Law of motion

• Static friction


  Force between two surfaces that prevents those surfaces from sliding or slipping across each other
• Static friction

  • Allows you to accelerate when you run, or let a car make a turn around a corner
  • Changes its value based on how much force is applied to the object
• Kinetic friction


  Force between two surfaces that are sliding or slipping across each other
• Kinetic friction


  • Always opposes the sliding motion and reduces the speed of sliding motion
• Coefficient of friction


  Characterizes the roughness between the two surfaces in contact
• Coefficient of friction


  • Depends only on the two surfaces in contact
  • Higher coefficient will prevent the object from moving more effectively
• Exercise


  1. Given information

  2. Draw force diagram

  3. Determine static and kinetic frictional forces
• An initially stationary 110 kg box sits on the floor
• The coefficient of static friction μs,max between the box and the floor is 0.60
• The coefficient of kinetic friction μk is 0.40
• A person pushes the box with the following forces: (i) Fpush = 400 N, (ii) Fpush = 600 N, (iii) Fpush = 800 N
• Law of acceleration and mass


  For an object of fixed mass M, a net force acting on the object will give it an acceleration directly proportional and the in the same direction as the net force
• Law of action and reaction 


  For every action, there is an equal and opposite reaction
• Law of inertia


  An object at rest will remain at rest, and an object in motion will remain in uniform motion unless an unbalanced force acts on it
• Frictional force


  An object moving on a surface experiences an apposing force that tends to slow down the motion