FRICTION
SLEDS, SANDPAPER, AND LOTS
of SLIDING
Friction
• Any force that resists motion
• It involves objects that are in contact with
each other.
• This is the force that keeps an object from
sliding down and incline plane.
• Some scientists believe that friction is
caused by uneven surfaces of the touching
objects – when rubbed together resistance is
offered.
• Experiments have shown that tiny particles
are actually torn from one surface and
imbedded in the other.
• If two surfaces were carefully polished,
there is a limit to the amount by which
friction may be reduced. If made too
smooth, the friction between them actually
increases.
Examples of Friction
•
•
•
•
Tires on the road
Walking/Running
Nails into wood
Sled on hill
Principles of Friction
• Friction acts parallel to the surface that are
in contact.
• The direction that friction acts is
OPPOSITE the direction of the motion (or
intended motion).
Fnormal
Ffriction(k)
Fapplied
Fweight
There are two types of friction:
Static Friction – the frictional force present
just before motion begins. Starting Friction
is when Static Friction is at is maximum.
Kinetic Friction – the frictional force
present with motion
Note: Static Friction is usually higher than
Kinetic Friction
• Friction depends on the nature of the
material in contact and the smoothness of
their surfaces.
• Static Friction is usually higher than Kinetic
Friction because it is harder to get
something started than it is to keep it going.
• Friction is practically independent of the
area of contact
The Coefficient of Friction
Frictional Force, both static and kinetic, is directly
proportional to the force pressing the two surfaces
together. The more weight, the more friction.
Ffriction  Fnormal
The missing link to the above equation is the
Coefficient of Friction, or μ:
Ffriction  Fnormal
static 
Ffriction (static)
Fnormal
kinetic 
Ffriction (kinetic)
Fnormal
PROBLEMS
A 75 kg crate is to be pushed up an incline plane
5 m long that makes an angle of 20° with the
horizontal. If the coefficient of static friction
between the crate and the inclined plane is 0.20,
how much force must be given to get it started
up the incline? If the coefficient of kinetic
friction is 0.15, how much applied force is
needed to keep it going at a constant speed up
the incline.
Given: m=75 kg; l=5 m; θ=20°; μs=0.20; μk=0.15
Fn=mg(cosθ)
Fp=mg(sinθ)
Ff
θ
Fw=mg
Fa
Fa  Fsf  Fp
Fsf   s Fn   s m g(cos )
Fp  m g(sin  )
 138.13 N
 251.38N
Fa  389.51 N
* note: thisis the threshhold. AT LEAST thismuch forceis needed
Fa  Fkf  Fp
Fsf   k Fn   s m g(cos )
Fp  m g(sin  )
Fa  354.98 N
* note: T hisis for CONST ANTspeed.
 103.60 N
 251.38N
Take into account the last problem. After you
have the crate moving up the incline at a
constant speed, you want to give it an
acceleration of 1.7 m/s2. What is the new
force needed to accomplish this task?
Fa  Fkf  Fp  m a
Fsf   k Fn   s m g(cos )  103.60 N
Fp  m g(sin  )
 251.38N
Fa  Fkf  Fp  ma
Fa  482.48 N
A 300 kg sled is pulled at constant speed over
a level, horizontal, snow covered surface. The
rope that is used to pull the sled makes a 30°
angle with the horizontal. If the coefficient of
friction is 0.10, find the force required.
Given: m=300 kg; θ=30°; μk=0.10
Fn
Fa
Fy (sin θ)
Ff
θ
Fx (cos θ)
Fw
Fax-Ff=max *note: ax is 0 because we are at a
constant speed
Fay+Fn-Fw=may *note: ay is 0 because the sled
never moves up and down