P5 - Forces

• Scalar - has magnitude only
• Vector - has magnitude and direction
• Contact force - occurs when the two objects are actually touching one another
• Non-contact force - occurs when objects are not touching eachother
• Contact forces:

  • Friction
  • Air resistance
  • Tension
  • Upthrust
• Non-contact forces:

  • Gravitational force
  • Electrostatic forces
  • Magnetic force
• Gravity - a force of attraction between all masses
• Mass - relates to the amount of mass an object contains and is constant
• Weight - the force acting on an object due to gravity. Depends on grav field strength and is directly proportional to its mass
• Weight of an object may be considered to act at a single point known as centre of mass
• Resultant force - when more than one force acts on an object, these can be seen as a single force that has the same effect as all the forces acting together
• Vector diagrams:

  • Add the length of arrows together to get the resultant force
  • If they are in different directions then the resultant force is diagonal
• Work done is measured in Joules
• Force is measured in Newtons
• Work done - when a force causes an object to move
• To change an object's shape, more than one force must be applied
• Elastically deformed - an object returns to its original shape after the forces have been removed
• Inelastically deformed - the object does not return to its original shape
• Extension is directly proportional to the applied force
• Once limit of proportionality has been exceeded:

  • Doubling the force will no longer exactly double the extension
  • The relationship becomes non-linear
  • A force extension graph will stop being a straight line
• Spring constant - indicates how easy it is to stretch or compress a spring. The higher the spring constant, the stiffer the spring
• In a spring:

  • A force that stretches or compresses it stores elastic potential energy in it
  • The amount of work done and the energy stored are equal (as long as the spring is not passed the limit of proportionality)
• Force/Extension Required practical:

  1. Hang a spring from a stand and hang a mass holder from it
  2. Add 100g to the holder
  3. Measure and record the extension with a ruler
  4. Repeat with a range of masses from 100g to 1000g
  5. Repeat and calculate means
• Moment - the turning effect when a force causes an object to rotate around a pivot point.
• Moment is measured in Newton Metres
• Levers and gears can be used to:

  • Transmit the rotational effects of forces
  • Magnify the size of the applied force
  • Magnify the distance the force moves over
• When the applied force moves further than the transmitted force, the force is increased
• When the applied force is greater than the transmitted force, the distance is increased
• Pressure in a fluid:

  • As fluid particles move, they collide with the surface of objects or the container
  • Collisions create a force normal (right-angled) to the surface
• Pressure is measured in Pascals
• Atmospheric pressure:

  • Atmosphere - a thin layer of air around the earth
  • The greater the altitude, the less dense the atmosphere and the lower the pressure
• Pressure at a particular point in a column of liquid depends on:

  • Height of the column
  • Density of the liquid
• The higher the column and more dense the liquid above a point:

  • The greater the weight above the point
  • The greater the force on the surface at that point
  • The greater the pressure
• Upthrust:

  • When an object is submerged in liquid, there is more liquid above the bottom surface than the top
  • The bottom experiences greater force than the top and this creates a resultant force upwards
  • The upwards force is called upthrust
• An object floats when its weight is equal to the upthrust and sinks when weight is greater than upthrust
• Density of an object indicates if it will float or sink
• An object less dense than the liquid:

  • Displaced a volume of liquid greater than its own weight so it will rise to the surface
  • Will float with some of the object remaining below the surface
  • Displaces liquid of equal weight to the object
• If an object has low density, more of it will remain below the surface
• An object denser than the surrounding liquid sinks
• Distance:

  • Scalar
  • How far an object moves
  • Does not count direction