Topic 2 ~ Motion and Forces

• Distance

  • Scalar
• Displacement


  • Vector
• Speed


  • Scalar
• Vector


  Has magnitude and direction
• Generally, scalar cannot be negative, but vectors can be, as a certain direction is positive
• Scalar


  Has just magnitude
• Velocity

  • Vector
• Momentum


  • Vector
• Mass


  • Scalar
• Force


  • Vector
• Acceleration

  • Vector
• Energy


  • Scalar
• Time

  • Scalar
• Imagine a ball is thrown off a cliff, the displacement is 0 at height of cliff, above the cliff the ball has positive displacement, and below the clifftop the ball has negative displacement
• In long answer questions, you may be able to decide where the “0” point of a vector may lie, for example you could set zero to be bottom of cliff, so the ball will never have negative displacement
• Displacement Time Graphs

  1. Gradient is velocity

  2. Sharper gradient means faster speed

  3. Negative gradient is returning back to starting point

  4. Horizontal line means stationary

  5. 0 Distance means that it is back to starting point

  6. Area under line = nothing

  7. Curved Line means the velocity is changing (acceleration)
• Velocity Time Graphs

  1. Gradient is acceleration

  2. Sharper gradient means greater acceleration

  3. Negative gradient is deceleration

  4. Horizontal line, constant speed

  5. 0 velocity means that it is stationary

  6. Area under line = distance travelled

  7. Curved Line means that the acceleration is changing
• Average Speed


  1. This is for when the speed changes during the motion

  2. Use overall distances and timings to work out average speed
• Methods to Determine Speeds

  1. For constant speeds: Measure distance travelled, Use stopwatch for time taken, Use speed = distance/time

  2. For average speed: Work out total distance travelled, Find the time taken for the whole journey, Use speed = distance/time

  3. Using light gates: Set up two, one at start and one at end, Measure distance between them, As soon as the object passes through the first, it will measure the time taken to reach the second, Then use speed = distance/time. This is more accurate as it removes reaction time and human error with a stopwatch
• Acceleration due to gravity: 𝑔 = 10𝑚𝑠−2
• If the resultant force is zero: No acceleration, moving at constant velocity, or the object is at rest
• Object moving in a circle with constant speed: The speed is constant, but the direction is always changing, so the velocity is always changing, and it is accelerating
• Acceleration

  1. Change in velocity over time

  2. The velocity will change

  3. The direction or speed of the object will change (or both)
• For motion in a circle, there must be a force which supplies this acceleration, called centripetal force, directed towards the centre of the circle
• Weight is measured using a force meter or weighing scales and is used to work out the mass of an unknown object
• Newton’s Second Law: Force = mass × acceleration, F = ma, where force is in Newtons, N, mass is in kg and acceleration in ms-2
• Newton’s First Law: An object has a constant velocity unless acted on by a resultant force
• If a resultant force acts on the object, it will accelerate
• The greater the gravitational field strength, the greater the weight
• Momentum is always conserved in a collision where there are no external forces like friction, air resistance, electrostatic attraction, etc.
• Momentum


  Momentum = mass × velocity, p = mv, where p is the momentum in kgms-1, m is the mass in kg, and v is the velocity in ms-1
• In collisions, total momentum before = total momentum after
• Newton’s Third Law: Every action force has an equal and opposite reaction force
• Ruler Drop Experiment

  Someone else holds a ruler just above your open hand, drops it at a random time, records the distance from the bottom of the ruler to the point where it was caught, averages this, and 1cm is 50ms, 2cm 60ms, and so on
• Human Reaction Time: There is a delay between a human observing an event and acting
• Average human reaction time is 0.25 seconds (250 milliseconds)
• Factors affecting Thinking Distance

  • Speed
  • Reaction time
  • Concentration
  • Tiredness
  • Distractions
  • Influence of drugs/alcohol
• Speed and Braking Distance


  Greater the speed, the greater distance travelled during the same time (reaction time)
• Factors affecting Braking Distance

  • Speed
  • Poor road conditions (icy, wet)
  • Bald tires (low friction)
  • Worn brake pads
  • Mass (more passengers)
• Experiment

  1. Someone else holds a ruler just above your open hand

  2. They drop it at a random time

  3. Record the distance from the bottom of the ruler to the point where it was caught

  4. Average this, and 1cm is 50ms, 2cm 60ms, and so on