Transcript PowerPoint

Physic² 121:
Fundament°ls of Phy²ics I
September 22, 2006
D. Roberts
University of Maryland
PHYS 121
Outline
• Worked Example Problem
• More Newton’s Laws of Motion
D. Roberts
University of Maryland
PHYS 121
Heads-Up on Problem 9
• For the part of the problem where you are asked
to determine the two times, it is possible for
WebAssign to mark on time as correct and the
other time incorrect even though the two times
add up to 90s.
• This is not a bug in WebAssign: Issue is that
WebAssign will count a numerical answer as
correct as long as it is within 1~2% of the correct
answer. So, the time marked as correct isn’t
REALLY correct, and that throws off the other
time
D. Roberts
University of Maryland
PHYS 121
Chapter 2, Problem 58
• A ranger in a national park is driving at 35.0 mi/h
when a deer jumps into the road 200 ft ahead of
the vehicle. After a reaction time t the ranger
applies the brakes to produce an acceleration
a = -9.00 ft/s2. What is the maximum reaction
time allowed if she is avoid hitting the deer?
D. Roberts
University of Maryland
PHYS 121
Newton’s First Law
• An object at that is at rest will remain at rest, or
an object this is moving will continue to move in
a straight line with constant velocity, if and only if
the net force acting on the object is zero
• New ideas we need to understand:
– Force
– Net Force
• This Law basically says no cause is needed for
an object to move.
– Uniform (constant) motion is an object’s natural state
Translation from original Latin: “Every body perseveres in its state of
rest, or of uniform motion in a right (straight) line, unless it is
compelled to change that state by forces impressed thereon”
D. Roberts
University of Maryland
PHYS 121
Newton’s Second Law
• The acceleration of an object is directly proportional to
the net force acting on it and inversely proportional to its
mass.
– F and a are both vectors
• Can also be applied three-dimensionally
D. Roberts
University of Maryland
PHYS 121
Newton’s Third Law
• If object 1 and object 2 interact, the force
exerted by object 1 on object 2 is equal in
magnitude but opposite in direction to the
force exerted by object 2 on object 1.
– F12  F21
– Equivalent to saying a single isolated force
cannot exist
D. Roberts
University of Maryland
PHYS 121
Newton’s Third Law cont.
• F12 may be called the action
force and F21 the reaction
force
– Actually, either force can be
the action or the reaction
force
• The action and reaction
forces act on different
objects
D. Roberts
University of Maryland
PHYS 121
Gravity as a Force
• Question: Does gravity affect all objects the same?
– How do you know?
– If not the same, what does it depend on?
– (book, paper)
• Experimentally, it has been observed that all objects
experience the same acceleration due to gravity near the
surface of the earth, when gravity is the only force
involved (i.e. neglecting air resistance)
– Galileo dropped objects of different mass from the Leaning
Tower of Pisa to show this
– Test…
• The acceleration is:
– g = 9.8 m/s2
– Note: units of acceleration (not force)
– If we call “up” positive, then the acceleration is:
• a  g
D. Roberts
University of Maryland
PHYS 121
Gravity as Force
• Gravitational Force
– Fgrav = -mg
(from N2, F = ma with a = -g)
– Be careful of sign! g is a positive number!
– Value of g would change if you weren’t on surface of
earth (on the moon, for example)
– Value of mass doesn’t change
• Weight is magnitude (absolute value) of grav
force, mg
– Unit of weight is Newton (just like force)
D. Roberts
University of Maryland
PHYS 121
How smart is your table?
Consider a book sitting calmly on a table.
Does the table (an inanimate object) exert
a force on a book that is resting on it?
Yes, an upward force exactly
equal to the weight of the
book.
No, an inanimate object
cannot exert a force.
It’s just “in the way.”
Yes, a frictional force.
Yes, a Newton-3 force – a
reaction
to the action of gravity on the
block.
No, there doesn’t need to be
a force for the table to hold it
up.
None of the above
1
2
3
4
5
an
o,
N
Y
es
,a
n
6.
fo
rc
ni
...
m
es
a
te
,a
ob
fr
Y
ic
es
j..
t
.
io
,a
n
N
al
ew
N
fo
o,
to
rc
th
ne.
er
3
fo
e
do
rc
e.
es
.
n’
N
on
tn
e
ee
of
...
th
e
ab
ov
e
5.
Y
3.
4.
17% 17% 17% 17% 17% 17%
ar
d
2.
up
w
1.
in
a
?
Some Action-Reaction Pairs
•
n and n '
– n is the normal force, the
force the table exerts on the
TV
– n is always perpendicular to
the surface
– n 'is the reaction – the TV on
the table
– n  n '
D. Roberts
University of Maryland
PHYS 121
More Action-Reaction pairs
• Fg and Fg'
–Fg is the force the
Earth exerts on the
object
'
F
– g is the force the
object exerts on the
earth
– Fg  Fg'
D. Roberts
University of Maryland
PHYS 121