Physics 121C Mechanics Lecture 8 More Newtons Laws October 15, 2004

1
Physics 121C - MechanicsLecture 8More
Newtons LawsOctober 15, 2004

• John G. Cramer
• Professor of Physics
• B451 PAB
• cramer_at_phys.washington.edu

2
Announcements

• Homework Assignments 2 and 3 are both due by
  900 PM on Wednesday, October 20. Homework up to
  24 hours late will receive 70 credit.
• Because of a scheduling conflict, from now on I
  will have my office hours in the Study Center on
  Monday and Friday (not Wednesday) before class
  from 930 to 1020 AM. The Physics 121C Syllabus
  on the Web has been modified to reflect this
  change.
• Exam 1 will be held in this room on October 22,
  one week from today. We will have a review of
  the material covered on the exam on October 20.

3
Lecture Schedule (Part 1)
You are here!
4
Clicker Question 1
A Martian Lander is approaching the surface of
Mars. It is slowing its descent by firing its
rocket motor. Which of the free-body diagrams on
the right correctly describes the system?
5
Mass and Weight
Mass and weight are not the same thing, but
they are related. Consider thr falling body
represented by the free-body diagram to the
right. The only force acting on it is its
weight, the downward pull of gravity, so Fnet
w. From Newtons 2nd Law, Fnet w ma
Galilelo discovered that any
object,regardless of its mass, has the same
acceleration afree-fall (9.80
m/s2, downward) (g, downward) Therefore,
w (mg, downward). The magnitude of the weight
force, in other words, the weight, is directly
proportional to the mass m, where g is the
constant of proportionality, so that w mg.Note
that m is invariant, while g depends on location.
6
Mass Measurements
Beam BalanceMass measurement
Spring ScalesWeight measurements
7
Apparent Weight
8
Weightlessness
If an object falls so as to freely respond to the
force of gravity, its acceleration in the
vertical y direction will be ay -g. Therefore,
the apparent weight of the object is
wapp w(1 ay/g) w(1 - g/g) 0 In this
situation, the apparent weight of the object
vanishes and it is weightless.

Does weightlessness mean that the object has
no weight, i.e. that no gravitational force is
acting on it? NO. The force of gravity is still
present, but since the object and its
surroundings are freely responding to it, the
apparent acceleratrion and force are
zero. Example A man is on a spring scale
in an elevator. The cable breaks, and the
elevator falls without friction(ay-g). On the
way down, what will he see as his weight, as
indicated by the scale? Zero.
9
Static Friction
10
Coefficients of Friction
fs Fpush
Object does not slip.
fs msn
fs max msn
Object begins to slip.
fk mkn
Typically, msgtmk
11
Kinetic Friction
12
Rolling Friction
13
A Model of Friction
14
Friction ExampleHow far does a box slide? (1)
Carol pushes a 10.0 kg wood box across a
wood floor at a steady speed of 2.0 m/s. How
much force does Carol exert on the box? If she
stops pushing, how far will the box slide before
coming to rest?
15
Friction ExampleHow far does a box slide? (2)
16
Friction ExampleDumping a file cabinet (1)
Steel on dry steel Þ
Free-body diagram
A 50.0 kg steel file cabinet is in the back
of a dump truck. The trucks bed, also made of
steel, is slowly tilted. What is the size of the
static friction force when the trucks bed is
tilted by 200? At what angle will the file
cabinet begin to slide?
17
Friction ExampleDumping a file cabinet (2)
File cabinet will begin to slide when
18
The Causes of Friction
At the microscopic level, friction is the
collision of the high points of one rough surface
jammed against the high points of another rough
surface as an object slides over a surface.
At the molecular level, friction is the repeated
bonding and breaking of molecular bonds between
the two materials. Lubrication places a thin
film of liquid between the two materals and
allows one to float past the other without
direct contact, reducing the friction force.
Friction is a very complex phenomenon, which in
Physics 121 we will treat with simple and rather
naïve models.
19
Drag
The cross-sectional area A

• The drag force D appears when an object moves
  through a liquid or a gas.
• It increases in magnitude with increasing
  velocity v.
• It acts in the v direction.
• In Physics 121, we will use a simple model
  for drag that is approximately valid only under
  the following conditions
• The objects size must be between a few
  millimeters and a few meters.
• The objects speed must be less than a few
  hundred meters per second.
• The object is moving through air near the
  Earths surface.
• We then will assume that the drag force has
  the following form
• D -¼A v2 (v) AÞm2, vÞm/s

20
Air Resistance vs.Rolling Friction
A typical 1500 kg passenger car has a front
profile of 1.6 m by 1.4 m. At what speed does
the magnitude of the drag equal the magnitude of
the rolling friction?
21
Drag Force on a Ball

• The rising ball slows down more rapidly than it
  would in a vacuum.
• The falling ball speeds up more slowly than it
  would in a vacuum.

22
End of Lecture 8

• Before the next lecture, read Knight,Chapters
  6.1 through 6.4.
• Homework Assignments 2 and 3 should be
  submitted on the Tycho system by 900 PM,
  Wednesday, Oct. 20.(24 hours late Þ 70 credit)
• If you have not already done so, registeryour
  clicker athttp//faculty.washington.edu/jcramer/
  ph121c/Clicker