Transcript Slide 1

Homework:
9.
a)
v = d/t
b)
v = d/t
Over entire
20 sec.
= 90. m
15 s
= 40. m
5.0 s
= 6.0 m/s
= 8.0 m/s
v = d/t = 130 m
20 s
= 6.5 m/s
Average velocities:
d = displacement here
e)
f)
v = d/t
v = d/t
g/ Over entire
20 sec.
= 90 m, N
15 s
= 6.0 m/s, N
40. m
90 m
50. m
= 40. m, S
5.0 s
= 8.0 m/s, S
v = d/t
= 50. m, N
20 s
= 2.5 m/s, N
10.
50. m
120 m
a)
v = d/t
= 50. m
5.0 s
v = d/t
= 120 m
20. s
Over entire
25 sec.
= 10. m/s
= 6.0 m/s
v = d/t = 170 m
25 s
= 6.8 m/s
b/ Average velocities:
v = d/t
v = d/t
50. m
= 50. m, W
5.0 s
= 10. m/s, W
= 120 m, E
20. s
= 6.0 m/s, E
70 m
120 m
Over entire
25 sec.
v = d/t
= 70. m, E
25 s
= 2.8 m/s, E
11. a/
d1 = vt = (30. m/s)(9.0 s)
= 270 m
d2 = vt = (40. m/s)(3.0 s)
= 120 m
dT = d1 + d2 = 270 m + 120 m
= 390 m
b/ avg. speeds are given for first 2 legs:
v1 = 30. m/s
v2 = 40. m/s
For entire trip:
v = d/t
= 390 m
12 s
= 32 m/s
c/ Displacement = distance + direction
d1 = 270 m, N
120 m
270 m
d2 = 120 m, S
dT = R = 150 m, N
R
d/ velocity = speed + direction
v1 = 30. m/s, N
v2 = 40. m/s, S
vT = total displacment
time
vT = 150 m, N
12 s
vT = 12.5 m/s, N
Now, that’s acceleration!
of change in velocity
acceleration, a = rate
____________________________
a vector (has mag. and dir.)
=______________________________
average a =
where Δv =
Any time that _____________ changes, there
is___________________. And because:
velocity =
speed
+
direction,
changing either _____________ or ______________
or _________ results in acceleration. In this
section, we only consider changes in __________ .
The _________ speed changes, the _________ the a.
PhysRT, last page:
of change in velocity
acceleration, a = rate
____________________________
a vector (has mag. and dir.)
=______________________________
average a = Δv/t
where Δv = vf - vi
velocity
Any time that _____________
changes, there
acceleration
is___________________.
And because:
velocity =
speed
+
direction,
speed
direction
changing either _____________
or ______________
both
or _________
results in acceleration. In this
section, we only consider changes in __________
speed .
The _________
more the a.
faster speed changes, the _________
Ex: Ms. Rudd accelerates her jet skis from a
speed of 5.0 m/s to a speed of 17 m/s in 3.0 s.
Find the magnitude of her acceleration.
vi = 5.0 m/s
vf = 17 m/s
t = 3.0 s
a=?
a = Δv/t
= (vf – vi)/t
= (17 m/s – 5.0 m/s)
3.0 s
= 12 m/s
3.0 s
= 4 m/s2
= 4 m/s
s
SI units for a:
other units:
m/s = m/s2
 derived
s
cm , km/h
mph ,
s2
s
s
Using brackets [ ] for units:
[a] = [speed] = [distance]/[time]
[time]
[time]
From last problem:
a = 4 m/s2
= 4 m/s
s
= [distance]
[time]2
4 m/s of
speed
_________
gained
each
second
_________
direction
Because a = _________
, the ________________
Dv/t
direction
of a is the same as the __________________
change
of the ________________in
v: Dv = vf - vi.
Ex 1: An object moving to the right
accelerates to a faster speed.
vi
vf
Dv = vf – vi
= vf + (-vi)
vf
Dv
-vi
to the right
Since Dv ____
> 0 (which is _________________),
then also the acceleration a ____
> 0.
Ex 2: An object moving to the right is
decelerating
slowing down, or ________________________.
vi
Dv = vf – vi
= vf + (-vi)
vf
vf
Dv
-vi
Since Dv ____
< 0, then also a ____
< 0.
Note that the direction of the acceleration
______________
always the same as the
is NOT
velocity
direction of the ___________________
.
Ex 3: An object moving up
but slowing down:
Dv = vf – vi
= vf + (-vi)
vf
vf
Dv
-vi
vi
downward
 a is ______________
or_______________.
negative
Conclusions:
velocity and the________________
1.If the ___________
acceleration
same
are in the __________
direction, then the object is
accelerating
speeding up ( __________________)
_________________
.
acceleration
velocity and the________________
2. If the ___________
opposite
are in _______________
directions, then the object
decelerating
is _________________
( __________________)
.
slowing down
Directions
_______________can
be confusing, but remember:
displacement
1. The __________________
is always from the
initial
final
_________________
to the _____________
points.
initial
final
2.The ________________
always has the same
velocity
displacement
direction as the object's ____________________
same direction it __________________).
(the _________
is moving
acceleration
3.The ___________________
has a direction given
change
by the direction of the ____________
in the
velocity, Dv , which may or may not be the
______________
velocity
same as the direction of the ________________
.
magnitude
Ex: The _________________of
the acceleration is
acceleration
also called the__________________________.
a = 2.0 m/s2, east
mag.
= + 2.0 m/s2
dir.
In review:
scalar
distance
speed
acceleration
vector
…is the
magnitude
of the…
displacement
velocity
acceleration
In word problems, remember:
1) “starts from rest” means 
2) “comes to rest” means 
vi =0
vf =0
uniform
3) When an object is in ______________motion,
constant
it means it has a _________________velocity.
In
vi
that case: __________
and ____
vf = _____
v = _____
a=0
light and sound
Examples: The speed of _____________________
constants
given in the PhysRT are ___________________.
4) up/right are___________________,
+
_
5) down/left are____________________.
Ex: A ball is dropped. It accelerates from rest
to a speed of 29 m/s in 3.0 seconds. Finds its
acceleration.
“from rest”  vi = 0
a = Δv/t
vf = -29 m/s (down)
a = (vf – vi)/t
t = 3.0 s
a = -29 m/s -0
3.0 s
a=?
a = -9.7 m/s2
What are the magnitude and direction of a?
9.7 m/s2
down (neg.)
How much speed does the ball gain each
second?
9.7 m/s
Starting from:
a = Δv/t
a = (vf – vi)/t
at = vf – vi
vf = vi + at
PhysRT, last page:
Ex: What is the speed of a giraffe, initially moving
at a speed of 21 m/s, that accelerates at 5.0 m/s2
for 2.0 s?
vf = vi + at
vi = 21 m/s
= 21 m/s + 5.0 m/s2(2.0 s)
a= 5.0 m/s2
= 21 m/s + 10. m/s
t = 2.0 s
vf = ?
= 31 m/s
If it remains at this final speed, how long will it
take to travel 100. m?
v = d/t
vf = v = 31 m/s
31 = 100 / t
d = 100. m
t = 3.2 s
Ex: Chuck Norris accelerates from a speed of
4.0 m/s to 10. m/s in 4.0 seconds. Find his
average speed during that time.
v = (vi + vf)
not in PhyRT
2
= (4.0 m/s + 10. m/s) = 7.0 m/s
2
How far does he travel in the 4.0 s?
v = d/t
d=vt
= (7.0 m/s)(4.0 m/s)
= 28 m
Why can't you use vi or vf to find d?
Open your 3-ring binder to the
Worksheet Table of Contents.
Record the title of the worksheet:
Acceleration WS