Friction & Applying Newton’s 2ndLaw
Chapter 6.2
System
What is Friction?
•Friction is a force that is parallel to thesurfaces of two objects that are in contactwith one another that resists the relativemotion of the two objects.
Ff
What causes friction?
•Friction is caused by the temporaryelectrostatic bonds created betweentwo objects in contact with oneanother.
Friction
•How does friction affect the motion ofobjects?
–It can slow an object down
like the friction between
the tires and the road.
–It is responsible for
increasing the speed of an
object like a car.
–It is also responsible
for objects being able to
change direction.
Static Friction (StationaryObjects)
System
Fapplied
Ffriction
Fground-on-crate
Fgravity
Fapplied
Ffriction
Fnet = Fapplied – Ffriction
Since the crate is not accelerating, Fnet = 0
Fapplied = Ffriction
Note: As long as the crate does not move, Fapplied = Ffriction
•Static Friction:
–The resistive force that keeps an object frommoving.
–Since v = 0, a = 0.
Kinetic Friction (Objects inMotion)
System
Fapplied
Ffriction
Fground-on-crate
Fgravity
•Kinetic Friction:
–The resistive force that opposes the relativemotion of two contacting surfaces that are movingpast one another.
–Fnet may or may not be 0.
Fapplied
Ffriction
Fnet = Fapplied – Ffriction
Fnet
Note: If the crate moves at a constantvelocity, then Fapplied = Ffriction and Fnet = 0.
Determining the Frictional Force(The Coefficient of Friction, )
•The force of friction (Ff) is proportional to the normal force (FN)and a proportionality constant ( - pronounced mu) called thecoefficient of friction.
•For static friction:
–0 < Ff, static < sFN
•For kinetic friction:
–Ff, kinetic = kFN
•As per the formula, the greater , the greater thefrictional force.
•Note: FN = the force normal (perpendicular) to thefrictional force on the object.
• is dimensionless (NO UNITS)
•Ff, static > Ff, kinetic
Ff
FN
The Normal Force
•The normal force is a force that most oftenopposes the Earth’s gravitational attractionand is perpendicular to the surface that anobject rests or is moving on.
–For a horizontal surface, FN = Fg = mg.
–For a surface that is not horizontal, as seen in thefigure below, FN = Fgcos
FN
The Normal Force
FN
Fg
FN
Fg
FN = Fg = mg
cos = adj/hyp
FN = Fg cos = mg cos
Example 2: Determining Friction(Balanced Forces)
•Assume that the man in the figure ispushing a 25 kg wooden crate across awooden floor at a constant speed of 1 m/s.
–How much force is exerted on the crate?
System
Fapplied
Ff
FN
Fg
Diagram the Problem
System
Fapplied
Ff
FN
Fg
Fapplied
Ff
FN
Fg
y-direction: FN = Fg
x-direction: Fnet = Fapplied - Ff
Since the crate is moving with constant speed,
a = 0, Fnet = 0, and Fapplied = Ff
+y
+x
State the Known and Unknowns
•What is known?
oMass (m) = 25 kg
oSpeed = 1 m/s
oAcceleration (a) = 0 m/s2
ok = 0.3 (wood on wood from referencetable)
•What is not known?
oFapplied = ?
Perform Calculations
•y-direction:
oFg = FN = mg(Note: horizontal surface)
•x-direction: Since a = 0, Fnet = 0
oFnet = Fapplied – Ff
oFapplied = Ff
oFapplied = kFN Fapplied = kmg
oFapplied = (0.3)(25 kg)(9.8 m/s2)
oFapplied = 73.5 N
0
Example 3: Determining Friction(Unbalanced Forces)
•Assume that the man in the figure is pushing a25 kg wooden crate across a wooden floor at aspeed of 1 m/s with a force of 73.5 N.
–If he doubled the force on the crate, what wouldthe acceleration be?
System
Fapplied
Ff
FN
Fg
Diagram the Problem
System
Fapplied
Ff
FN
Fg
Fapplied
Ff
FN
Fg
y-direction: FN = Fg
x-direction: Since a > 0, Fnet = Fapplied - Ff
+y
+x
State the Known and Unknowns
•What is known?
oForce = 147 N
oMass (m) = 25 kg
oSpeed = 1 m/s
ok = 0.3 (wood on wood)
•What is not known?
oa ?
Perform Calculations
•y-direction:
oFg = FN = mg
•x-direction: a > 0
oFnet = Fapplied – Ff
oma = Fapplied – Ff
oa = 147N – 73.5N
25kg
oa = 2.9 m/s2
Key Ideas
•Friction is an opposing force that existsbetween two bodies.
•Friction is proportional to the normalforce and the coefficient of friction;static or kinetic.
•The force required to overcome staticfriction is greater than that required toovercome kinetic friction.