physics

• If there was no air resistance or drag, falling objects would differ in that the forces would remain unbalanced and the object would continue to accelerate downwards at a constant rate
• Mass


  The measure of the quantity of matter in an object at rest (relative to the observer)
• Weight


  A gravitational force acting on an object that has mass
• Weight


  Unit = Newtons
• Equation linking weight, mass and gravitational field strength


  1. Weight = mass x gravitational field strength

  2. W = m x g
• If the gravitational field strength increases


  Weight increases
• Terminal velocity of a skydiver

  1. Skydiver accelerates (Weight >> air resistance)

  2. As speed increases, air resistance increases. Acceleration slows (Weight > air resistance)

  3. Terminal velocity reached. Forces balanced (Weight = air resistance)

  4. Parachute opened. Skydiver decelerates (Weight < air resistance)

  5. New, slower terminal velocity reached (Weight = air resistance)
• How weights and masses are measured

  Using a balance
• Equation linking density, mass and volume

  1. Density = mass / volume

  2. ρ = m / V

  3. Units = kg/m3
• Measuring the density of a regular solid object


  1. Use a balance to measure object's mass

  2. Use a ruler to measure object's volume (length x depth x height)

  3. Density = mass ÷ volume
• Measuring the density of an irregular solid object


  1. Use a balance to measure object's mass

  2. Lower object into known volume of liquid

  3. Measure new volume of liquid (or volume of liquid removed from container)

  4. Density = mass ÷ volume
• Measuring the density of a liquid

  1. Measure mass of empty beaker using a balance

  2. Measure mass of beaker now containing liquid

  3. Density = mass ÷ volume
• What determines whether an object will float or sink in water

  • The object's density
  • If density of object < density of water → object floats
  • If density of object > density of water → object sinks
• What determines whether a liquid will float on top of another liquid

  • The liquid's density
  • If density of liquid A < density of liquid B → liquid A floats on top of liquid B
• Effect of a force on an object


  • Changes object's speed
  • Changes object's direction
  • Changes object's shape
• Hooke's law


  The extension of a spring is directly proportional to the force applied, provided its limit of proportionality (elastic limit) is not exceeded
• Elastic behaviour


  Ability of a material to revert to its original shape after forces causing deformation have been removed
• If the elastic limit/limit of proportionality is exceeded, the material no longer reverts to original shape after the forces have been removed
• Investigating whether a spring obeys Hooke's law


  1. Measure original length of spring

  2. Add a known weight and measure force using a Newton meter

  3. Measure the new length using a ruler and calculate extension

  4. Repeat for range of weights

  5. Plot graph of extension against force

  6. Graph should be a straight line through origin

  7. Force proportional to extension

  8. If graph is curved, spring does not obey Hooke's law
• Spring constant


  • Force per unit of extension
  • A measure of the stiffness of a spring
  • Up to its limit of proportionality
• Calculating the spring constant


  1. Spring constant = force / extension

  2. k = F / x

  3. Force (F) in Newtons (N)

  4. Spring constant (k) in N/m

  5. Extension (x) in metres (m)
• Newton's 1st Law


  • If forces acting on an object are balanced, the resultant force is zero
  • Object at rest → stays stationary
  • Object moving → continues to move in same direction and at same speed
• Newton's 2nd Law


  • Acceleration is proportional to resultant force
  • Inversely proportional to mass of object
• Calculating the resultant force acting along a line


  1. Add up all forces acting in useful direction

  2. Subtract all forces acting in the opposite direction
• Equation linking force, mass and acceleration


  • Force = mass x acceleration
  • F = m x a
  • Force and acceleration must be acting in the same direction
• Friction


  • Force between two solid surfaces that opposes motion
  • Kinetic energy converted into thermal energy
• Drag


  • Friction that acts on an object moving through a liquid or gas
  • In gas, this is also called air resistance
• Circular motion of an object orbiting at a constant speed

  • Speed is constant
  • Direction of travel is changing as object moves along circular path
  • Therefore velocity is always changing
• Change in motion in a circular path if the perpendicular force increases


  • As force increases, speed increases
  • If mass and radius are constant
  • As force increases, radius decreases
  • If mass and speed are constant
• Change in force acting on an object moving in a circular path if the object's mass increases


  • Force must also increase
  • If speed and radius are constant
• Newton's 3rd Law


  Every force has an equal and opposite reaction
• Moment


  • The turning effect of a force
  • Units = Newton metres (Nm)
• Equation to calculate the moment of a force


  Moment = force x perpendicular distance from pivot
• How to increase the moment of a force

  • Increase distance
  • Increase force
• The relationship between the clockwise and anticlockwise moments on a balanced object
• Experiment to demonstrate that there is no resultant moment on an object in equilibrium


  1. Suspend a metre rule horizontally from its centre (O) using a strong thread

  2. Suspend two differing masses (W1 and W2) on either side of the thread

  3. Adjust the distances of two weights until the ruler is balanced and horizontal

  4. Calculate the clockwise moment = W2 × L2

  5. Calculate the anti-clockwise moment = W1 × L1

  6. If the ruler is horizontal (in equilibrium), clockwise moment = anti-clockwise moment
• Centre of gravity


  The point on an object where its weight appears to act through
• Experiment to determine the position of the centre of gravity of an irregularly shaped plane lamina


  1. Suspend a plumb line (thread with a weight on the end) from a clamp stand

  2. Make three holes near edge of lamina

  3. Suspend lamina through one hole and hang behind plumb line

  4. Trace plumb line to draw line of equilibrium

  5. Repeat for other two holes in lamina

  6. Point of intersection of the three lines is the centre of gravity
• Effect of the position of the centre of gravity on the stability of an object


  • Low centre of gravity = stable object
  • High centre of gravity = unstable object
  • If centre of gravity is above base = stable object
  • If centre of gravity is outside of base = unstable object
• Equation for momentum


  • Momentum = mass x velocity
  • p = m x v
  • Units = kg m/s