Forces and Motion

• Resultant force


  The single force that is equivalent to all the other forces acting on a given object
• Newton's first law for a stationary object


  If the resultant force on a stationary object is zero, the object will remain at rest
• Newton's first law for a moving object

  If the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction)
• Newton's Second Law (equation)

  Resultant force = Mass x Acceleration

  F = ma
• Newton's Second Law (words)

  An object's acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass
• Weight


  The force that acts on an object due to gravity and the object's mass
• Quantities weight depends on

  • The object's mass

  The gravitational field strength at the given position in the field
• Unit for weight


  Newton (N)
• Equipment to measure weight

  A calibrated spring-balance or newton-meter
• Scalar quantity


  • A quantity that only has a magnitude
  • A quantity that isn't direction dependent
• Vector quantity


  A quantity that has both a magnitude and an associated direction
• How vector quantities are represented

  • Using vector arrows
  • The length of the arrow represents the magnitude
  • The arrow points in the associated direction
• Vector quantities


  • Velocity
  • Displacement
  • Force
• Velocity


  The speed of an object in a specific direction
• Gradient of distance/time graph

  The speed
• Gradient of displacement/time graph

  The velocity
• Calculating speed at a given time from a distance-time graph for an accelerating object

  1. Draw a tangent to the curve at the required time

  2. Calculate the gradient of the tangent
• Calculating distance from a velocity-time graph

  It is equal to the area under the graph
• Gradient of velocity/time graph

  The acceleration
• Typical value for speed of sound
• Typical value for human walking speed
• Typical value for human running speed
• Typical value for human cycling speed
• Approximate value for acceleration of an object in free fall under gravity near the Earth's surface
• Centripetal force


  The resultant force that acts on an object moving in a circle, and it acts towards the centre of the circle.
• Inertial mass


  A measure of how difficult it is to change a given object's velocity

  The ratio of force over acceleration
• Newton's Third Law
• Momentum


  Momentum = Mass x Velocity
• Unit for momentum
• In a closed system, what can be said about the momentum before and after a collision? (Higher)
• Equation linking change in momentum, force and time (Higher)


  Force x Time = Change in Momentum

  F Δt = mΔv
• Newton's first law states that if there are no forces acting on an object, it will continue to move at constant velocity or remain stationary.
• The force on an object is the product of its mass and acceleration.
• How can you measure human reaction times?

  Using the ruler drop test:

  Person A and B hold each end of a ruler with the 0 cm mark at the bottom

  Person A drops the ruler without telling person B

  Person B catches it

  The distance travelled corresponds to their reaction time
• Stopping distance of a vehicle

  The sum of thinking distance and braking distance.
• For a given braking distance, if the vehicle's speed is increased


  The stopping distance is increased with an increase in speed.
• Typical range of values for human reaction time
• Factors which can affect a driver's reaction time

  • Tiredness
  • Drugs
  • Alcohol
• Factors which may affect braking distance

  • Adverse (wet/icy) road conditions
  • Poor tyre/brake conditions
• Energy transfers when a car applies its brakes

  Work is done by the friction force between the brakes and wheel

  Kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs