Chapter 3 Linear Motion 1.MOTION IS RELATIVE Everything moves, at least with respect to some...
-
Upload
darlene-haynes -
Category
Documents
-
view
223 -
download
2
Transcript of Chapter 3 Linear Motion 1.MOTION IS RELATIVE Everything moves, at least with respect to some...
1. MOTION IS RELATIVE1. MOTION IS RELATIVE
Everything moves, at least with respect to some reference point.
To describe motion we shall talk about
Speed
Velocity
Acceleration
= distance/timeUnits - m/s, ft/s, etc.
Instantaneous Speed is the speed you would
read from a speedometer.
2. Speed2. Speed
Average Speed
Example of Average SpeedExample of Average Speed
You take a trip from A to B and back to A. You want to average 60 mph for the
round trip A to B to A.
A B2 miles
From A to B you average 30 mph.
What is your average speed on the return trip from B to A?
30 mph
?
Example of Average SpeedExample of Average Speed
A B2 miles
30 mph
? 60 mi/hr is 60 mi/(60 min) or 1 mi/min. To average 1 mi/min for a 4 mi trip would
require4 min.
30 mi/hr is 30 mi/(60 min) or 1 mi/(2 min). A 2 mi trip would take 4 min. See a problem???
Sorry, Ma’am, but youwere doing 45 mph in a30 mph zone.
But I haven’t driven 45miles yet.
Speeding Little Old Lady
Okay, okay, would you believe that I haven’t been driving for an hour yet?
3. Velocity3. Velocity
Average Velocity = Displacement/time
Units - m/s, ft/s, etc.
Instantaneous Velocity of an object is its instantaneous speed plus the direction it is traveling.
Velocity is a vector.
Displacement and Average VelocityDisplacement and Average Velocity
D
tD
v
Distance traveled is the length of the path taken.
ntDisplacemeD
Average velocity =
Acceleration = "change" in velocity/time
Units – (m/s2),
4. Acceleration4. Acceleration
ss
m
ss
ft(ft/s2), etc.
Acceleration is also a vector.
Demo - Ball on incline and ball on tableDemo - Ball on incline and ball on table
We can sense acceleration by comparing
observations from a constant velocity frame of
reference to observations from an
accelerating frame of reference.
Interpretation - we can feel acceleration if
there is a “support” force or contact.
Velocity and Acceleration
Galileo used inclined planes to study accelerations.
He found constant accelerations for inclines: the steeper the incline, the greater the acceleration. (It was too hard to measure time for free-falls.)
He also found that the size of the objects didn't matter.
Relationships Between v and a for Linear Motion.Relationships Between v and a for Linear Motion.
t
vva 0
atvv 0
atvv 0
atvIf initial velocity is zero, then
ExampleExample
A jogger starts at zero velocity with an acceleration of 3 ft/s2. How fast is she moving after 4 seconds? (Let’s see if we can first do this without using any equations.)
0v0 atv2s/ft3a
s4t )s4(s/ft3v 2
s/ft12v
What is the average speed of a horse that gallops a round-trip distance of 15 km in a time of 30 min?
What is the average speed of a horse that gallops a round-trip distance of 15 km in a time of 30 min?(a) 0(b) 0.5 km/h(c) 30 km/h(d) 500 m/s(e) None of the above
What is the average velocity for the round-trip of the horse in the previous question?
What is the average velocity for the round-trip of the horse in the previous question?(a) 0(b) 0.5 km/h(c) 30 km/h(d) 500 m/s(e) None of the above
5. FREE FALL
Motion near the surface of the earth in the absence of
air resistance.
The acceleration of an object is
g = 32 ft/s2 = 9.8 m/s2.
How FarHow Far
BC and how deep is a well.Michael Jordan – 3 s hang time??
tvd
tvd2
tgt
d2
22
2
1 tgtd 16
(If initial velocity is zero)
Free FallFree FallTime of Fall
(s)Velocity Acquired
(ft/s)Distance Fallen
(ft)
1
2
3
4
5
32
64
96
160
128
400
256
144
64
16
Demonstrations
Demo - Coin and feather in Demo - Coin and feather in vacuumvacuum
Film - Galileo's Experiment on the Film - Galileo's Experiment on the MoonMoon
Demo - Reaction timerDemo - Reaction timer Demo - Paper and book dropDemo - Paper and book drop
Free Fall - How Quickly How Fast Changes
Free Fall - How Quickly How Fast Changes
Acceleration Is How Quickly How Fast Changes.
Acceleration is difficult to understand because it is a rate of a rate.
What is a rate of a rate of a rate? JERK
You throw a stone downward. It leaves your hand with a speed of 10 ft/s. What is its speed two seconds after leaving your hand? (Neglect air resistance.)
You throw a stone downward. It leaves your hand with a speed of 10 ft/s. What is its speed two seconds after leaving your hand? (Neglect air resistance.)
(a) 10 ft/s(b) 32 ft/s(c) 42 ft/s(d) 64 ft/s(e) 74 ft/s