Density , Pressure ,Moments

• Density is the mass per unit volume
• Pressure is force per unit area
• The equation to calculate pressure is P=F/A
• The moment of a force about a point is the product of the magnitude of the force and its distance from the point.
• The equation to calculate moments is M = F x d
• The equation to calculate density is D = m / V
• A moment is the turning effect produced when a force acts at some distance from an axis or pivot
• Materials with tightly packed atoms have high sensitivity while those with space have low density
• Pressure increases if the surface area decreases
• Oil floats i.e. it is less dense than water so it can be separated from water
• A straight line through the origin shows that two quantities are directly proportional.
• Measuring density of a regular object

  1. Measure the length, width and height of a block using a ruler

  2. Calculate its volume using the equation V = Lxbxh

  3. Using an electronic balance record the mass of the block

  4. Calculate the density of the block using the equation density = mass + volume

  5. Repeat for a range of different materials of block
• As liquids are materials which do not remain fixed shape, the previous method for measuring density will not be possible
• Measuring density of liquids

  1. Measure the mass of an empty measuring cylinder using an electronic balance

  2. Add water to the cylinder and record the mass

  3. Measure the mass of the water and cylinder. Subtract the cylinder's mass to get the water's mass

  4. Repeat with different volumes of water to get 5 pairs of results

  5. Plot a graph of mass (y-axis) against volume (x-axis)
• Gradient: g/cm³
• If a solid is an irregular shape then it is not possible to calculate the volume of it using Lxwxh. Another method for finding the volume must be found
• Measuring the density of irregular solids

  1. Measure the mass of the object using an electronic balance

  2. Add water to a measuring cylinder and record the volume

  3. Carefully lower the object into the water and record the new volume. Subtract the original volume to get the volume of the object

  4. Use the equation density = mass / volume
• Why must the object be lowered carefully into the cylinder? So that the water doesn't spill therefore providing a lower volume = bigger density
• the accuracy of the experiment =, use more accurate apparatus
• the reliability of the experiment be improved=Repeat and take an average
• FAPSO

  Force, Area, Pressure, SO
• FAPSO examples

  • Skier's weight over large area = low pressure, tank's weight over large area = low pressure, force of bite over small area = high pressure
• Lever

  Used to move difficult or heavy objects by allowing the user to apply force at a greater distance from the object
• Experiment: Clockwise and anticlockwise moments

  1. Balance the paper plate on one finger

  2. Add two sweets and balance the plate again

  3. Measure the distance from the centre of each sweet to the pivot

  4. Measure the mass of each sweet using an electronic balance

  5. Calculate the moment created by each sweet

  6. Repeat to get 5 sets of results
• The meter stick is balanced when the clockwise and anticlockwise moments are equal
• Principle of Moments

  When an object is in equilibrium, the sum of the clockwise moments about a point is equal to the sum of anticlockwise moments about the same point