Forces and Motion

• Distance and Displacement

  . Distance is a scalar quantity:

  - It is how far an object moves

  - It does not take into account the direction an object is travelling in or even if it ends up back where it started

  . Displacement is a vector quantity:

  - It has a magnitude, which describes how far the object has travelled from the origin, measured in a straight line

  - It has direction, which is the direction from the straight line
• What is a key point to remember about distance and displacement?

  . Distance and speed are scalar quantities, so they only have magnitude

  . Displacement is a vector quantity, so it has direction as well as magnitude
• Speed

  A measure of how fast an object is travelling
• Speed

  • It is a scalar quantity
  • Measured in metres per second (m/s)
• Factors affecting speed

  • Age
  • Fitness
  • Terrain
  • Distance
• Typical speeds

  • Walking, 1.5m/s
  • Running, 3m/s
  • Cycling, 6m/s
  • City driving, 12m/s
  • Motorway driving, 30m/s
  • High speed trains, 75m/s
  • Commercial aircraft, 250m/s
  • Speed of sound in air, 330m/s
• Most things (including sound) do not travel at a constant speed, so it is often the average speed over a period of time that is used
• Distance travelled

  1. Distance = speed x time

  2. s = vt
• Distance (s)

  Measured in metres (m)
• Speed (v)

  Measured in metres per second (m/s)
• Time (t)

  Measured in seconds (s)
• Velocity

  A vector quantity, the speed of an object in a given direction
• Velocity

  • When travelling in a straight line, an object with constant speed also has constant velocity
  • If the object is not travelling in a straight line, e.g. it is turning a corner, the speed can still be constant but the velocity will change, because the direction has changed
  • An object moving in a circle is constantly changing direction, so it is constantly changing velocity, and is accelerating even if it is travelling at a constant speed
• Orbiting planets

  • An example of an object constantly changing velocity due to the force of gravity causing acceleration
• What is a key point to remember about velocity?

  Velocity is a vector quantity
• What is a key point to remember about an object travelling in a circle?

  An object travelling in a circle can have constant speed, but its velocity is still changing
• Newton's First Law

  An object will remain in the same state of motion unless acted on by an external force
• When the resultant force acting on an object is zero

  1. If the object is stationary, it remains stationary

  2. If the object is moving, it continues to move at the same speed in the same direction, i.e. at constant velocity
• Inertia

  The tendency for objects to continue in the same state of motion
• The velocity (speed or direction) of an object will only change if there is a resultant force acting on it
• For a car travelling at a steady speed

  The driving force is balanced by the resistive force
• Distance-Time Graphs

  . A distance-time graph can be used to represent the motion of an object travelling in a straight line

  . The speed of the object is found from the gradient (slope) of the line

  . If an object is accelerating, the distance-time graph will be a curve

  . For an accelerating object, its speed at a particular time is found by:

  - drawing a tangent to the curve at the point in time

  - working out the gradient of the tangent
• What is a key point to remember about distance-time graphs?

  A distance-time graph can be used to calculate speed