Presentation on theme: "Lecture 5, Pre-Flight Questions 3&4 total An airplane is flying from Willard airport to O'Hare. Many forces act on the plane, including weight (gravity),"— Presentation transcript:
Lecture 5, Pre-Flight Questions 3&4 total 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 lift weight drag thrust correct
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 weight drag thrust F= 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.
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. 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 Because we all love physics
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 l A force F acting on a mass m 1 results in an acceleration a 1. The same force acting on a different mass m 2 results in an acceleration a 2 = 2a 1. What is the mass m 2 ? (a) (b) (c) (a) 2m 1 (b) m 1 (c) 1/2 m 1 Fa1a1 m1m1 F a 2 = 2a 1 m2m2 F=ma F= m 1 a 1 = m 2 a 2 = m 2 (2a 1 ) Therefore, m 2 = m 1 /2 Or in words…twice the acceleration means half the mass
2) Compare the magnitudes of the acceleration you experience, a A, to the magnitude of the acceleration of the spacecraft, a S, while you are pushing: 1. a A = a S 2. a A > a S 3. a A < a S 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, F S, to the magnitude of the force exerted by the spacecraft on you, F A, while you are pushing: 1. F A = F S 2. F A > F S 3. F A < F S correct a=F/m F same lower mass give larger a Third Law!
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? The answers: F hA >F BA F BA =F AB F B >F A Explain (FBD)
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 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. Fn = mg + ma. If the acceleration in downward, then a is negative. This will make F-n the smallest.
FNFN W a F y = ma y F N - W = ma y F N = W + ma y Note: if free fall, a = -g Then F N = W- mg = 0! y If a < 0 Then F N < W
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 Office hours are a great opportunity for one-on-one teaching. Take advantage of it whenever you think it is necessary. 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… Do more problems in class.Will try do (when there is time) “Too hard”, “Too easy”, “slower”, “faster” Not much we can do about this– some of you are taking physics for the first 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). Are you open to individual meeting times set up in advance?Of course 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 !