topic 15 forces and matter

• bending, stretching and compression

  • forces can deform and change the shape of an object
  • if it requires more than one force to stretch , bend and compress an object

  
• elastic vs inelastic objects

  • an elastic object will return to its original shape once the force acting on it are removed
  • inelastic object will keep its new shape once the force acting on it is removed
• elastic vs inelastic

  • elastic objects return to their original shape when the force acting on them are removed E.g a diving board
  • inelastic objects do not return to their original shape when forces are removed - they are permanently deformed e.g paper clip
  • some objects are elastic when forces are small but behave inelastic when the forces are too big e.g springs
• to devise an investigation to measure the extension of a spring when force is applied

  • equipment : spring , masses and mass hanger , clamp , boss and clamp stand , ruler ( measure new length )
  • independent variable : force applied
  • dependent : extension of spring
• hooke's law : the extension of a spring ( or other object ) is the change in length when a force is applied

  the extension depends on :

  • the force applied
  • how stiff the spring is (the spring constant)
  • the relationship between forces and extension first become non-linear . this is known as the limit of proportionality
  • then the elastic limit is reached , where by the material will not return to its original shape

  
• rubber extension increases , force increases

  • shows a non-linear relationship between extension and force
  • stiffness of rubber varies
  • hooke's law is not obeyed

  
• spring constant

  • the spring constant for a spring is the force needed to produce on extension of 1 m
  • spring constant (N/m) = load (N) / extension ( m )
• energy transferred in stretching

  E = 1/2 x K x x^2

  energy transferred ( J) = 1/2 x spring constant (N/m) x extension^2 ( m^2)
• pressure in fluids

  atmospheric pressure : the pressure of air acting down on an object

  • pressure is due to weight of fluid above
  • hence the greater the height of atmosphere above the greater the pressure
• pressure at sea level

  • pressure is 100000 Pa / 100KPa
  • pressure is higher at sea level due to more particles exerting force in a set area
  • pressure decreases as you move higher as there are less particles exerting a force in a set area
• pressure ( Pa ) = force perpendicular to the surface ( N ) / area of the surface ( m 2 )
• pressure in fluids

  • pressure exerted by a fluid depends on the depth of the fluid
  • the deeper you are , the more weight of a fluid is above you to exert a pressure
• pressure also depends on the density of the fluid
• pressure due to a column of liquid ( Pa ) =

  • height of column x density ( kg/m 3 ) x gravitational field strength
  • p = h x p x g
• pressure is also additive

  total pressure = atmosphere + water
• upthrust : the upward force a liquid or a gas exerts on a body floating in it , equal to the weight of the fluid displaced