Pressure, Gravity, and Moments

• DENSITY

  measure of mass per unit volume
• DENSITY FORMULA AND UNITS

  • ρ=m/v
  • kg/m3
• UNIT CONVERSION

  • 1 m3 = 1 x 10^6 cm3
  • 1 cm3 = 1 x 10^-6 m3
• PRESSURE

  force per unit area
• PRESSURE FORMULA AND UNITS

  • p=f/a
  • N/m2
• UNIT CONVERSION

  • 1 m2 = 1 x 10^4 cm2
  • 1 cm2 = 1 x 10^-4 m2
• PRESSURE DUE TO A LIQUID FORMULA

  P=ρgh (density, gravity, vertical height)
• the difference in pressure (P) between two points in a fluid depends upon

  • density of the liquid
  • acceleration due to gravity
  • difference in height between the two points
• BUOYANCY/UPTHRUST

  upward force exerted by a fluid that opposes the weight of a partially or fully immersed object
• PRINCIPLE/LAW OF FLOATATION

  when an object displaces a weight of water equal to its own weight, it floats
• ARCHIMEDES' PRINCIPLE

  when a body is partially or fully immersed in a fluid, it experiences an apparent loss in weight that is equal to the weight of the fluid displaced by the immersed part of the body
• hydrometer

  instrument used to measure the relative density of liquid based on the concept of buoyancy (often used to measure salt levels in swimming pools)
• ATMOSPHERIC PRESSURE

  • indicator of weather
  • low pressure = cloudiness, wind, precipitation
  • high pressure = fair, calm weather
• why is high pressure associated with good weather?

  • the blanket of air is heavy and it is therefore harder for water molecules to evaporate
  • there is now less water vapour in the air, so less clouds, which means less rain and more sunlight
• PRESSURE AND BOILING POINT

  • as you ascend, the air pressure decreases
  • as pressure decreases, liquid water finds it easier to turn into a gas, to boil
• HYPERBARIC/DECOMPRESSION CHAMBER

  • the bends
  • nitrogen in our blood comes out of the solution when the pressure drops quickly
  • this then forms little gas bubbles which travel with the blood
• BOYLE'S LAW

  the pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature
• BOYLE'S LAW FORMULA

  p is proportional to 1/v
• according to boyle's law

  • if the pressure of a gas goes up, the volume goes down in the same proportion
  • p1 v1 = p2 v2
• IDEAL GAS

  • no volume
  • no molecular attractions
  • follows gas laws at all pressures and temperatures
• for a fixed mass of gas, at a constant temperature, pV=k, where k is a constant
• VERIFICATION OF BOYLE'S LAW

  • air pump, pressure gauge, air column, volume scale, oil column
  • using the pump, decrease the volume of the column of air in increments of 1 cm3
  • obtain at least 6 readings for graphing
  • after increasing the pressure, wait 5 minutes before increasing it again to allow the temperature of the gas to return to room temperature (BOYLE'S LAW, constant temperature)
• PRECAUTIONS in boyle's law experiment

  • eye level with pressure gauge to avoid parallax
  • work in a room where the temperature remains constant throughout
• what does boyle's law reveal about the relationship between pressure and volume?

  they have an opposite relationship, if one goes down the other goes up
• why is there no atmosphere on the moon?

  the gravitational force is too weak to sustain an atmosphere
• NEWTON'S LAW OF GRAVITY


  the force of gravity between two bodies is proportional to their masses and inversely proportional to the square of their separation. the force of gravity is always attractive.
• NEWTON'S LAW OF GRAVITY FORMULA

  Fg = G M1 M2 / R2
• why do astronauts appear weightless?

  they are in a state of free fall, gravity cannot be felt
• TO CALCULATE NET MOMENT

  • calculate anti clockwise moments
  • calculate clockwise moments
  • subtract smaller from bigger and take direction fo the bigger
• CENTER OF GRAVITY

  point through which the force of gravity acts on an object or system
• LAW OF LEVERS

  a lever works by reducing the amount of workload needed to move an object or lift a load. a lever does this by increasing the distance through which the force acts.
• LEVER DEFINITION

  rigid body which is free to rotate about a fixed point
• MOMENT FORMULA

  M = F X D (moment (Nm), force (N), perpendicular distance between pivot and point where force is applied (m))
• TORQUE

  measure of the force that can cause an object to rotate about an axis. just as force is what causes an object to accelerate in linear kinematics, torque is what causes an object to acquire angular acceleration.
• difference between torque and moment

  • torque = revolution
  • moment = static, non rotational
• COUPLE

  two parallel forces with the same magnitude acting in opposite directions to prevent the turning of a body
• LAWS OF EQUILIBRIUM

  1. when a body is in equilibrium, the vector sum of the forces acting on it is zero
  2. when a body is in equilibrium, the sum of the moments about any point is zero
• MOMENT DEFINITION

  measure of the force's tendency to rotate an object
• TORQUE FORMULA

  • the moment of a couple is a torque
  • therefore t = f x d
• INVESTIGATING THE LAWS OF EQUILIBRIUM FOR CO-PLANAR FORCES (1)

  • retort stands x2, spring balances x2, weights
  • find the centre of gravity of the metre stick (pivot)
  • find the mass and hence the weight of the metre stick