Physics 2A
Lecture 8: Feb 2, 2017
Kaixuan Ni
UCSD Physics

•Kinematics:
–
Language
for describing motion
•displacement, velocity, acceleration
•Dynamics:
–Cause of change in motion
•New Concepts : Force and Mass
•Newton
±
s Three Laws of Motion
–Obtained from first principles
!
Fundamental
– Based on experimental observations of
motion, NOT product of some math
derivation

The Force Vector
•
Force is a
push or pull
on some object
– Contact or no contact (gravity, EM)
•
Gives quantitative description of interaction
between an object(s) & its environment
– i.e: pulling a box
•
Force is a vector:
– has direction and magnitude
– SI unit of magnitude is the
Newton (N)
– adding and decomposing force vectors

Newton
±
s First Law Of Motion
•
Actually first noticed by Galileo in his
experiments with smooth surfaces (technology!)
•
Later generalized by Newton:
When there is no net force acting on
an object, the object maintains its motion
with a constant velocity (
zero acceleration
)
Conversely:
When an object moves with constant velocity,
the net force acting on it must be zero
Tendency of an object to maintain its state of rest
or uniform motion is called
inertia

Inertial Frame Of Reference
•
A frame of ref. where Newton
±
s 1
st
law is valid is called
an
inertial frame of reference
•
Any reference frame that moves with
constant velocity
w.r.t an inertial frame is also an inertial frame (Newton
±
s
1
st
law is valid in this ref. frame too)
•
Reference frame where Newton
±
s 1
st
law does
not
hold
"
non-inertial frames
–
e.g: An accelerating frame of reference

Relative Perspectives !
Consider 3 inertial frames of reference & their experiences
1
Observer A claims he is at rest
B & C are moving towards him
with v =v
!
!
1
Observer B claims she & C
are at rest, A is moving towards
him with v = -v
!
!
Who is right ? !!
Everyone
!!

Perspective In Non-Inertial F.O.R Girl on frictionless skates initially at rest in a boxcar with frictionless floor Consider perspective of a observer stuck to boxcar as it accelerates from v=0 What does he think of girl±s motion compared to the ground? What does the girl think?

Newton
±
s 2
nd
Law Of Motion
F
ma or
m
inertial mass= measure of an object's resistence
to
F
a
cha
in its mo
tion (The "I
F
a
m
ner
nge
;
tia" of 1st law
)
=
∝
=
=
!
!
!
!
!
!

At all points in circular motion,
a &
F are in same direction:
towards the c
enter
∑
!
!
Newton
±
s 2
nd
Law Of Motion
F
a
∝
!
!

Interpreting Newton
±
s 2
nd
Law
•
Vector equation:
•
refers to
external
forces exerted on the object by
the environment
– impossible for object to affect its own motion by
applying force on itself !
• Can’t lift yourself to ceiling by pulling up you belt
•
Valid only in inertial frames of reference
•
Only true when
mass m = constant
– think of acceleration of a fuel tank due to a
constant force when the tank is leaking fuel ??
x
x
y
y
z
z
F
ma
F
ma ,
F
ma ,
F
ma
=
⇒
=
=
=
∑
∑
∑
∑
!
!
!
!
!
!
!
!

Mass Of An Object
F
Ratio
is
& is constant
a
regardless of the magnitu
property o
de of the
f the obje
applied f

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