phy 231 1 physics 231 lecture 2: motion in 1 dimension, part i remco zegers
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PHY 2311
PHYSICS 231Lecture 2: Motion in 1 dimension,
part I
Remco Zegers
PHY 2312
Galileo (1564-1642)
What mathematical forcesgovern accelerated motion?
A wooden and stone sphere are dropped from the tower of pisa. Which one reaches the earth first?
Answer: same time!
PHY 2313
The inclined plane experiment
Distance traveled goes with the square of time:x(t)~t2
Time measured with water!
PHY 2314
Displacement
X=0 X=30 X=45
t=0 t=1X=30
X=0 X=30 X=45
t=0t=1X=-30
Displacement is a vector and thus has direction (sign)Displacement is not equal to Distance
PHY 2315
Average Velocity
Average velocity=Displacement
Time interval
if
if
tt
xx
t
xv
X=0 X=30 X=45
t=0 t=1 t=2
t=0 t=1 t=2
Which object has the largest average velocity over 2 s?
Same!
PHY 2316
Graphically x(m)
t(s)
The average velocity is the slope of the line connecting begin end end point in the x-t graph. Ignore what happens in between.
if
if
tt
xx
t
xv
= slope
PHY 2317
Vectors and Scalars...
X=0 X=30 X=45
t=0t=1X=-30
if
if
tt
xx
t
xv
Average velocity:vector=-30
if
if
tt
xx
t
xspeed
=+30Average speed:scalar
PHY 2318
Instantaneous Velocity
“But officer, my average speed was only 25 miles per hour...”
Sometimes we want to know the speed at one particularpoint in time.
PHY 2319
PHY 23110
Instantaneous velocity x(m)
t(s)
if
if
tt
xx
t
xv
What is the Velocity at t=2.0 s?
Consider the average velocity for a very small time intervalwith t=2.0 s in the center!
PHY 23111
Instantaneous velocity
x(m)
t(s)
t
xv
t
0lim
t
x
PHY 23112
Instantaneous velocity
x(m)
t(s)
t
xv
t
0lim
t
x
v=slope of the tangent to the x-t curve at t=2.0 sThink about the sign!
tangent
t
xv
PHY 23113
Question
t
x
Which object had the largest average velocity?Which object had the largest instantaneous velocity?
A B
B
if
if
tt
xx
t
xv
tangent
t
xv
B
PHY 23114
Acceleration
t
x
A change in slope in thex-t graph…
t
v…means a change in velocity:Acceleration
PHY 23115
Acceleration
if
if
tt
vv
Average acceleration
t
va
t
0lim
tangent
t
va
Instantaneous acceleration:
PHY 23116
Question
t
v t=0,v=20
t=1, v=5
What is the average acceleration between t=0 and t=1
A) 15 B) -15 C) 0 D) infinity
PHY 23117
Constant positive velocity
PHY 23118
Positive velocity and acceleration
PHY 23119
Question
Which v-t diagram matches which a-t diagram?
GALILEO!