Lecture 5, Pre-Flight Questions 3

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Lecture 5, Pre-Flight Questions 34

• 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
  upward2. is pointing downward 3. is pointing
  forward 4. is pointing backward5. is zero

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Lecture 5, Pre-Flight Questions 34 (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.

SF ma m0 0
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.
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Lecture 5, Pre-Flight 3-4(common misconceptions)
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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).
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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?

(a) 2m1 (b) m1 (c) 1/2 m1

• Fma
• F m1a1 m2a2 m2(2a1)
• Therefore, m2 m1/2
• Or in wordstwice the acceleration means half the
  mass

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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 pushing1. FA FS 2. FA gt FS3. FA
  lt FS

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 gt aS 3. aA lt aS
aF/m F same ? lower mass give larger a
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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 FhAgtFBA FBAFAB FBgtFA
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Lecture 6, Pre-Flight Questions 78

• 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.

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.
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y
SFy may FN - W may FN W may
FN
a
W
If a lt 0 Then FN lt W
Note if free fall, a -g Then FN W- mg 0!
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Lecture 6, Pre-Flight Questions 910

• 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.

The normal force is minimized when the elevator
accelerates downward, minimizing the friction on
the box.
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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 week3. Never

Office hours are a great opportunity for
one-on-one teaching. Take advantage of it
whenever you think it is necessary.
Dont hesitate to contact me or your TA if you
need help outside of regular office hours!
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• Comments on any part of the class so far