Download The Nature of Friction

The Nature of Friction
OR
Here’s where things start to get rough
(30S version)
Friction - a force that resists motion. It involves
objects that are in contact with each other.
Causes of Friction
•an object deforms a surface and vice versa
•a "plowing" force is required to move the
object over the deformations
•irregularities interlock and offer resistance to
movement
Starting friction - the maximum frictional force
between stationary objects
Sliding Friction - the frictional force between
objects that are sliding with respect to each
other
Properties of Friction
1. Friction acts parallel to the surfaces that are
in contact and in the direction opposite to the
motion of the object or to the net force tending
to produce such motion.
2. Friction depends on the nature of the materials
in contact and the smoothness of their surfaces.
3. Sliding friction is less than or equal to starting
friction.
4. Friction is practically independent of the
area of contact
5. Starting or sliding friction is directly proportional to
the force pressing the two surfaces together.
Coefficient of sliding friction - the ratio of the
force of sliding friction to the normal force

Ff
FN
Example 1: To move a 45 kg crate at a constant speed
along a surface, requires a 125 N force. What is the
coefficient of friction between the 2 surfaces?
Ff = FA= 125 N – constant velocity
FN = Fw= mg
= 45(9.8)
= 441 N

Ff
FN
125

441
  0.283
Example 2: A box weighing 450 N is pulled along a level
floor at constant speed by a rope that makes an angle
of 30 with the floor as shown. If the force on the rope is
260 N
a) What is the horizontal component (Fh) of this force?
b) What is the normal force (FN)?
c) What is the coefficient of sliding friction?
Fa=260
Ff
30
Fw = 450 N
Solution
a) Fh = Fa cos
= 260cos(300)
= 225N
c)  
Ff
FN
225

320
  0.703
b) The normal force
(FN) is the difference
between
the downward force of
the block's weight (Fw)
and the vertical
component of the force
of the rope (Fv).
FN = Fw - Fa sin 
= 450 - 260sin(300)
= 320 N