Physics 121C Mechanics Lecture 6 Forces Change Motion October 11, 2004

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Physics 121C - MechanicsLecture 6Forces
Change MotionOctober 11, 2004

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

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Announcements

• The due date for Homework Assignment 1 has been
  rescheduled for 900 PM on Wednesday, October 13.
  Homework up to 24 hours late will receive 70
  credit.
• Homework Assignment 2 has now been placed on
  Tycho, can be done any time, and is due by 900
  PM on Wednesday, October 20.
• 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.

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Lecture Schedule (Part 1)
You are here!
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Defining Force

• Questions
• What is a force?
• What is the connection between force and motion?
• Definitions
• A force is a push or pull on an object.
• A force is a vector. It has both magnitude and
  direction.
• A force requires an agent. Something must do the
  pushing and pulling.
• A force is either a contact force or a long-range
  force. Gravity is the only long-range force we
  will consider until Physics 122..

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Drawing Force Vectors
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Examples of Force Vectors
Pull (contact force)
Push (contact force)
Gravity (long-range force)
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Two Forces Applied to a Box
superposition of forces
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Clicker Question 1
Two forces are exerted on an object.Which third
force wouldmake the netforce point to the left?
(a)
(b)
(c)
(d)
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A Catalog of Forces (1. Weight)
Weight The falling box is pulled toward the
Earth by the long-range force of gravity. The
gravitationa pull on a object on or near the
surface of the Earth is called weight, for which
we use the symbol w. Weight is the only long
range force that we will consider in Physics 121.
The agent for the weight force is the entire
Earth pulling on the object. Weight acts equally
on objects at rest or in motion. The weight
vector always points vertically downward, and can
be considered to act on the center of mass of the
object.
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A Catalog of Forces (2. Spring)
Spring Force A stretched or compressed
spring exerts one of the most common contact
forces. A spring can either push (when
compressed) or pull (when stretched). In either
case, the tail of the vector force is attached to
the contact point. There is no special symbol for
the spring force, but we can use Fsp.
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A Catalog of Forces (3. Tension)
Tension Force A string or rope exerts a
contact force on an object when it pulls on it.
We call this a tension force, representd by the
symbol T.
Microscopic View A powerful microscope would
see that the string was made up of atoms
connected by molecular bonds, that can be thought
of as tiny springs holding the atoms together.
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A Catalog of Forces (4. Normal)
Normal Force A solid object resists the
action of another force which compresses it with
what we call the normal force. The normal force
always acts outward and perpendicular to the
surface of the compressed object. The symbol for
the normal force is n.
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A Catalog of Forces (5. Friction)
Friction At the molecular level, surfaces
tend to stick together, impeding motion. This
produces the force we call friction. It comes in
two varieties Kinetic friction, denoted by the
symbol fk , appears when an object slides across
a surface. This is a force that opposes the
motion and points in the opposite direction from
the velocity. Static friction denoted by the
symbol fs , is a force that keeps an object at
rest stuck to a surface and prevents its motion.
It points in the direction tha prevents motion.
Typically, it is larger than the kinetic friction
that appears after the object begins to move.
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A Catalog of Forces (6. Drag)
Drag Force A friction-like force that
opposes motion in a gas or liquid. Like kinetic
friction, it appears in the direction opposite
that of the motion. The symbol for drag is D.
Example air resistance.
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A Catalog of Forces (7. Thrust)
Thrust A contact force produces, for
example, when the exhaust gases press against the
inner surfaces of a jet engine. It has no
special symbol, so we will use Fthrust .
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A Catalog of Forces (8. EM)
Electric and Magnetic Forces Electricity
and magnetism, like gravity, produce long range
forces. We will study them in Physics 122. For
now, it is worth noting that the forces that hold
atoms and molecules together, molecular bonding,
are not little springs but electrical forces
acting between charged particles.
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Identifying Forces
? Identify the system and the environment.
The system is the object whose motion you wish to
study the environment is everything else. ?
Draw a picture of the situation. Show the
objectthe systemand everything in the
environment that touches the system. Ropes,
springs, and surfaces are all parts of the
environment. ? Draw a closed curve around the
system. Only the object is inside the curve
everything else is outside. ? Locate every point
on the boundary of this curve where the
environment touches the system. These are the
points where the environment exerts contact
forces on the object. ? Name and label each
contact force acting on the object. There is at
least one force at each point of contact there
may be more than one. When necessary, use
subscripts to distinguish forces of the same
type. ? Name and label each long-range force
acting on the object. For now, the only
long-range force is weight.
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The Forces on a Bungee Jumper
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The Forces on a Skier
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The Forces on a Rocket
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A Virtual Experiment (1)
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A Virtual Experiment (2)
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A Virtual Experiment (3)
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A Virtual Experiment (4)
Conclusion Acceleration is proportional to force
and inversely proportional to the number of
objects (mass). Therefore
a F/m
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Newtons Second Law
Newtons 2nd Law An object of a given mass m
subjected to forces F1, F2, F3, will undergo an
acceleration a given by a
Fnet/m where Fnet F1 F2 F3 The mass
m must be positive so that force and acceleration
are in the same direction.
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Clicker Question 2
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Newtons First Law
Newtons 1st Law In the absence of external
forces, an object at rest remains at rest an
object in motion remains in motion.
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Inertial Reference Frames
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End of Lecture 6

• Before the next lecture, read Knight,Chapters
  4.7 through 5.3
• Homework Assignment 1 should be submitted on
  the Tycho system by 900 PM, next Wednesday, Oct.
  13.(24 hours late Þ 70 credit)
• Homework Assignment 2 has been posted on Tycho
  and can be done any time.
• If you have not already done so, registeryour
  clicker athttp//faculty.washington.edu/jcramer/
  ph121c/Clicker