chapter 5 is over terminal Velocity, force, friction,angles,drag force, and delta x

1. Chapter 5
Furter Applications of
Newton’s Laws

2. Frictional Forces
Friction has its basis in surfaces that are not
completely smooth:
Always against the direction of motion

3. Normal Force

4. Different Material
• The tip of a probe is deformed sideways by frictional force as the probe is dragged
across a surface. Measurements of how the force varies for different materials are
yielding fundamental insights into the atomic nature of friction.

5. Frictional Forces
Kinetic friction: the friction experienced by
surfaces sliding against one another
coefficient of kinetic friction.
Constant, no unit.
Depends on surface type only, not on surface area.
N: normal force

6. Frictional Forces
Static friction: the friction by surfaces when
there is a tendency to move against one another
coefficient of static friction.
Constant, no unit.
Depends on surface type only, not on surface area.
N: normal force
Nf ss µ=
sµ
Maximum:
sff ≤≤0

7. 6-1 Frictional Forces

8. Many people have experienced the slipperiness of walking on ice. However, many
parts of the body, especially the joints, have much smaller coefficients of friction—
often three or four times less than ice
Other natural lubricants include saliva produced in our mouths to aid in the
swallowing process, and the slippery mucus found between organs in the body,
post-op x rays of the right knee
joint replacement. (credit: Mike
Baird, Flickr)

9. The static frictional force keeps an object from
starting to move when a force is applied.
Frictional Forces
No friction
Static friction increase
to balance out pulling
force
Object start to move,
kinetic friction

10. Ex: sled resting on a patch of snow, µs =0.350,
m(sled+person)=38kg. Minimum force F to have the sled
moving. After the sled started moving v=4m/s, stop pulling on
the sled. How far will the sled go before stopping (µk= 0.05)
Free body diagram:
X:
Y:
130N,ax=-0.49m/s2
, x=16.3m
CYU: when car moving with tire rolling, static or kinetic
friction? When sliding?

11. ABS, when car about to slide, brake release and reapply in
rapid succession. Kinetic friction is less than static.

12. Friction forces, same or different?
Ex: crate rest on a plate, when it is raised to an angle of
23.2o
, crate begin to slide, µs=?

13. Ex: a child slide down icy hill θ=30o
, with constant
velocity. find the kinetic coefficient of friction.

14. Consider the 65.0-kg ice skater being pushed by two others
(a) Find the direction and magnitude of the total force exerted on her by the
others, given that the magnitudes F1 and F2 are 26.4 N and 18.6 N,
respectively.
(b) What is her initial acceleration if she is initially stationary and wearing
steel-bladed skates that point in the direction of F total?
(c) What is her acceleration assuming she is already moving in the direction
of F total? (Remember that friction always acts in the direction opposite that
of motion or attempted motion between surfaces in contact.)

15. Drag force
FD∝v2
FD=1/2CρAv2
,
C : drag coefficient,
A : area facing the fluid
ρ: the density of the fluid.
Object C
Airfoil 0.05
Toyota Camry 0.28
Ford Focus 0.32
Honda Civic 0.36
Ferrari Testarossa 0.37
Dodge Ram pickup 0.43
Sphere 0.45
Hummer H2 SUV 0.64
Skydiver (feet first) 0.70
Bicycle 0.90
Skydiver (horizontal) 1.0
Circular flat plate 1.12LZR racer: reduced 10%

16. Terminal Velocity
Fnet=mg−FD=ma=0.
mg=FD.
mg=12ρCAv2
.
Air density: ρ=1.21 kg/m3. m= 75-kg
skydiver descending head first A=0.18m2
;C=0.70.
V=98 m/s= 350 km/h
CYU:
1.Compare two tennis balls, on filled with iron pellet, one not, dropped
from a very tall building. Which one is going to terminal velocity first,
which one is going to reach ground first?
2.Why can a squirrel jump from tree and will not harmed, while human
can not?

17. Which way to push if boxes are fragile?
Acceleration and contact force ?
SKIP, Elasticity, Stress and Strain

18. Ex: m1=30kg, m2=10kg.
Find a and tension in string.

19. Ex: m1=20kg, m2=10kg. Find
acceleration and tension in the string.
m1m2