Friction

1
Friction
Physics 131 - Physics for Health, Life and
Environmental Sciences

• At a microscopic level surfaces are not as smooth
  as they seem.
• When stationary they lock together cold
  welding.
• If a force is applied to the object, the surface
  pushes back until the object pops out of the
  groove.
• Thereafter the object hops along the surface,
  making it easier to push.

Ffr
Fapp
2
Friction
Physics 131 - Physics for Health, Life and
Environmental Sciences

• Two types of friction
• Static Friction
• Reaction Force
• Has a threshold above which the object begins to
  move
• Kinetic Friction
• Occurs when the object is in motion
• Constant force
• What effects friction
• The roughness of the surface
• The weight of the object

Ffr
Fapp
µs - Coefficient of static friction µk -
Coefficient of kinetic friction
3
Friction
Physics 131 - Physics for Health, Life and
Environmental Sciences

• A tractor intends to move a block of mass of
  150kg from the side of a road. µs 0.3 and µk
  0.15
• What force will the tractor have to apply in
  order to begin moving the block?
• What force will be required to move the mass at
  constant velocity thereafter?

4
Friction
Physics 131 - Physics for Health, Life and
Environmental Sciences

• A car of mass 1000kg has coefficients of friction
  µs 0.3 and µk 0.15 between its tires and the
  road.
• What is the maximum acceleration it can achieve?

5
Friction
Physics 131 - Physics for Health, Life and
Environmental Sciences

• A box of mass 8kg is pushed along a surface at
  constant velocity with a force of 20N. Find the
  coefficient of kinetic friction between the box
  and surface.

20N