forces and motion

• Distance

  • how far an object moves.
  • Distance does not involve direction.
  • Distance is a scalar quantity.
• Displacement

  • includes both the distance an object moves and the direction of that straight line.
  • measured in a straight line from the start point to the finish point
  • Displacement is a vector quantity.
• Speed

  does not involve direction.

  Speed is a scalar quantity.
• Typical speeds: 

  1. Normal walking speed: 1.5 m/s
  2. Running speed: 3 m/s
  3. Cycling speed: 6 m/s 

  • Speed depends on age and fitness level & terrain (flat ground= fast)  
  • car on main road: 13 m/s, fast train in UK: 50 m/s, Cruising aeroplane: 250 m/s, sound in air 330: m/s 
• For an object moving at constant speed the distance travelled in a

  specific time can be calculated using the equation:


  distance travelled = speed × time

  s = v t

  distance, s, in metres, m

  speed, v, in metres per second, m/s

  time, t, in seconds, s
• The velocity of an object

  speed in a given direction.

  Velocity is a vector quantity
• If velocity is moving in a circle:

  • if an object moves at a constant speed in a circle then its velocity is constantly changing, even if speed is constant 
• If an object moves along a straight line, the distance travelled can

  be represented by a distance–time graph.


  The speed of an object can be calculated from the gradient of its

  distance–time graph
• If an object is accelerating(distance time graph)


  its speed at any particular time can be determined by drawing a tangent and measuring the gradient of the distance–time graph at that time.
• The acceleration of an object can be calculated using the

  equation:


  acceleration = change in velocity/time taken

  a = ∆ v/t

  acceleration, a, in metres per second squared, m/s2

  change in velocity, ∆v, in metres per second, m/s

  time, t, in seconds, s
• velocity time graph

  • The distance travelled by an object (or displacement of an

  object) can be calculated from the area under a velocity–time

  graph

  • The acceleration of an object can be calculated from the gradient of a velocity–time graph
• The following equation applies to uniform acceleration: (constant velocity changes)


  final velocity 2 − initial velocity 2 = 2 × acceleration × distance

  v2 − u2 = 2 a s

  final velocity, v, in metres per second, m/s

  initial velocity, u, in metres per second, m/s

  acceleration, a, in metres per second squared, m/s2

  distance, s, in metres, m
• Near the Earth’s surface any object falling freely under gravity has

  an acceleration of about 9.8 m/s2
• terminal velocity-the maximum velocity that an object can attain when it is falling through a fluid, such as air or water


  An object falling through a fluid initially accelerates due to the force

  of gravity. Eventually the resultant force will be zero and the object

  will move at its terminal velocity.
• Newton’s First Law:


  If the resultant force acting on an object is zero and:

  • the object is stationary, the object remains stationary

  • the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity.
• inertia


  The tendency of objects to continue in their state of rest or of uniform motion

  inertial mass = a measure of how difficult it is to change the

  velocity of an object

  • inertial mass is defined as the ratio of force over acceleration
• Newton’s Second Law:

  The acceleration of an object is proportional to the resultant force

  acting on the object, and inversely proportional to the mass of the

  object
• resultant force = mass × acceleration

  F = m x a

  force, F, in newtons, N

  mass, m, in kilograms, kg

  acceleration, a, in metres per second squared, m/s2
• Required Practical: Acceleration

  1)Attach car to a piece of string and loop around a pulley - the other end is attached to a 100g mass

  2)The weight of the mass will provide the force acting on the toy car, on the desk draw chalk lines at equal intervals (eg every 10cm) hold the toy car at the starting point, let go of car when ready - as there's a resultant force acting through the string the car will accelerate along the bench, must record the time the car crosses each mark

  3)repeat experiment but decrease the mass at end of the string (80, 60, 40 g) - the force it decreases each time
• Newton’s Third Law:


  Whenever two objects interact, the forces they exert on each other

  are equal and opposite.
• The stopping distance of a vehicle

  the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance).

  • For a given braking force the greater the speed of the vehicle, the greater the stopping distance
• Reaction times vary from person to person.

  • Typical values range from 0.2 s to 0.9 s.
  • A driver’s reaction time can be affected by tiredness, drugs,

  alcohol and distractions
• • Measure reaction time: using a ruler, ruler drops and person has to catch ruler with their fingers, the further ruler falls the longer the reaction time 
• The braking distance of a vehicle


  can be affected by adverse road and weather conditions and poor condition of the vehicle.

  • include wet or icy conditions. Poor condition of the vehicle is limited to the vehicle's brakes or tyres
• • When a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temp of the brakes increases.
  • The greater the speed of a vehicle the greater the braking force needed to stop the vehicle in a certain distance.
  • The greater the braking force the greater the deceleration of the vehicle.
  • Large decelerations may lead to brakes overheating and/or loss of control