GCSE Physics Topic 1 - Motion and Forces

• Vector


  Quantity with both magnitude and direction
• Scalar


  Quantity with only magnitude
• Examples of scalars


  • Speed
  • Distance
  • Time
  • Mass
  • Energy
• Examples of vectors


  • Velocity
  • Displacement
  • Acceleration
  • Force
  • Momentum
• Generally, scalars cannot be negative, but vectors can be, as a certain direction is positive
• Displacement at the height of a cliff

  Zero
• Displacement above the cliff

  Positive
• Displacement below the cliff

  Negative
• Speed is only velocity when given a direction
• Displacement-Time Graph


  • Gradient is velocity
  • Sharper gradient means faster speed
  • Negative gradient is returning back to starting point
  • Horizontal line means stationary
  • 0 Distance means that it is back to starting point
  • Curved Line means the velocity is changing (acceleration)

  
• Velocity-Time Graph


  • Gradient is acceleration
  • Sharper gradient means greater acceleration
  • Negative gradient is deceleration
  • Horizontal line, constant speed
  • 0 velocity means that it is stationary
  • Area under line = distance travelled
  • Curved Line means that the acceleration is changing

  
• Average Speed


  For when the speed changes during the motion, 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

  
• Typical speeds


  • Wind: 5-7 m/s
  • Sound: 340 m/s
  • Walking: 1.4 m/s
  • Running: 3 m/s
  • Cycling: 4 m/s
  • Bus: 14 km/h
  • Train: 125 miles/h
  • Plane: 900 km/h
• Acceleration due to gravity: g = 10 m/s^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. If the resultant force is zero, no acceleration, so moving at constant velocity or at rest.
• Newton's Second Law


  Force = mass × acceleration
• Weight


  Measured using a force meter or weighing scales, and is used to work out mass of unknown object. The greater the gravitational field strength, the greater the weight.
• Circular Motion


  Object moving in a circle, with constant speed. The speed is constant, but direction always changing, so the velocity is always changing, so it is accelerating. There must be a force which supplies this acceleration, called centripetal force, and is directed towards the centre of the circle.
• Inertial Mass


  A measure of how difficult it is to change the velocity of an object (including from rest), calculated as force/acceleration.
• Newton's Third Law


  Every action force has an equal and opposite reaction force.
• Momentum


  Momentum is always conserved in a collision (where there are no external forces like friction, air resistance, electrostatic attraction etc.), calculated as mass × velocity.
• For Newton's Second Law, Force = change in momentum/time = (mv - mu)/t
• Human Reaction Time


  There is a delay between a human observing an event, and acting. Average human reaction is 0.25 seconds (250 milliseconds).
• Vehicle Stopping Distances


  • Thinking Distance: Affected by speed, reaction time, concentration, tiredness, distractions, influence of drugs/alcohol
  • Braking Distance: Affected by speed, poor road conditions, bald tires, worn brake pads, mass

  
• Typical Stopping Distances (Physics only)
• Greater the speed


  Greater distance travelled during the same time (reaction time)
• Dangers of Large Decelerations

  When in a crash, there is a large deceleration over a very short time as you stop moving from a high speed. This large deceleration means a great force is exerted on the car, and the passengers, which can cause injury.
• To estimate the forces felt on a road, use known values of mass and acceleration to calculate force. Average mass of a car ~1500kg.

  
• Work Done to Stop (Physics only)

  The work done to stop a vehicle is equal to the initial KE of the vehicle, as all the kinetic energy the car had has to be transferred to friction for it to stop. Braking distance ∝ (initial velocity)^2.

  
• Mathematical skills


  • Convert units
  • Interpret distance/time and velocity/time graphs
  • Calculate distance, speed and time for uniform speed, uniform acceleration, and non-uniform motion
  • Estimate stopping distances for a car at a range of speeds
  • Calculate force, mass and gravitational field strength using formulae
  • Calculate force, mass, velocity and acceleration using formulae
  • Estimate the speed, accelerations and forces involved in large accelerations for everyday road transport