Slipping & Sliding (College Board AP® Physics 1: Algebra-Based)

Conditions for slipping & sliding

• Slipping and sliding refer to situations in which two surfaces are moving relative to each other

  • Sliding occurs when two surfaces in contact move relative to each other and kinetic friction applies

  • Slipping is a special case of sliding and occurs when an object that normally rolls does not, as covered in the study guide on Rolling

• Static friction applies to an object that is rolling

  • Considering a rotating bike wheel, each point on the wheel makes contact with one point on the road as it rolls along

Slipping

• Slipping occurs when an object moves translationally but without rotation, so it does not roll

  • When the brakes are applied hard to a bike wheel the surface of the wheel slides over the ground and kinetic friction now applies

  • The center of mass of an object moves faster translationally than the point of contact with ground

• Static friction adopts the value and direction required to prevent an object from slipping or sliding on a surface

Derived equation

• There exists a maximum value for which static friction will prevent an object from slipping on a given surface

Derivation:

Step 1: Identify the fundamental principle

• The static friction force equation is:

• Where:

  • = magnitude of static friction, measured in newtons

  • = coefficient of static friction

  • = normal reaction force between the two contact surfaces, measured in newtons, in

Step 2: Apply the specific conditions

• The magnitude of static friction applied on an object increases until it reaches the point of slipping

• When an object is right on the point of slipping static friction is at its maximum magnitude

  • Equal in magnitude and opposite in direction to any applied force

• The inequality means that the maximum value of static friction is equal to the product of the coefficient of static friction and the normal force

• It is not possible for the magnitude of static friction to be greater than the product of the coefficient of static friction and the normal reaction force because this would mean

  

  • The object would then move in the direction of the static friction