 06 - Chapter 6 - Force and Motion 2 Fric...
06_-_Chapter_6_-_Force_and_Motion_2_Friction_Drag_Circular_Motion.pdf-Chapter 6 Force and Motion-II (Friction,
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06 - Chapter 6 - Force and Motion 2...
06_-_Chapter_6_-_Force_and_Motion_2_Friction_Drag_Circular_Motion.pdf-Chapter 6 Force and Motion-II (Friction,
##### Page 6
Sample Problem
Assume that the constant acceleration
a was due only to a
kinetic frictional
force on the car from the road, directed
opposite the direction of the car’s motion. This results in:
where
direction
of the kinetic frictional force.
Calculations:
The frictional force has the magnitude
f
k
=
μ
k
F
N
,
where F
N
is the magnitude of the normal
force on the car
from the road. Because the car is not accelerating vertically,
F
N
=
mg.
Thus,
f
k
k
F
N
= μ
k
mg
a =
f
k
/m =
μ
k
mg/m =
μ
k
g,
the negative direction. Use
where (
x
-
x
0
) = 290 m, and the final speed is 0.
Solving for v
o
,
We assumed that
v = 0 at the far end of the skid marks.
Actually, the marks ended only because the Jaguar left
the road after 290 m. So
v
0
was at least 210 km/h.

##### Page 7
Sample Problem: Friction applied at an angle

##### Page 8
Sample Problem: Friction applied at an angle

##### Page 9
6.4 The Drag Force and Terminal Speed
When there is a relative velocity between a fluid and a body (either because the body
moves through the fluid or because the fluid moves past the body), the body
experiences a
drag force,
D
,
that opposes the relative motion and points in the direction
in which the fluid flows relative to the body.

##### Page 10
6.4 The Drag Force and Terminal Speed
For cases in which air is the fluid,
and the body is blunt (like a
baseball) rather than slender (like a
javelin), and the relative motion is
fast enough so that the air becomes
turbulent (breaks up into swirls)
behind the body,
where
ρ
is the air density (mass per
volume),
A
is the
effective cross-
sectional
area of the body (the area
of a cross section taken
perpendicular to the velocity), and C
is the drag coefficient
.
When a blunt body falls from rest through air, the
drag force is directed upward; its magnitude
gradually increases from zero as the speed of the
body increases. From Newton’s second law along
y
axis
where m is the mass of the body. Eventually,
a
= 0,
and the body then falls at a constant speed, called
the
terminal speed
v
t
.

##### Page 11
Some typical values of terminal speed
6.4 The Drag Force and Terminal Speed