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Microsoft PowerPoint - apb209day3hints Day 3 Homework
Day 3 Homework
Hints
Hints
Physics AP
Physics AP
-
-
B
B #35
#35
We talked A LOT about average velocity in class.
We talked A LOT about average velocity in class.
What you need to realize is that, for constant
What you need to realize is that, for constant
acceleration, finding the average velocity by adding
acceleration, finding the average velocity by adding
the initial to the final and dividing by 2 is a fair
the initial to the final and dividing by 2 is a fair
thing to do.
thing to do.
What I want you to also realize is that the average
What I want you to also realize is that the average
velocity will become the instantaneous velocity at
velocity will become the instantaneous velocity at
one instant.
one instant. #35 cont.
#35 cont.
When is the average velocity the instantaneous
When is the average velocity the instantaneous
velocity?
velocity?
IN THE MIDDLE OF THE
IN THE MIDDLE OF THE
TIME
TIME
INTERVAL!
INTERVAL!
For example, if an object accelerates (uniformly)
For example, if an object accelerates (uniformly)
from 12
from 12
m/s
m/s
at t = 1 sec to 18
at t = 1 sec to 18
m/s
m/s
at t = 5 sec.
at t = 5 sec.
The average velocity is 15
The average velocity is 15
m/s
m/s
, and it will be
, and it will be
moving at this velocity exactly when t = 3 sec!
moving at this velocity exactly when t = 3 sec! #35 cont.
#35 cont.
What does that have to do with this problem?
What does that have to do with this problem?
The problem gave you the time it took a train to
The problem gave you the time it took a train to
cross an intersection. Knowing a displacement
cross an intersection. Knowing a displacement
and a time, you can easily get the average
and a time, you can easily get the average
velocity. You can now use this as an
velocity. You can now use this as an
instantaneous velocity (like the initial velocity),
instantaneous velocity (like the initial velocity),
but you must remember that the train is going at
but you must remember that the train is going at
this velocity 1.2 seconds into the intersection.
this velocity 1.2 seconds into the intersection. Don
Don
t look at these hints until you
t look at these hints until you
ve
ve
thought through the problems.
thought through the problems.
If you have, and still need a little boost, turn the page.
If you have, and still need a little boost, turn the page. #45
#45
Did you find out how far the object fell in 1.2
Did you find out how far the object fell in 1.2
seconds?
seconds?
If this was halfway, don
If this was halfway, don
t you know the entire
t you know the entire
distance?
distance?
Can you find the time it takes to fall all the way?
Can you find the time it takes to fall all the way? #51
#51
Don
Don
t try to work the problem from the instant
t try to work the problem from the instant
the man sees the block until it hits his head.
the man sees the block until it hits his head.
That is the hard way.
That is the hard way.
Instead, work the problem from the very top
Instead, work the problem from the very top
down.
down.
(When
(When
v
v
o
o
= 0, the equations are rather nice.)
= 0, the equations are rather nice.) #51 cont.
#51 cont.
Do you know how long the block has been falling
Do you know how long the block has been falling
before the man notices it?
before the man notices it?
Do you know how long it would take for the block
Do you know how long it would take for the block
to fall from the top of the building to his head?
to fall from the top of the building to his head?
Hmmmm
Hmmmm
.
.