physics forces

• Distance


  Can be measured in mm, cm, m and km
• Time


  Measured in ms, s, mins and hours
• Calculating speed


  speed = distance / time
• Vector


  Has magnitude and direction
• Scalar


  Has just magnitude
• Velocity


  Vector as it gives speed in a given direction
• Distance-Time Graphs


  • Gradient shows velocity
  • Steeper gradient = faster speed
  • Negative gradient = object returning to starting point
  • Horizontal line = object stationary
• Velocity-Time Graphs


  • Gradient shows acceleration
  • Steeper gradient = greater acceleration
  • Negative gradient = object decelerating
  • Horizontal line = constant speed
• Area under velocity-time graph

  Gives total distance travelled
• Calculating average speed


  average speed = total distance / total time
• Electrostatic interaction


  Force of attraction or repulsion between charged particles
• Gravitational attraction


  Force between particles with mass
• Contact forces


  Forces experienced in the opposite direction to contact, such as friction
• Free body force diagrams

  • Show direction of forces on an object
  • Reaction force acts normal to line of contact
  • Friction acts opposite to movement
  • Weight acts vertically downwards
• Scale drawings of forces

  • Length of arrows represents force magnitude
  • Larger arrow shows resultant force
  • Equal length arrows in opposite directions = equilibrium
• Newton's First Law


  An object has constant velocity unless acted on by a resultant force
• Resultant force acts on an object

  Object will accelerate
• Inertia


  Measure of how difficult it is to change an object's velocity based on its mass
• Momentum


  Depends on mass and velocity
• In an elastic collision, momentum and kinetic energy are conserved
• Newton's Third Law


  Every action force has an equal and opposite reaction force
• Work done


  Force x distance
• Stretching an object


  • Requires more than one force applied in opposite directions
• Elastic deformation


  Object returns to original shape when load removed
• Plastic deformation


  Object does not return to original shape when load removed
• Hooke's Law


  F = kx, where F is force, k is spring constant, x is extension
• Force-extension graph


  • Linear section obeys Hooke's Law
  • Elastic limit where linearity stops
  • Non-linear section shows non-elastic behaviour
• Calculating work done on a spring

  Work Done = 1/2 kx^2
• Gravitational field


  All matter with mass has a gravitational field that attracts other matter
• Weight


  Force exerted on a mass by a gravitational field, W = m x g
• Acceleration in free fall is due to gravity and is the same as g (9.8m/s^2)
• Gravitational potential energy


  GPE = m x g x h
• Moment of a force

  Force x perpendicular distance
• Equilibrium


  • Sum of anticlockwise moments = sum of clockwise moments
• Gears


  • Lower gear with fewer teeth turns faster with less force
  • Higher gear with more teeth turns slower with greater force
• Pressure


  Force / Area
• Pressure in fluids causes a net force at right angles to any surface
• Hydraulic brakes


  Force applied to narrow cylinder is multiplied in wider brake cylinders due to constant pressure in fluid