Forces

• force

  A push or pull that acts on an object due to its interaction with another object

  measured in newtons

  are vector quantities
• Contact forces

  Objects are physically touching

  e.g friction, air resistance, tension and normal contact force (when equal and opposite forces are applied)
• Non-contact forces

  Don’t require objects to be touching and can act through empty space

  e.g gravitational, magnetic and electrostatic
• scalars

  Only have magnitude (size)

  Measured with numerical value

  e.g speed, distance, mass, temperature and time
• Vectors

  Have both magnitude and direction

  e.g velocity, displacement, acceleration, force and momentum
• Free body diagrams

  Show all forces acting on an object

  drawn with force arrows representing all the forces

  can see magnitude form length of arrows

  can see direction from direction of arrows
• Resultant force

  Overall force on an object

  if no resultant force the object is at equilibrium
• Deformed

  When a object changes shape

  inelastic (plastic) = stays deformed

  elastic = returns to original shape
• Extension

  -Increase in length of a spring when it’s stretched

  -measure how springs length changes as we add downwards force

  natural length - stretched length = extension

  -force is directly proportional to extension

  -extension depends on spring constant
• Spring constant

  Tells us how many newtons needed to stretch the object by 1 meter

  so higher spring constant = stiffer material
• Hooke’s law

  force and extension are directly proportional

  but there is an elastic limit / limit of proportionality when Hooke’s law no longer applies + now inelasticly deformed
• Elasticity F=ke

  F = force

  k = spring constant

  e = extension
• Elasticity Ee=1/2ke squared

  Ee = elastic potential energy (J)

  k = spring constant

  e = extension
• Moments


  Rotational or turning effect of a force

  M = Fd

  d = perpendicular distance (
• Levers


  -Transmit the turning effect of a force

  -if output + input force are on different sides of pivot = act in different directions

  -if output + input forces are on same sides of pivot = act in same direction

  -output force is generally closer to the pivot = larger force
• Gears


  Transmit turning effects

  gears turn in opposite directions and are different sizes (gear A radius is 2x that of gear B = 2x turning effect of gear B)
• Pressure


  Force per unit of area

  P=f÷a

  Measured in pascals
• Pressure in fluids


  Particles move and collide with walls of containers and apply a force = increase pressure
• Perpendicular force


  Component of the force which is at a right angle to the surface its colliding with

  (Used for pressure)
• Liquid pressure


  P = hpg

  (h) height of column of liquid

  (p) density

  (g) gravitational field strength
• Upthrust


  Surrounding water exerts force on object in water

  As the bottom is deeper it will experience a larger upwards force than the top = rusultant force upward is upthrust
• Weight and upthrust


  If weight is larger object will sink

  If upthrust is larger object will float
• Factors affwcting pressure from liquid

  Depth increased = more pressure

  Density increased = more pressure

  Gravitational field which is 9.8N/m
• Atmospheric pressure


  Particles are more dense closer to earth = more collisions + force = more pressure

  As well weight of all particle above push down so force is larger = more pressure
• Speed


  Scalar so only has magnitude e.g distance is also scalar
• Velocitcy


  Vector so has both direction and magnitude e.g displacment is also a vector
• Speed equation


  S = d÷t

  Speed = distance ÷ time
• Velocity equation


  V = s÷t

  Velocity = displacment ÷ time
• Acceleration


  Rate of change in velocity (how quickly something speeds up or slows down)

  a= change in V ÷ t

  Change in v = final - initial
• Constant acceleration


  If accelerates at same rate while time
• 2as = v² - u²

  a = acceleration

  S = distance

  v = final velocity

  u = initial velocity
• Distant time graph


  Gradient of line at any point = speed of object

  Straight line = constant

  Curved up = acceleration

  Curved down = decceleration

  If curved draw tangent that huts that point
• Inertia 


  Tendency for the motion of an object to remain unchanged

  So unless acted on by force objects at rest stay at rest and in motion stay in motion
• Inertial mass


  How difficult it is to change an objects velocity

  F=ma

  So large mass e.g moon has lots of inertia
• Terminal velocity


  When an object is falling and the velocity increases as well as air resistance until its equal to weight so there is zero resultant force so no change in velocity
• Newtons 1st law


  Resultant force is required to change a motion of an object

  Works on both stationary and moving objects
• Newtons 2nd law


  If a non-zero resultant force acts on an object it will cause acceleration

  And resultant force is directly proportional to acceleration
• 2nd law on motion

  If stationary it starts to move it

  If moving right it speeds it up

  If moving left it slows it down

  If slowly moving left it stops it

  And it can change direction without changing speed = circular motion
• Newtons 3rd law


  When two forces interact the forces they exert are equal (magnitude) and opposite (direction)
• Stopping distance


  Minimum distance required to stop a vehicle in an emergancy

  Stopping d = thinking d + breaking d