Download Lecture 9

SI
M 9:30 - 10:30 AM
CR 138
M 12:00 - 1:00 PM
CR 136
W
12:00 - 1:00 PM
CR 136
W
1:15 - 2:15 PM
CR 138
12:00 - 1:00 PM
CR 136
F
Forces
Observe and Represent
Any push or pull, any interaction of objects
Force is a vector!!!
F
How to measure forces?
[F] = N (Newton)
Represent: Free Body Diagram
FH on B

Represent an object with a dot (a point
particle) and use an arrow to represent
each interaction of another object with the
object of interest. The tail of the arrow is
on the dot.

F
Label each force arrow with an
that
has two subscripts. The first subscript is
for the object that exerts a force on the
object of interest, and the second
subscript is for the object of interest.
Pay attention to the lengths of the arrows
on each free-body diagram.
Forces
Pick up a notebook and
hold it stationary in your
hand. Then pick up a
textbook and hold it the
same way. Do you feel
any difference?
For each situation
(notebook and textbook)
draw the free-body
diagram.
Free-body diagram
Notebook

FHonNB

FEonNB
Textbook

FHonTB

FEonTB
The textbook is now on the
inclined board. Draw a free body
diagram for the book
Textbook
N

FHonTB

FEonTB
θ
W
Normal force is always
perpendicular to the surface
Net Force
Vector sum of all forces
exerted on an object

Net Force  F
Applied Force
5N
5N
5N
5N
5N
10N
10N
0N
5N
Force and Motion
Experiment
Steadily push the back of
a cart along a smooth
surface so that it moves
faster and faster
Stop pushing the cart and
let it coast. The cart does
not slow down
Push gently on the front
of the moving cart so that
it moves slower and
slower
A motion diagram
for the cart

v

v

v

v

v  0

v

v

v

v
A free-body diagram
for the cart
Ffr
FTonC
FHonC
FEonC

v
FTonC
FEonC
Ffr
FTonC
FHonC
FEonC
Force and Motion
The object always changes its motion in
the direction of the net force.
The direction of the acceleration is in
the same direction as the net force.
Force and Motion
Aristotle:
The normal state of objects is one of rest
Galileo: ???
Galileo Galilei
(1564 – 1642)
Slope downward – speed increases
Slope upward – speed decreases
No slope – does the speed change?
Aristotle:
The normal state of objects is one of rest
Galileo: ???
Galileo Galilei
(1564 – 1642)
Where is the final position?
Initial position
Final position
Galileo:
There is property of a moving object to
continue moving. This property called
inertia
Galileo Galilei
(1564 – 1642)
Isaac Newton
(1642 - 1727)
Newton’s First Law
Every object continues in its state
of rest, or uniform motion in a
straight line, unless it is compelled
to change that state by forces
impressed upon it.
Mass
The greater the mass of an object
the greater its inertia
[m] = kg
Weight of an object is the force
of gravity acting on the object
[W] = lb
N
Mass ≠ Weight
A steel ball is attached to a string and is
swung in a circular path in a horizontal plane
as illustrated in the accompanying figure. At
the point P indicated in the figure, the string
suddenly breaks near the ball. If these
events are observed from directly above as
in the figure, which path would the ball most
closely follow after the string breaks?
No Homework!!!
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