Topic 2 - Motion & Forces

• Distance

  • Scalar
• Scalar


  Has just magnitude
• Speed

  • Scalar
• Time

  • Scalar
• Force

  • Vector
• Momentum

  • Vector
• 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
• 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
• Acceleration due to gravity: 𝑔 = 10𝑚𝑠−2
• Mass

  • Scalar
• Velocity

  • Vector
• Acceleration

  • Vector
• Vector


  Has magnitude and direction
• Energy

  • Scalar
• Displacement


  • Vector
• 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)
• 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
• Newton’s First Law: An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force
• Average Speed

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

  2. Use overall distances and timings to work out average speed
• Recall typical speeds

  • Wind: 5 − 7𝑚𝑠−1
  • Sound: 340𝑚𝑠−1
  • Walking: 5km/h = ~1.4𝑚𝑠−1
  • Running: ~6 𝑚𝑖𝑙𝑒𝑠 𝑝𝑒𝑟 𝑕𝑜𝑢𝑟 = ~3𝑚𝑠−1
  • Cycling: 15km/h = ~4𝑚𝑠−1
  • Bus: 14km/h
  • Train: 125miles/h
  • Plane: 900km/h
• Acceleration

  1. Change in velocity over time

  2. The velocity will change

  3. The direction or speed of the object will change (or both)
• Newton’s Second Law: Force = mass × acceleration
• Newton’s First Law: An object has a constant velocity unless acted on by a resultant force
• The greater the gravitational field strength, the greater the weight
• The speed is constant, but the direction is always changing
• Inertial Mass

  A measure of how difficult it is to change the velocity of an object (including from rest)
• For motion in a circle, there must be a force which supplies this acceleration
• Newton’s Third Law: Every action force has an equal and opposite reaction force
• Momentum

  Momentum = mass × velocity
• Human Reaction Time: There is a delay between a human observing an event and acting
• If a resultant force acts on the object, it will accelerate
• If the resultant force is zero
• Weight is measured using a force meter or weighing scales and is used to work out the mass of an unknown object
• Inertial Mass

  Measured by 𝐢𝐧𝐞𝐫𝐭𝐢𝐚𝐥 𝐦𝐚𝐬𝐬 = 𝐟𝐨𝐫𝐜𝐞/𝐚𝐜𝐜𝐞𝐥𝐞𝐫𝐚𝐭𝐢𝐨𝐧
• Object moving in a circle with constant speed
• Moving at constant velocity (same speed and same direction) or the object is at rest (no speed)
• This force is called centripetal force and is directed towards the centre of the circle
• Momentum is always conserved in a collision (where there are no external forces like friction, air resistance, electrostatic attraction etc.)
• Average human reaction time is 0.25 seconds (250 milliseconds)
• So the velocity is always changing, and it is accelerating