Download 6 Applications of Newton`s Laws 6-1 Frictional Forces

6 Applications of Newton’s Laws
6-1 Frictional Forces
What is friction?

F
To slide one such surface across another requires a
force large enough to overcome the resistance of
microscopic hills and valleys bumping together.
Kinetic Friction
For an object moving
f k  k N
 k : coefficient of kinetic friction;
independent of area
N: Normal force
Ski on snow 0.05
µ k=
Wood on leather 0.4
Rubber on concrete 0.8
Properties of kinetic frictional force
• Opposes motion in direction of surface
• Depends on smooth of surface
• Increase as force pressing surface
increasing
How do we apply?

F

fk

W

N

v   xˆ
f k   xˆ
y

fk

N
x

W

F
Ex.
Driving at 12 m/s slow on brake, m=1000kg,
the coefficient µk is 0.8, what is the
acceleration? How long to stop?
Static Friction
• For a stationary object

fs
V=0

F
 
 f s  F Object is static, until F>fs(max) object starts motion
f s (max)   s N  s   k
coefficient of static friction
Ski on snow 0.1
µs=
Wood on leather 0.5
Rubber on concrete 1-4
• Independent of the area of contact
• Static friction is not relative with the normal
force, (direction and magnitude), except fs(max)
• Direction of fs is parallel to the surface of
contact, and opposite to the direction the object
would move if there were no friction, (opposite
to the direction of the tendency of motion)
y
x
θ
Change the tilt angle θ, let the static friction get the
maximum value, but the box still rest, find the coefficient
of static friction?