Stuff you asked about:
I believe i got it all right. Assuming i did and didn't mess something up i found it easy.
Most likely because this is very similar to what i did in AP Physics B in High school.
I was confused about the dog running on the conveyor belt.
I would like to give some attention to how the centripetal motion equation is derived
intuitively.
release an angry dog on a moving walkway in class.
I found the reference frames and centripetal acceleration concepts the most difficult.
Please go over relative motion and the general equation for it.
I've always had trouble with reference frames. They're probably the
most annoying part of physics.
the a = v^2/R formula
The fact that for centripetal acceleration, the vector is directed in towards the center
of the circle is confusing. I understand it is true, because the fuzzy dice show the
evidence, but I don't get why.
Can we demonstrate centripetal acceleration by doing donuts in a parking lot?
I want to know why some idiot is driving a smart car to show physics. If that thing gets
hit by a rock, the force of the rock is greater that the force of the smart car, which
would cause a fatal accident.
I would like to spend more time on relative motion since it has always confused me.
I'm pretty sure I'm correct about the answer to the Rock on the String question, but
my reasoning is pretty shoddy. Could we go over that particular question tomorrow?
Actually, I think my grasp on the concept of angular velocity and acceleration is not
that strong. Could we go over angular/circular motion in general as well.
Homework Types & Scoring
Getting it done…
Its your responsibility to get stuff finished online before the deadlines.
The best way to avoid problems because your internet goes down or
because the server restarts etc. is not to leave it to the last minute.
FYI: The smartPhysics servers are run by a professional provider that gets
paid $$ and in return guarantees very high uptime & availability. We have
never had a serious outage.
If you need help go to office hours.
We really cant help you using email...
But remember, you get 80% credit up to a week late.
Time spent viewing prelctures…
Most of you are doing it right!
“Why are there questions built
into the prelectures now?
Isn't that what these
checkpoints are for?”
“I found that I need to actually
read the entire problem in the
questions portion if I want to
get them right.”
Physics 211
Lecture 3
Today’s Concepts:
a) Relative motion
b) Centripetal acceleration
Relative Motion
What is Mike’s velocity relative to the ground?
What you just did
v ac  v ab  v bc
v Mike ,Ground  v Mike ,Train  vTrain ,Ground
Student Suggestion
“Can you give more examples on how
v(a,b)=v(a,c)+v(c,b) is used?”
You are flying east on an airplane, that is traveling 150 m/s due east w.r.t. the air.
There is a 30 m/s wind that is traveling due west w.r.t the ground.
You carry your food tray toward the back of the plane with a speed of 3 m/s w.r.t.
the plane.
How fast are you moving relative to the ground?
A) 183 m/s
B) 177 m/s
C) 123 m/s
D) 117 m/s
Vyou,ground = Vyou,plane + Vplane,air + Vair,ground
Vyou,ground = -3 m/s + 150 m/s -30 m/s = 117 m/s
East
Prelecture 3, Question 1 (Red’s POV)
Question number #1 from the pre-lecture was a bit tricky! Can you please
go over it!?!?!
Prelecture 3, Question (Blue’s POV)
Red
Blue
Blue
Checkpoint
A girl stands on a moving sidewalk (conveyor belt) that is moving to the right at a
speed of 2 m/s relative to the ground. A dog runs on the belt toward the girl at a
speed of 8 m/s relative to the belt.
What is the speed of the dog relative to the ground?
vdog,belt = 8 m/s
vbelt,ground = 2 m/s
A) 6 m/s
B) 8 m/s
C) 10 m/s
“I'm having trouble understanding how we're supposed to know when to add and
to subtract the vector quantities, like in the checkpoint question with the
woman and the dog”
What is the speed of the dog relative to the ground?
vdog,belt = 8 m/s
vbelt,ground = 2 m/s
A) 6 m/s
B) 8 m/s
C) 10 m/s
+x
vdog, ground = vdog, belt + vbelt, ground
= (-8 m/s) + (2 m/s) = -6 m/s
Checkpoint
A girl stands on a moving sidewalk (conveyor belt) that is moving to the right at a
speed of 2 m/s relative to the ground. A dog runs on the belt toward the girl at a
speed of 8 m/s relative to the belt.
What is the speed of the dog relative to the girl?
vdog,belt = 8 m/s
vbelt,ground = 2 m/s
A) 6 m/s
B) 8 m/s
C) 10 m/s
I get this stuff when I go by common sense, but for examples like the dog and lady problem, I
wouldn't be able to do it without a visual, or I would have trouble doing it mathematically..
What is the speed of the dog relative to the girl?
vdog,belt = 8 m/s
vbelt,ground = 2 m/s
A) 6 m/s
B) 8 m/s
C) 10 m/s
A) Vdog,girl = Vdog,belt + Vbelt,girl= -8 + 2 = -6
B) Vdog,girl = Vdog,belt + Vbelt,girl= -8 + 0 = -8
C) Vdog,girl = Vdog,belt + Vbelt,ground = 8 + 2 = 10
Clicker Question
A man starts to walk along the dotted line painted on a moving sidewalk toward
a fire hydrant that is directly across from him. The width of the walkway is 4 m,
and it is moving at 2 m/s relative to the fire-hydrant. If his walking speed is 1
m/s, how far away will he be from the hydrant when he reaches the other side?
A) 2 m
B) 4 m
C) 6 m
D) 8 m
“The boat problem is always very confusing for me.
Can we do more examples?”
If the sidewalk wasn’t moving:
Time to get across:
Dt = distance / speed
= 4m / 1m/s
=4s
v man , sidewalk
Just the sidewalk:
v sidewalk , hydrant
Combination of motions:
v m an , hydrant  v m an , sidew alk  v sidew alk , hydrant
D
D = (speed of sidewalk) ∙ (time to get across)
=
(2 m/s)
∙
(4 s)
= 8m
Clicker Question
Three swimmers can swim equally fast relative to the water. They
have a race to see who can swim across a river in the least time.
Relative to the water, Beth swims perpendicular to the flow, Ann
swims upstream at 30 degrees, and Carly swims downstream at 30
degrees.
Who gets across the river first?
Ann
A) Ann B) Beth C) Carly
y
x
Beth
Carly
Look at just water & swimmers
Time to get across = D / Vy
B
D
A
C
30o 30o
Vy,Beth = Vo
y
Vy,Ann = Vocos(30o)
x
Vy,Carly = Vocos(30o)
Clicker Question
Three swimmers can swim equally fast relative to the water. They
have a race to see who can swim across a river in the least time.
Relative to the water, Beth swims perpendicular to the flow, Ann
swims upstream at 30 degrees, and Carly swims downstream at 30
degrees.
Who gets across the river second?
A) Ann B) Carly C) Both same
Ann
y
x
Carly
Checkpoint (& Demo)
A girl twirls a rock on the end of a
string around in a horizontal circle
above her head as shown from above
in the diagram.
A
B
If the string brakes at the instant
shown, which of the arrows best
represents the resulting path of the
rock?
“Once the string is released, no more force
is acting on the rock, so the rock will travel
in a straight line”
C
D
Top view looking down
Centripetal Acceleration…
I thought it was confusing how the object inside the car moved in a direction
opposite to the direction of the centripetal acceleration. It seems that the
direction of the acceleration should go outward along with the object because
the object must be pushed by something.
Centripetal Acceleration
Angular Velocity w
v = wR
w is the rate at which the angle q changes:
dq
w
dt
q
Once around:
v  Dx / Dt = 2pR / T
w  Dq / Dt = 2p / T
v = wR
Another way to see it:
dq
R
v dt =R dq
dq
vR
dt
v = Rw
“I'm not sure angular velocity is a good topic to touch too much on at our
current level, as I think it's a pretty difficult concept to grasp. However,
I get that it is necessary to explain centripetal acceleration”
Earth’s Rotation
Earth’s Rotation
When standing on the equator, what is the direction of your acceleration due to the
rotation of the earth?
A)
B)
C)
D)
E)
East
West
Toward Center of Earth
Away from Center of Earth (toward sky)
North