chap 3 - forces and pressure

• relationship between pressure, area and force = the greater the magnitude of force, the greater pressure
• forces is a push and pull, not an energy

  • when force is exerted, it can

  > start/ stop an object from moving

  > slow/speed up the object

  > change the direction of the object
• contact force

  • exists when an object are in contact

  > eg: friction (oppose motion)

  air resistance (frictional force by air)

  normal force ( push exerted by a surface on an object pressing it)

  tension ( pull exerted by a stretch spring)
• non-contact force

  • objects do not need to be in contact to experience it
  • feel instead of touch

  > eg: gravitational force ( pull exerted by earth´s gravity)

  electrostatic force ( attractive/repulsion between electric charges)

  magnetic force ( attraction/repulsion between magnets)
• gravitational field

  • weight is the effect of a gravitational field on a mass
  • exists in the region except shadow(cuz no mass)
  • mass experience force due to gravitational attraction
  • strongest at the surface of the earth and gets weaker further away
• pressure

  • force acting per unit area
  • scalar quantity
  • formula : pressure = force/ contact area
  • SI UNIT = N/m² OR pascal (P)
• relationship between pressure, force and area

  • greater the magnitude of force, the greater pressure
  • smaller the contact area, the larger the pressure
• mass vs weight

  - completely different quantities

  • mass

  > a measure of the amount of matter in a body

  > does nit change with its location, shape and speed of the body

  > depends on the no. and composition of atoms and molecules that make up the body

  > SI unit : kg

  > scalar quantity
• • weight

  > gravitational force acting on an object that has mass

  > SI unit : newton (N)

  > vector quantity

  > direction of weight is downwards, towards the centre of the earth
• gravitational field strength

  • weight of an object depends on the strength of gravitational force acing on it
  • gravitational field strength is defined as the gravitational force per unit mass placed at that point
  • gravitational field strength on earth = 10N/kg
  • gravitational field strength on moon - 1.6N/kg
  • formula : g=w/m
• relationship between mass and weight

  • weight (aka gravitational force) acting on an object is directly proportional to its mass
  • formular : w=m x g
• gravitational field strength and acceleration due to gravity

  • formula : resultant force (f) = mass (kg) x acceleration (a)
• common instruments

  • weight

  > spring balance, bathroom scale, electronic balance

  > calibrated to give readings in grams or kilograms for use on earth

  • mass

  > beam balance compares the gravitational force acting on an object with that acting on standard masses

  > both object and standard masses experience the same gravitational field strength

  > the mass reading is taken for a given object, whether on earth or on moon will be the same
• density

  • mass per unit volume
  • formula : mass/volume
  • SI unit : kg/m³
• density of liquid

  • find the mass of an empty measuring cylinder
  • pour the liquid into the cylinder and find the mass of the liquid and cylinder
  • find the volume of the liquid in the cylinder and calculate density using the formula
• density of solid

  • pour an amount of liquid into the cylinder and find the volume
  • drop the irregular solid into the liquid and find the difference before and after dropping the liquid
  • the mass of the irregular solid can be measured using a electronic balance