Transcript Physics 106P: Lecture 1 Notes

Lecture 5, Pre-Flight Questions 3&4
An airplane is flying from Willard airport to O'Hare. Many forces act
on the plane, including weight (gravity), drag (air resistance), the trust
of the engine, and the lift of the wings. At some point during its trip
the velocity of the plane is measured to be constant (which means its
altitude is also constant). At this time, the total force on the plane:
1. is pointing upward
2. is pointing downward
3. is pointing forward
4. is pointing backward
5. is zero
correct
lift
drag
thrust
weight
Lecture 5, Pre-Flight Questions 3&4
(great answers)
Newton's first law states that if no net force acts on an object, then the
velocity of the object remains unchanged. Since at some point during the trip,
the velocity is constant, then the total force on the plane must be zero,
according to Newton's first law.
lift
SF= ma = m0 = 0
drag
thrust
weight
the upward and downward forces on the plane would have to be equal for it to
fly at a constant altitude, and the same is true of the forward and backward
forces for the plane to go at a constant speed.
Lecture 5, Pre-Flight 3-4
(common misconceptions)
The total force on the plane is pointing upward otherwise the plane
wouldn't be in the air.
With the force of gravity, there is a constant acceleration downwards.
Therefore the total force is only downwards
The plane is still traveling forward, so the net force on the plane must
be forward.
Because we all love physics
All other forces would be at equilibrium at constant velocity and
altitude, except since the plane is still moving forward it still faces air
resistance
Lecture 5, Pre-Flight Question 5
You are watching an old episode of Vampires from Outer Space, your
favorite Sci-Fi TV show, when you see the following scene: A starship is
shown cruising through space with a constant velocity, it's engines turned
on full blast. As the starship nears the space station it wants to visit, the
captain turns the engines off and the ship is shown gliding to a stop.
Looking at this through the eyes of a physicist, briefly explain what things
are wrong with this scene.
First, if the engines are on full blast, the starship is accelerating. Second, once
the engines are turned off, there isn't a force in space acting to slow the star
ship down. Third, vampires don't come from outer space. Fourth, I don't have
any favorite Sci-Fi shows.
First of all, if I am watching this particular show, it's way too late to be thinking
about physics (obviously the show comes on at a rather late hour). Neglecting
that fact, I, as a physicist, would notice that the ship would not be slowing down
at all if this were a real situation. In space there are no opposing forces that
would stop the craft. It would keep right on going into oblivion. Therefore, I
would immediately get up and go to bed feeling disgusted that I had ever liked
the show in the first place.
Vampires have never been proven to exist, nor is it known whether they
have space travel; this is yet another mistake (despite its irrelevance to physics).
Example 2

A force F acting on a mass m1 results in an acceleration a1.
The same force acting on a different mass m2 results in an
acceleration a2 = 2a1. What is the mass m2?
m1
F
(a)
2m1
a1
m2
F
(b) m1
(c)
a2 = 2a1
1/2 m1
• F=ma
• F= m1a1 = m2a2 = m2(2a1)
• Therefore, m2 = m1/2
• Or in words…twice the acceleration means half the mass
Lecture 5, Pre-Flight Questions 1 & 2
Suppose you are an astronaut in outer
space giving a brief push to a spacecraft
whose mass is bigger than your own (see
Figure 4.8 in text).
1) Compare the magnitude of the force
you exert on the spacecraft, FS, to the
magnitude of the force exerted by the
spacecraft on you, FA, while you are
pushing:
1. FA = FS
2. FA > FS
3. FA < FS
correct
Third Law!
2) Compare the magnitudes of the acceleration
you experience, aA, to the magnitude of the acceleration
of the spacecraft, aS, while you are pushing:
1. aA = aS
2. aA > aS
3. aA < aS
correct
a=F/m
F same  lower mass give larger a
Lecture 6, Pre-Flight Questions 1-6
A
B
• What are forces on A?
• What are forces on B?
• How does net force on A compare with net force on B?
Explain
(FBD)
The answers:
FhA>FBA
FBA=FAB
FB>FA
Lecture 6, Pre-Flight Questions 7&8
Suppose a box sits next to you on the floor of an elevator. During which
of the following situations is the normal force exerted on the box by
the floor of the elevator smallest? [see text 4.8]
1. When the elevator is accelerating upward.
2. When the elevator is moving upward with constant speed.
3. When the elevator is stationary.
4. When the elevator is moving downward with constant speed.
5. When the elevator is accelerating downward.
correct
Fn = mg + ma. If the acceleration in downward,
then a is negative. This will make F-n the smallest.
Objects experiencing a net force accelerate.
Therefore, for an object to accelerate downward,
its net force must also be in that direction,
meaning the normal force is less than the force
of gravity.
y
SFy = may
FN - W = may
FN
a
FN = W + may
W
If a < 0
Then FN < W
Note: if free fall, a = -g
Then FN = W- mg = 0!
Lecture 6, Pre-Flight Questions 9&10
Now suppose the box next to you on the elevator floor is annoying you,
and you want to push it out of the way. Because of static friction, you
need to push on the box with a minimum force F before it will start to
move. During which of the following situations is the force F required
to move the box smallest? [see text 4.9]
1. When the elevator is accelerating upward.
2. When the elevator is moving upward with constant speed.
3. When the elevator is stationary.
4. When the elevator is moving downward with constant speed.
5. When the elevator is accelerating downward.
correct
The normal force is minimized when the
elevator accelerates downward, minimizing
the friction on the box.
Lecture 7, Pre-Flight Questions 7
How often do you go to office hours?
1. At least once per week
2. From time to time but less than once per week
3. Never
9%
13%
77%
0%
20%
40%
60%
Office hours are a great opportunity
for one-on-one teaching. Take advantage
of it whenever you think it is necessary.
80%
Don’t hesitate to contact me or your TA
if you need help outside of regular office hours!
Comments on any part of the class so far…
“Too hard”, “Too easy”, “slower”, “faster”
Not much we can do about this– some of you are taking physics for the first time.
Do more problems in class.
Will try do (when there is time)
…post your lectures before class…
Will do
…in terms of discussion, it stinks. Physics is tough stuff. I think the TA's should
spend more time going over the material.
We have done years of research on this. Having the TA “lecture” is not an
effective way to teach for most students.
(We really believe the way we are doing it is best, or we would change).
Timing of material in lecture, HW and discussion.
After years of experience, we believe the overall structure is the best
approach. Some early weeks not ideal…should get better.
I think it would be a cool idea to have a section at the end of the preflights to
say which parts of the material we are the most unsure of. I’ll try it !
Are you open to individual meeting times set up in advance? Of course