speed in a particular direction.
  • In velocity-time graphs sloping lines represent steadily increasing or decreasing velocities.
  • Horizontal lines represent movement at constant velocities.

Speed is distance divided by time taken. (http://www.bbc.co.uk/schools/gcsebitesize/science/images/add_aqa_speed_dist.gif)

  • To calculate the gradient of the line on a distance-time graph, you need to divide the change in the vertical axis by the change in the horizontal axis.

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Distance-Time graphs

  • Features: the steeper the line, the greater the speed of the object.
  • The blue line is steeper than the red line because it represents an object moving faster than the object represented by the red line.
  • The line representing the return journey slopes downwards.
  • Changes in distances in one direction are positive, and negative in the other direction.

 time (s) on x axis, distance (m) on y axis (http://www.bbc.co.uk/schools/gcsebitesize/science/images/ph_forces01.gif)

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A force occurs when two objects interact. eg. A person pushing a wall. The two objects are called interaction pairs.

The theory behind interaction pairs is stated in Newton's Third Law:

All actions have an equal and opposite reactions meaning if object 1 pushes against object 2, then object 2 exerts an equal force in an opposite direction to object 1.

All moving objects experience friction whether it is between two solid surfaces gripping onto each other (static friction), sliding past each other or the resistance from gas or liquid (drag)

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Forces and Motion

  • The momentum of an object is its mass multiplied by its velocity. The larger the mass and velocity the larger the momentum.
  • Forces change momentum - the larger the force the more quickly the momentum changes.
  • The resultant force is the overall result of all forces acting on an object.


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Balanced and Unbalanced forces

  • A car or bicycle has a driving force pushing it forwards. There are always counter forces of air resistance and friction pushing backwards.
  • If the driving force is greater than the counter forces, there is a resultant force forwards. This will make the car speed up.
  • If the driving force is less than the counter forces, there is a resultant force backwards. This will make the car slow down.
  • If the driving force is the same as the counter forces, there is no resultant force, and so no change in velocity.
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Forces and Momentum change

The momentum of a moving object depends on its mass and its velocity:

momentum (in kg m/s) = mass (in kg) × velocity (in m/s)

A resultant force acting on any object changes the momentum of that object.

The size of the change in momentum depends on the size of the resultant force and the time for which the force acts:

change of momentum (in kg m/s) = resultant force (in newton, N) × time for which it acts (in s).

To give the same change of momentum, you can have:

  • a larger force for a shorter time, or
  • a smaller force for a longer time
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Speed 2

  • The average speed of a jouney is calculated by using the total distance covered and the total time taken.
  • Instantaneous speed is the speed at a particular moment (eg. the speed on a car speedometer).
  • speed-time graphs
    • flat sections are steady/constant speed.
    • acceleration and deacceleration are represented by upward and downward sections respectively.
  • velocity-time grpahs
    • gradient is the acclerations.
    • flat sections are steady/constant speed.
    • acceleration and deaccelerations are represented by upward and downward sections respectively.
    • curves represent acceleration or deacceleration.
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