PH 211 Winter 2014
Friday February 14
Lab calendar update
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<< Thursday 2/13
• Tuesday 2/18
• Wednesday 2/19
• Thursday 2/20
Lab 5 >>
Lab 4 >>
NO LAB
MIDTERM 2
Lab 5
Midterm pickup
Tuesday 14:00 – 15:30
• Compare your exam and solutions.
• Come to office hours for questions.
• If asking for re-grading, use form from website, and follow procedure.
Google moderator
Which topic would you like to discuss in class on
Monday?
http://www.google.com/moderator/#16/e=20e
597
Google moderator
Which topic would you like to discuss in class on
today?
http://www.google.com/moderator/#16/e=20e
889
On question 6.44, the problem states that a
bag is dropped on a conveyor belt moving at a
constant velocity. I am confused about how to
determine the acceleration and/or force
imparted on the bag."
"Can you explain how to do problem 6.28 from
MasteringPhysics? "A 1400 kg steel beam is
supported by two ropes." rope 1 is 20° left of
vertical, rope 2 is 30° right of vertical. What is
the tension in ropes 1 and 2?"
A box on a rough surface is
pulled by a horizontal rope
with tension T. The box is
not moving. In this situation:
A. fs > T.
B. fs = T.
C. fs < T.
D. fs = smg.
E. fs = 0.
Slide 6-75
A box on a rough surface is pulled by a horizontal rope
with tension T. The box is not moving. In this situation:
A. fs > T.
B. fs = T.
C. fs < T.
D. fs = smg.
E. fs = 0.
A box has a weight of 100 N.
It sits on the floor. It is then
pulled by a 30 N horizontal
force.
Does the box move?
A. Yes
B. No
C. Not enough information to say.
Slide 6-78
A box has a weight of 100 N. It sits on the floor. It is then
pulled by a 30 N horizontal force.
Does the box move?
A. Yes
80%
B. No
C. Not enough information to say.
17%
in
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no
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3%
An object is held in place by friction on an
inclined surface. The angle of inclination is
increased until the object starts moving. If the
surface is kept at this angle, the object
1.
2.
3.
4.
5.
slows down.
moves at uniform speed.
speeds up.
none of the above
There’s not enough information to answer this
An object is held in place by friction on an inclined
surface. The angle of inclination is increased until the
object starts moving. If the surface is kept at this angle,
the object
76%
slows down.
moves at uniform speed.
speeds up.
none of the above
There’s not enough
information to answer this
21%
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A.
B.
C.
D.
E.
Friction depends on normal force
Static friction: fs≤μsFnormal, opposes cause
Kinetic friction: fk=μkFnormal, opposes velocity
But how about this?
𝑣, 𝑎
In case a the weight of the hanging block is 10 N, in case b the
tension force is 10 N. Compare the accelerations of the two
systems. (the big block is the same in both cases) The
acceleration is:
1. The same
2. Greater on the left
3. Greater on the
right
4. I have no idea
In case a the weight of the hanging block is 10 N, in case b the
tension force is 10 N. Compare the accelerations of the two
systems. (the big block is the same in both cases) The
acceleration is:
1. The same
2. Greater on the left
3. Greater on the
right
4. I have no idea
A compact car and a large truck collide head on
and stick together. Which vehicle undergoes the
larger acceleration during the collision?
1.
2.
3.
4.
Car
Truck
Both experience the same acceleration.
Can’t tell without knowing the final velocity
of the combined mass.
A compact car and a large truck collide head on and
stick together. Which vehicle undergoes the larger
acceleration during the collision?
54%
31%
10%
6%
el
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A. Car
B. Truck
C. Both experience the
same acceleration.
D. Can’t tell without
knowing the final
velocity of the
combined mass.
Newton 3
𝐹𝑎,𝑏 = −𝐹𝑏,𝑎
Act on different objects!
a
b
𝐹𝑏,𝑎
𝐹𝑎,𝑏
𝑇=
1
(𝐹
2 𝑎,𝑏
𝐹𝑛𝑒𝑡,𝑎 = 𝐹𝑏,𝑎 − 𝑇
+ 𝐹𝑏,𝑎 )
𝐹𝑛𝑒𝑡,𝑏 = 𝑇 − 𝐹𝑎,𝑏
If Fab and Fba are not the same we get
spontaneous motion!
Action-reaction pairs
𝑚𝑎 𝑚𝑏
𝐺 2
𝑑
Gravity: 𝐹𝑎,𝑏 = 𝐹𝑏,𝑎 =
Tension: 𝐹𝑎,𝑏 = 𝐹𝑏,𝑎 = 𝑇
And now …. this…..It is moving!
T1
m1
m1g
And now …. this…..Still moving
T2
Fn
𝜃
m2
fk
m2g
And now …. this…..It is moving!
T1 = T 2
T2
T1
s
z
Δz=Δs→a1=a2
T1
𝑇1 − 𝑚1 𝑔 = 𝑚1 𝑎1
m1
m1g
−𝑇2 + 𝑚2 𝑔 sin 𝜃 + 𝑓𝑘 = 𝑚2 𝑎2
T2
Fn
𝜃
m2
fk
m2g
𝐹𝑛 = 𝑚2 𝑔 cos(𝜃)
𝑇1 − 𝑚1 𝑔 = 𝑚1 𝑎1
−𝑇2 + 𝑚2 𝑔 sin 𝜃 + 𝑓𝑘 = 𝑚2 𝑎2
𝑓𝑘 = 𝜇𝑘 𝑚2 𝑔 cos(𝜃)
𝑎1 = 𝑎2 < 0
𝑇1 = 𝑇2
𝑇1 − 𝑚1 𝑔 = 𝑚1 𝑎1
−𝑇1 + 𝑚2 𝑔 sin 𝜃 +
𝜇𝑘 𝑚2 𝑔 cos(𝜃) = 𝑚2 𝑎1
−𝑚1 𝑎1 − 𝑚1 𝑔 + 𝑚2 𝑔 sin 𝜃 +
𝜇𝑘 𝑚2 𝑔 cos(𝜃) = 𝑚2 𝑎1
𝑚1 + 𝑚2 𝑎1 =
𝑚2 𝑔 (sin 𝜃 + 𝜇𝑘 cos(𝜃)) − 𝑚1 𝑔
Demo
http://www.physics.umn.edu/outreach/pforce/c
ircus/
Hammer and nail