Transcript physics140-f07-lecture5 - Open.Michigan

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Physics 140 – Fall 2007
lecture 5 : 18 Sep
Mathematics is the language of precise thinking.
– Richard W. Hamming (1915-1998)
Ch 4 topics:
• Newton’s laws of motion (I + II)
• dynamics: force and acceleration
Newton’s First Law: the Law of Inertia
“Every body continues in a state of rest, or uniform
motion in a straight line, unless it is compelled to
change that state by outside forces impressed upon it.”
In the “language of precise thinking”, we can say
SF=0
Source: Undetermined
velocity v is constant
where S F represents the sum of all external forces acting on
an object with velocity v.
A valid inertial reference frame is one in which objects move at
constant velocity unless forced to do otherwise.
Why is there a minimum vertical
distance (called B in the figure)
when hanging a frame by wire?
What will happen to the
tension measured by the
spring scale when I attach
the opposite end of the
string to the other,
identical “salami”?
A.
B.
C.
D.
?
The tension will stay the same.
It will double.
It will be halved.
It will change by a factor different from two.
Newton’s Second Law: Force and Acceleration
“The change in the quantity of motion is proportional
to the motive force impressed and is made in the
direction of the line in which that force is impressed.”
In short,
SF=ma
(Unit: 1 Newton (N) = 1 kg m/s2)
The vector sum of forces SF acting on a body cause it to
accelerate in the direction of SF.
The magnitude of the body’s acceleration depends inversely on its
inertial mass m.
Mass is a measure of inertia (resistance to change in motion).
Inertia
F = ma
density
r = mass/volume
google aerogel to find out
what this funny stuff is
Source: Undetermined
Source: NASA
? mass ?
Energy
E = mc2
Source: The Scientific Monthly (1921)
Higgs
particle
CC: BY University of Michigan
some common forces
Weight from near-Earth gravity, W
• magnitude W=mg
• directed to Earth’s center (defines downward)
Normal/contact Force, N
• occur at interfaces
• act perpendicular to interface (come in pairs)
• situation-dependent magnitude
Tension in rope or string, T
• acts at contact point
• directed along rope/string
• light (“massless”) strings have constant
tension along their lengths
(act as force conduits)
N
W
T
On a horizontal, frictionless surface, the blocks above are
being acted upon by two opposing horizontal forces, as shown.
What is the magnitude of the net force acting on the 3kg block?
A.
B.
C.
D.
E.
zero
2N
1.5 N
1N
More information is needed.