Transcript Friction - AHS Physics

Friction
Objective
• Calculate static and kinetic friction.
• Calculate the angle of repose.
Homework
Size 2. Show your solution.
• Answer no. 97 of page 142.
• Answer no. 104 of page 143.
– Additional Question
– If the sled is on an inclined plane covered with
snow, what is the max angle of inclination that
would still make the sled stay at rest?
Types of Friction
• Static Friction
– This is the force exerted on one surface by
another when there is no motion between the
two surfaces.
• Kinetic Friction
– This is the force exerted on one surface by
another when the two surfaces rub against each
other because one or both of them are moving.
Types of Kinetic Friction
• Sliding Kinetic Friction
• Rolling Kinetic Friction
Friction
Ff = µFn
Normal
Force
Coefficient of Friction
Frictional Force
Coefficient of Friction
Ff = µFn
Fn
Kinetic Friction
Ffk = µkFn
Coefficient of Kinetic
Friction
Static Friction
Ffsmax = µsFn
Coefficient of
Kinetic Friction
The force required to start it moving. However,
you need MORE than this amount to effectively
move it.
Coefficient of Friction
Surfaces
µs
µk
Rubber on dry concrete
0.80
0.65
Rubber on wet concrete
Wood on wood
0.60
0.40
0.50
0.20
Steel on steel (dry)
0.78
0.58
Steel on steel (with oil)
0.15
0.06
Example
a. You push a 25.0-kg wooden box across a wooden
floor a constant speed of 1.0 m/s. How much
force do you exert on the box?
b. If the force you exert on the 25.0-kg box is
doubled, what is the resulting acceleration of the
box?
Note: µk = 0.200
a. 49.0 N
b. 1.96 m/s2
Seatwork Size 4
• A 60.0-kg crate rests on a level floor at a
shipping dock. The coefficients of static and
kinetic friction are 0.760 and 0.410,
respectively. What horizontal pushing force is
required to (a) just start the crate moving and
(b) to keep it moving at a constant speed.
Angle of Repose
• The angle of repose is the measure of an
angle of inclination wherein a body at rest
remains at rest and a body is motion will
travel at a constant speed.
Angle of Repose
Θrepose =
-1
tan (µ
)
s
Example
If a wooden block is at rest on an inclined plane
whose surface is made of wood. the coefficient
of static friction between them is 0.500. At what
maximum angle of inclination would the block
stay at rest?
Question
What is the angle of inclination is greater than
the angle of repose?
The body moves downward
at an increasing speed.
Seatwork Size 4
• A 60.0-kg crate rests on a level floor at a
shipping dock. The coefficients of static and
kinetic friction are 0.760 and 0.410,
respectively. What horizontal pushing force is
required to (a) just start the crate moving and
(b) to keep it moving at a constant speed.