The Force of Friction

1
The Force of Friction
2
What Do We Already Know about forces?

• Newtons Laws
• An object at rest remains at rest until acted
  upon by an unbalanced force
• Fma
• Every force has an equal and opposite force

3
Friction Opposes the Applied Force

• Static Friction
• The resistive force that keeps an object from
  moving
• As long as an object does not move, the force of
  static friction is always equal to and opposite
  in direction to the applied force.

Fs -Fapplied
F
Fs

• When the applied force is as great as it can be
  without causing the object to move, the force of
  static friction reaches its maximum value, Fs,
  max

4
What happens when the applied force on an object
exceeds Fs,max?

• The object begins to move with an acceleration in
  the direction of the applied force.
• There still is a frictional force, but the force
  is less than Fs,max.
• The frictional force on the object is then called
  the force of kinetic friction (Fk)

a
Fk
F
5
Coefficient of Friction

• Frictional forces depend on the surface on which
  the object is in contact with.
• The coefficient of friction is the ratio of the
  force of friction to the normal force acting
  between two objects (µ ).

Fk µkFn
Fs,max µsFn
6
WHEN DOES THE CAR BEGIN TO MOVE AT A CONSTANT
SPEED?
Weak applied force, car remains at rest
No applied force, car at rest
Fn
Fn
Fa
Fs
Fg
Fg
Fs is less than µsFn
Stronger applied force, car just about to move
Car moving at a constant speed
Fn
Fn
a
Fa
Fa
Fs
Fs
Fg
Fg
Fs µsFn
Fk µkFn
7
What friction force is stronger?
What is the frictional force on a 10kg rubber
block on concrete just before it begins to slide?
µs0.90
Fn(9.81m/s2)(10kg)98.1N
Fs, max µsFn (0.90)(98.1N)88.29N
What is the frictional force on a 10kg rubber
block on concrete when it moves at a constant
acceleration?
µk0.68
Fn(9.81m/s2)(10kg)98.1N
Fk µkFn (0.68)(98.1N)66.71N
8
Practice Problem

• A 24kg crate initially at rest on a horizontal
  floor requires a 75N horizontal force to set it
  in motion. Find the coefficient of static
  friction between the crate and the floor.

Fn
Fs,max 75N
m24kg
Fs,max µsFn
24kg
Fs,max
Fa
µs Fs,max _ 75N_____ 0.32 Fn
24kg x 9.81m/s2
Fg
9
Practice Problem Overcoming Friction

• A student moves a box of books by attaching a
  rope to the box and pulling with a force of
  90.0N. The box of books has a mass of 20.0kg,
  and the coefficient of kinetic friction between
  the bottom of the box and the sidewalk is 0.50.
  Find the acceleration of the box.

Fa
Fg mg 20kg9.81m/s2 196.2N SFy -Fg Fn
-196.2N Fn 0 Fn 196.2N
20kg
Fa
Fk
Fa 90.0N SFx -Fk Fa ma SFx -Fnµk 90.0N
ma SFx-196.2N0.50 90.0N20kga a
-196.2N0.50 90.0N -9.41m/s2
20kg
Fg