Lect05 - Physics 211 Lecture 5 Today's Concepts: a) b) c)...

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Unformatted text preview: Physics 211 Lecture 5 Today's Concepts: a) b) c) d) Free Body Diagrams Force due to strings Force due to springs Force due to gravity Physics 211 Lecture 5, Slide 1 Physics Stuff you asked about: Nothing really too confusing A problem like the elevator problem, but in two dimensions or even 3 dimensions. So maybe the elevator is from Willy Wonka's factory, and can move in 3 dimensions I have always had a problem with springs. I don't know when to use the f=-kx... or the 1/2kx^2... The relationship between Newton's Universal Gravitational Law and velocity through the circular motion equations; and springs. I'd really appreciate seeing more examples like the elevator problem in order to make things more intuitive. Why gravitational force is so small compared to the other major forces? I completed preflight 3 and 4 before the due date, but on my gradebook on physics 211 homepage, my score for preflight 3 and 4 are both 0. Why is that? Physics 211 Lecture 5, Slide 2 Physics Hurricane explanation We observe that hurricanes rotate CCW in the northern hemisphere, CW in southern hemisphere. Physics 211 Lecture 5, Slide 3 Physics 1) If you are on some object rotating clockwise then clockwise then objects moving in a straight line relative to someone who is not rotating appear to be turning LEFT relative to you. LEFT relative 2) If you are on some object rotating counter clockwise then then counter objects moving in a straight line relative to someone who is not rotating appear to be turning RIGHT relative to you. RIGHT relative Physics 211 Lecture 5, Slide 4 Physics 3) Viewed from above the North Pole, the earth is rotating counter clockwise. This means if you are in the northern hemisphere, things will always appear to be turning right. SUN Physics 211 Lecture 5, Slide 5 Physics 4) The atmospheric pressure at the center of a hurricane is much lower than the atmospheric pressure in the air surrounding it. This means that air tries to moved from the outside to the inside. Physics 211 Lecture 5, Slide 6 Physics 5) Since the earth is rotating, all of these lines will turn to the right as they move toward the center. Physics 211 Lecture 5, Slide 7 Physics 6) All of these “right turns” make the hurricane rotate CCW Physics 211 Lecture 5, Slide 8 Physics Physics 211 Lecture 5, Slide 9 Physics Physics 211 Lecture 5, Slide 10 Physics x y Physics 211 Lecture 5, Slide 11 Physics Preflight A box of mass m is hung with a string from the ceiling of an elevator that is accelerating upward. Which of the following best describes the tension T in the string: A) T = mg B) T > mg C) T < mg a m Physics 211 Lecture 5, Slide 12 Physics Explanations from people who said T>mg Which do you like best? m A) Tension increases because the box will move downward as the elevator accelerates up. B) The force downward is just the weight of the box. The force upwards is the weight of the box and the mass*acceleration. C) In order for the mass to accelerate upwards, the net force must also be upwards, so the tension must be greater than the weight. a Physics 211 Lecture 5, Slide 13 Physics Preflight A box of mass m is hung by a spring is from the ceiling of an elevator. When the elevator is at rest the length of the spring is L = 1m. If the 1m elevator accelerates upward the length of the spring will be A) L = 1 m B) L < 1 m C) L > 1 m L m a Physics 211 Lecture 5, Slide 14 Physics Explanations from people who said L>1m Which do you like best? L m a A) Since the net force will be upward, that means that the spring force must be greater than mg. B) The acceleration upward will cause the object to feel as if it's pulled downward so the spring will stretch itself. C) The downward force exerted by the mass is greater, and thus, cause the spring to be longer. Physics 211 Lecture 5, Slide 15 Physics I don't like elevators. The are either hot, smelly, and icky or really cold. They give you motion sickness if they go to fast, but if they don't they take forever. And they have to stop at like every other floor. They are usually always cramped and enclosed. You can die in there or get trapped in there, or both. If you move up, you get squished, but if you move down you fly. Talk about elevators please. Physics 211 Lecture 5, Slide 16 Physics Act You are traveling on an elevator up the Sears tower. As you near the top floor and are slowing down, your acceleration A) is upward B) is downward C) is zero Physics 211 Lecture 5, Slide 17 Physics Act You are traveling on an elevator up the Sears tower, and you are standing on a bathroom scale. As you near the top floor and are slowing down, the scale reads A) More than your usual weight B) Less than your usual weight C) Your usual weight Physics 211 Lecture 5, Slide 18 Physics Preflight A block sits at rest on a horizontal frictionless surface. Which of the following sketches most closely resembles the correct free body diagram for all forces acting on the block? Each red arrow represents a force. A B C D The only forces acting on the box are gravity (acting downward) and normal force (acting upward). Physics 211 Lecture 5, Slide 19 Physics Act A block slides down a frictionless inclined plane. Which of the following sketches most closely resembles the correct free body diagram for all forces acting on the block? Each red arrow represents a force. A B C D Physics 211 Lecture 5, Slide 20 Physics Act A cart with mass m2 is connected to a mass m1 using a is using string that passes over a frictionless pulley, as shown below. The cart is held motionless. m2 The tension in the string is A) m1g B) m2g C) 0 m1 g Physics 211 Lecture 5, Slide 21 Physics Act A cart with mass m2 is connected to a mass m1 using a is using string that passes over a frictionless pulley, as shown below. Initially, the cart is held motionless, but is then released and starts to accelerate. a m2 After the cart is released, the tension in the string is A) = m1g B) > m1g C) < m1g m1 g Physics 211 Lecture 5, Slide 22 Physics Please discuss more about the universal gravity and the equations that were given there. They were confusing.. Do we need to know the calculations/proof of the universal law Do of gravitation? Physics 211 Lecture 5, Slide 23 Physics Physics 211 Lecture 5, Slide 24 Physics Physics 211 Lecture 5, Slide 25 Physics Physics 211 Lecture 5, Slide 26 Physics Act A block weighing 4 lbs is hung from a rope attached to a scale. The scale is then attached to a wall and reads 4 lbs. What will the scale read when it is instead attached to another block weighing 4 lbs? ? m m m A) 0 lbs. B) 4 lbs. C) 8 lbs. ...
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