Title: Physics 211 Lecture 18, Slide 1
1Physics 211 Lecture 18
Today's Concepts Static Equilibrium Potential
Energy Stability
Plus Ill work through a couple of the HW
problems
2Ladders on walls and tensions in ropes.
How to apply the concepts presented in
pre-lecutre in more word problems.
I just wanted to tell you how I experienced
physics this weekend. I was camping at Kickapoo
State Park, and I had hiked my way onto this
ledge overlooking a river
What is this footprint business? Thanks
go over the truck and box problem again.
i wish i did not leave this till 2 in the morning
None. This weather is making me too happy! I have
no problems in life!
It all seemed to make sense. Kudos to the
masterful prelecture.
How to determine if the torque is positive or
negative. I know the right hand rule as related
to the cross product, but some help would be
appreciated. Also, that was a very creative way
of saying the word "slip" to whoever does the
prelectures.
I'm starting to believe that you pick these
comments strictly at random because I've written
some pretty creative and funny comments (if I do
say so myself) in attempt to make it to the
almighty introduction lecture slide but to no
avail. However, I will never quit because
quitters never win, and winners never quit. I'll
cross my fingers and hope for the best.
3Act
- A (static) mobile hangs as shown below. The rods
are massless and have lengths as indicated. The
mass of the ball at the bottom right is 1kg. -
- What is the total mass of the mobile?
A) 4 kg B) 5 kg C) 6
kg D) 7 kgE) 8 kg
1 m
2 m
1 kg
1 m
3 m
4Act
- In which of the static cases shown below is the
tension in the supporting wire bigger? In both
cases M is the same, and the blue strut is
massless.
A) case 1 B) case 2 C)
same
T1
T2
300
300
L
L/2
M
M
case 1
case 2
5case 1
case 2
T1
T2
q
q
M
M
Balancing Forces in the y direction
Physics 211 Lecture 18, Slide 5
6case 1
case 2
d1
T1
T2
d2
q
q
L
L/2
M
M
Balancing Torques
Physics 211 Lecture 18, Slide 6
7Prelecture
- In which of the static cases shown below is the
tension in the supporting wire bigger? In both
cases the red strut has the same mass and length.
A) case 1 B) case 2 C)
same
T1
T2
300
300
L
L/2
case 1
case 2
55 got this right
8- In which of the static cases shown below is the
tension in the supporting wire bigger? In both
cases the red strut has the same mass and length.
T1
T2
300
300
L
L/2
case 1
case 2
- Case 1 because the lever arm distance is the
largest there - Both cases must have the same torque. Since case
1 has a longer length, it needs a smaller force
than case 2, which has a shorter length. - Both wires have to hold up a beam of the same
mass.
9Lets work our way through a couple of HW problems
10Parts (a) and (b) are just like the problem we
did last lecture
11Parts (a) and (b) are just like the problem we
did last lecture
We also know T1 T2 Mg
2 equations, 2 unknowns Solve for T1 T2
12Act
Now find T2 right after you cut the wire on the
left.
T2
M
x cm
Mg
13Act
The free side of the beam will start to fall. Use
this fact to figure out how T2 compares to
weight of the beam?
T2
A) T2 Mg B) T2 gt Mg C) T2 lt Mg
M
x cm
Mg
Physics 211 Lecture 18, Slide 13
14Find T2 right after you cut the wire on the left.
T2
M
x cm
d
Mg
Plug this into the expression for T2
15(No Transcript)
16Just like the ladder problem in the prelecture
q
q
17Just like the ladder problem in the prelecture
q
q
18Just like the ladder problem in the prelecture
q
19Only difference iswhere q is defined
20(No Transcript)
21General Case Ladder Example
- Bill (mass m) is climbing a ladder (length L,
mass M) that leans against a smooth wall (no
friction between wall and ladder). A frictional
force f between the ladder and the floor keeps it
from slipping. The angle between the ladder and
the wall is f. - How does f depend on the angle of the ladder and
Bills distance up the ladder?
L
M
Bill
m
q
f
22- Balance forces
- x Fwall f
- y N Mg mg
Fwall
Balance torques
L/2
Mg
axis
d
mg
q
f
N
23y
L/2
M
m
d
q
f
x
Physics 211 Lecture 18, Slide 23
24Prelecture
In the two cases shown below identical ladders
are leaning against frictionless walls. In which
case is the force of friction between the ladder
and the ground the biggest? A) Case 1 B) Case
2 C) Same
Case 1
Case 2
70 got this right
Physics 211 Lecture 18, Slide 24
25In the two cases shown below identical ladders
are leaning against frictionless walls. In which
case is the force of friction between the ladder
and the ground the biggest? A) Case 1 B)
Case 2 C) Same
Case 1
Case 2
- F(1/2) mg cot(theta)
- case 2 has a larger force of friction because a
larger percentage of the center mass is in the
vertical direction. - The coefficient of friction and normal force are
the same in both cases.
Physics 211 Lecture 18, Slide 25
26Box on Truck...
27Stability
28Prelecture
- Suppose you hang one end of a beam from the
ceiling by a rope and the bottom of the beam
rests on a frictionless sheet of ice. The center
of mass of the beam is marked with an black spot.
Which of the following configurations best
represents the equilibrium condition of this
setup?
A) B) C)
65 got this right
29- Which of the following configurations best
represents the equilibrium condition of this
setup?
A) B) C)
- The center of mass is inside the footprint of the
object on the sheet of ice. - The center of mass is directly below the point on
the ceiling where the rope is attached. - The ice is frictionless so it can't put a force
on the beam that could make the string hang at an
angle