Transcript Physics 130 - University of North Dakota

10/1 Friction
Text: Chapter 4 section 9
HW 9/30 “Block on a Ramp” due Friday 10/4
Suggested Problems:
Ch 4: 56, 58, 60, 74, 75, 76, 79, 102
Talk about friction
Lab Questions?
Chapter 6 (energy) next
then 5 (circular motion) for next exam (10/17)
Static Friction
The maximum static friction force depends on the type of surface
and the contact force between the surfaces. The actual static
friction force depends on Newton’s second law!
fT,BMAX = sNT,B note subscripts!
s, called the “coefficient of static friction”, is a number that that
is small for slippery surfaces.
Note this is not a vector equation as N is 90° to f and only relates
their magnitudes.
Kinetic Friction:
Always acts parallel to the surface of
contact.
Relative motion of surfaces (sliding)
fa,b = kNa,b always
(Remember, static by second law only)
k is the coefficient of kinetic friction.
Jack pulls the box at constant
speed
Draw a FBD of the box.
a = ?0
v=
Since a = 0, Fnet = 0 and
TS,B = fF,B = kNF,B
NF,B
fF,B
TS,B
Box
WE,B
Jack pulls harder, what
happens?
Kinetic Friction
Jack pulls harder
Draw a FBD of the box.
a=?
v=
Fnet = TS,B - fF,B = ma
NF,B
y
fF,B
TS,B
x
Box
WE,B
The tension force increases
but the kinetic friction force
stays the same!
Kinetic friction does not
depend on velocity or
acceleration.
fF,B = kNF,B
Jack pulls with the same force
but in a different direction.
Draw a FBD of the box.
a=?
v=
NF,B
NF,B
fF,B
fF,B Box
TS,B
WE,B
How will the string angle
change the FBD?
Tension force now has a
vertical component which
means NF,B is smaller.
fF,B is proportional to the
normal force so it is smaller
also.
Static vs. Kinetic Friction
Static
fF,Bmax = sNF,B
Equation only gives the
maximum possible, not
what is is.
Find the force with the
2nd and 3rd laws always!
Kinetic
fF,B = kNF,B
Equation gives what is is.
Still may need to use the
2nd and 3rd laws, but
will always satisfy the
above equation.
Homework Problem
y
A 70 kg skier skis down a slope that
is angled 36.8° with respect to the
horizontal. The coefficient of
kinetic friction between the skis and
the snow is 0.10. What is the skiers
acceleration?
x
36.8°
Start with FBD, find Wx and Wy, apply
2nd law to x and y separately (hint:
must start with y first) (hint2: what is
ay?
If the instantaneous velocity is 16m/s, how far up the slope
did he start from rest?