investigating motion

• Falling objects eventually reach terminal velocity - where their resultant force is zero
• Stopping distances depend on speed, mass, road surface and reaction time
• Mass and acceleration experiment

  1. Vary the mass of an object

  2. Measure the acceleration produced by a constant force
• Falling objects eventually reach terminal velocity - where their resultant force is zero
• Stopping distances depend on speed, mass, road surface and reaction time
• Force and acceleration experiment

  1. Position an air track on a bench with a bench pulley at one end and two light gates above the track

  2. Cut an interrupt card to a known length (such as 10 cm) and attach it to an air track glider

  3. Connect the glider to a hanging mass by a string the length of the air track passing over the bench pulley

  4. Make sure the air track is level and that the card will pass through both gates before the mass strikes the floor

  5. Set the data logging software to calculate acceleration

  6. Add 5 × 20 g slotted masses (0.98 N of force) to the end of the string

  7. Release the glider, then record the weight and acceleration

  8. Repeat steps 6 and 7 two more times, and calculate a mean value for the acceleration

  9. Repeat steps 6 to 8, removing one of the slotted masses each time (giving forces of 0.78 N, 0.59 N, 0.39 N and 0.20 N
• Results table


  • Force (N) 0.98 Run 1 acceleration (m/s) 2 0.22 Run 2 acceleration (m/s) 2 0.27 Run 3 acceleration (m/s) 2 0.37 Mean acceleration (m/s) 2 0.29
  • Force (N) 0.78 Run 1 acceleration (m/s) 2 0.20 Run 2 acceleration (m/s) 2 0.29 Run 3 acceleration (m/s) 2 0.21 Mean acceleration (m/s) 2 0.23
  • Force (N) 0.59 Run 1 acceleration (m/s) 2 0.26 Run 2 acceleration (m/s) 2 0.11 Run 3 acceleration (m/s) 2 0.17 Mean acceleration (m/s) 2 0.18
  • Force (N) 0.39 Run 1 acceleration (m/s) 2 0.21 Run 2 acceleration (m/s) 2 0.10 Run 3 acceleration (m/s) 2 0.05 Mean acceleration (m/s) 2 0.12
  • Force (N) 0.20 Run 1 acceleration (m/s) 2 0.04 Run 2 acceleration (m/s) 2 0.06 Run 3 acceleration (m/s) 2 0.11 Mean acceleration (m/s) 2 0.07
• Control measures: Check mains cable and plug are not broken or wiring exposed before use. Use relatively small masses. Step back after releasing glider.
• Newton's laws surrounding forces were formulated hundreds of years ago, but are still used today - they help to describe the relationship between a body and the forces that act upon it.
• Free body diagrams


  Simplified drawing of an object or system showing the forces acting on it. The forces are shown acting away from the centre of a box or dot.
• Free body diagrams


  • Do not need to be drawn to scale but it can sometimes be useful if they are
  • It is important to label each arrow to show the magnitude of the force it represents
  • The type of force involved may also be shown
• Examples of free body diagrams

  • Weight and reaction force for a resting object
  • Weight, reaction force and friction for an object moving at constant speed down a hill
  • Weight, upthrust, thrust and air resistance for an accelerating speedboat
• Vector diagrams


  Used to resolve (break down) a single force into two forces acting at right angles to each other
• Using a scale diagram to find the resultant vector

  1. Draw the two vectors at right angles to scale

  2. Use Pythagoras' theorem to calculate the resultant vector
• In any right-angled triangle, the square of the longest side is the sum of the squares of the other two sides