Forces

• Forces


  Measured in Newtons
• Types of forces

  • Contact forces
  • Non-contact forces
• Contact forces

  • Two surfaces in contact
  • Examples: friction, air resistance/drag, tension, normal contact force
• Non-contact forces


  • Act over distance
  • Examples: gravitational attraction, electrostatic force, magnetic force
• Vectors

  Have size and direction
• Vectors vs scalars

  Vectors can be represented by arrows, scalars only have size
• Mass


  Measured in kilograms
• Weight

  A force caused by the mass of an object in a gravitational field, measured in Newtons
• Converting mass to weight

  Weight (N) = Mass (kg) x Gravitational field strength (N/kg)
• Gravitational field strength

  Measured in Newtons per kilogram, depends on location (e.g. 9.8 N/kg on Earth, 1.6 N/kg on Moon)
• Measuring weight


  Use a Newton meter
• Measuring mass

  Use a mass balance
• Weight acts at the center of mass
• Resultant force


  Single force that represents the overall effect of multiple forces acting on an object
• For a balanced object, the resultant force is zero
• Resolving a force into components


  Find the vertical and horizontal components of the force
• Work


  Force applied over a distance, measured in Joules
• Elasticity


  An object's ability to return to its original shape after a force is applied and removed
• Elastic vs inelastic/plastic

  Elastic: returns to original shape, Inelastic/Plastic: does not return to original shape
• Spring stiffness

  k, measured in Newtons per meter
• Moment


  Turning effect of a force, measured in Newton-meters
• For a balanced object, clockwise moments = anticlockwise moments
• Pressure


  Force per unit area, measured in Pascals (N/m^2)
• Buoyancy/Upthrust


  Upward force exerted by a fluid on an object immersed in it
• Atmospheric pressure


  Pressure exerted by the weight of the atmosphere, decreases with height
• Scalar quantities


  Have size only, e.g. speed, distance
• Vector quantities

  Have size and direction, e.g. velocity, displacement, acceleration
• Velocity can change while speed remains constant (e.g. circular motion)
• V

  Speed or velocity, measured in meters per second
• s

  Displacement or distance, measured in meters
• T


  Time, measured in seconds
• V = s/T

  Speed is equal to distance divided by time
• Velocity can change while speed stays constant, such as in circular motion
• Distance-time graph


  • Gradient represents speed
  • Curved line requires drawing tangent to find speed
• Velocity-time graph


  • Gradient represents acceleration
  • Area under graph represents displacement
• Acceleration


  Rate of change of velocity, measured in meters per second squared
• Falling object under gravity reaches terminal velocity when weight equals air resistance
• Newton's First Law

  • Stationary objects remain stationary, moving objects continue at constant velocity with no net force
• Newton's Second Law


  • Acceleration is proportional to net force and inversely proportional to mass
• Newton's Third Law

  • Forces between interacting objects are equal and opposite