Post on 18-Dec-2015
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