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Homework Help: On to Greater Things

  1. Nov 16, 2004 #1
    Thanks to everyone who have viewed my threads and helped me understand some basic physics concepts better. I passed my first on-line certification on a basic physics module. I am now proceeding to Module 2. I have complete the first section and I am working on the assessment questions. My answers are marked with an astericks. Please review for me and let me know the one that I got correct. Those that I missed please provide some information to get me thinking in the right direction. Thanks again. I am starting ot enjoy this work.

    1. Imagine riding a glass-walled elevator that goes up the outside of a tall building at a constant speed of 20 meters per second. Assuming that you drop a ball as you pass a window washer, the window washer will see the ball:

    a. fall starting from rest
    b. fall starting with an upward speed of 20 m/s*
    c. fall starting with a downward speed of 20 m/s
    d. remain stationary

    2. You can throw a ball vertically up in a car moving with a constant velocity and have it land back in your hand because:

    a. there ins no net horizontal force acting on the ball
    b. the reference system attached to the car is non-inertial*
    c. there is net force in the forward direction
    d. the force in the forward direction is cancelled by the inertial force

    3. An observer drops a ball in a train traveling along a straight, horizontal track with a constant acceleration in the forward direction. What would an observer in the train say about the horizontal force acting on the ball?

    a. There is not horizontal force*
    b. A force acts backward
    c. A force acts forward
    d. There is a centrifugal force

    4. An elevator is being accelerated upward with an acceleration equal to one-half that of gravity. If a man who weighs 160 lbs, when he is at rest on the earth, steps on a bathroom scale in the elevator, the reading will be:

    a. zero
    b. 80 lbs
    c. 160 lbs*
    d. 240 lbs

    5. Which of the following was NOT cited as evidence that the earth moves?

    a. The plane of pendulum rotates
    b. The sun rises and sets each day*
    c. Hurricanes turn counterclockwise in the Northern Hemisphere; clockwise in the Southern
    d. The stars exhibit parallax

    6. As a space ship approaches you in outer space at 50% of the speed of light, its rotating beacon sends out a pulse of light. An observer in the ship measures the speed of the light leaving the ship to be

    a. 50 % of c*
    b. 75 % of c
    c. equal to c
    d. 150 % of c

    7. As a friend passed you at a very high speed, she reported that she simultaneously exploded a firecracker at each end of her skateboard. Which one exploded first from your point of view?

    a. the one at the front
    b. the one at the back*
    c. they exploded simultaneously
    d. the answer depends on the speed of the skateboard

    8. Suppose a meterstick zips by you at a speed only slightly less than the speed of light. If you measure the length of the meterstick as it goes by, you would determine that it is ____________ one meter long.

    a. shorter than
    b. longer than
    c. still*
    d. The answer depends on the speed of meterstick

    9. Suppose two teams of astronauts who think they are accelerating through space are actually sitting on the surfaces of Earth and Mercury. The gravitational field of Mercury is much smaller than that of Earth. Which team thinks it has the larger acceleration?

    a. Mercury
    b. Earth
    c. The accelerations are the same*
    d. Neither team can determine their acceleration

    10. The general theory of relativity says that clocks run ________ as the gravitational force is increased.

    a. at the same rate
    b. slower*
    c. faster
  2. jcsd
  3. Nov 16, 2004 #2
    Answer explanation

    thats because the ball when dropped will have the same velocity as the elevator & you who dropped it and its 20m/s. In relation to you the ball is moving downwards but in relation to the stationary washer its moving upwards.
    Thats because as the car moves it moves you AND the ball at the same velocity.....as much the ball moves forward you hand will move forward too.

    Same reason as in 2nd question. The ball and the observer are moving at the same speed so the observer observes the ball to be stationary in the horizontal plane.

    Same reason the same upward force is applied to the man and the balance so itseems to those two things that there is no force acting on both of them

    thats because we cannot clearly prove by that whether sun rotates around earth or earth rotates about sun

    thats because the light emitted to us seems to have the speed of the spaceship + its speed.

    Thats because the one at the back was closer to you that the one at the front as she passed you.So the bang from the back end arrived to you earlier than the bang at the front.

    -------->this for now the rest In a while if you have any problem prv msg me :zzz:
  4. Nov 16, 2004 #3

    Doc Al

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    Staff: Mentor


    Incorrect. Hint: To keep the ball moving at a constant horizontal velocity, what must the horizontal force on it be?

    Incorrect. Hint: The observer is in a non-inertial frame.

    Incorrect. Hint: The scale measures the force that the elevator floor exerts on the man. What other forces act on the man? What must the net force be if he is accelerated?


    Incorrect. Hint: Every observer will see the light moving with the same speed with respect to themselves. What is that speed?

    Correct. According to you, her clocks are not synchronized: the clock in the back is ahead of the one in front.

    Incorrect. Hint: What does special relativity tell us about length measurements of moving objects?

    Incorrect. Hint: Assuming they can't look out the window, all they can do is measure the acceleration of an object released from rest inside the ship. Which one will see the object acceleration more?

  5. Nov 16, 2004 #4
    Thanks ColdRifle. I assume that the quotes you posted with the answers below are the correct answers. Is that right? If so then I just missed one of the first seven questions. I did not think that I did that good. Now, for question #6. I do not understand. You say the answer is d.) 150% of c. I follow your comments and understand them completely. I read the question as asking what would an observer on the ship measure the speed of the light emitted from the beacon. The ship is approching me, the ships speed is 50% c, the light is emitted from the ship, and the observer is measuring the speed of c. Since the observer is on the ship then the speed detected is in relation to him. The ships speed is 50% c and the emitted light travels at c. So the observed speed would be = lights speed-ships speed. Where did I go wrong? Thanks again.
  6. Nov 16, 2004 #5


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    Science Advisor

    Just a word of advice. Read Doc Al's comments. (Some of Coldrifle's are wrong.) And think hard about them. There seems to be a lot about inertial issues you do not yet understand. Your incorrect answer to #3 surprises me, since it is a common experience that something dropped in an accelerating frame does not fall straight down. For example, your head drops forcefully against the headrest and not straight down when you are in an accelerating car or an airplane on takeoff.
  7. Nov 16, 2004 #6
    DocAl to the rescue. Hello again DocAl. I have inertial issues! I will re-read my on-line tutorial. Anyway, lets begin.

    2.) The horizontal force is equal to the force acting on the car to keep it moving at a forward, constant velocity. With that in mind I select answer (c.) there is a net force in the forward direction.

    3.) I am not 100% sure what a non-inertial frame is. Can you explain?

    4.) The forces acting on the man in the elevator are the force exerted by the elevator floor along with his weight due to gravity. It would then seem like the net force would be the sum of these two, answer (d.) 240 lbs.

    6.) If all observers see light moving with the same speed with respect to themselves then it would seem the answer is (c.) equal to c.

    8.) I believe that special relativity tells us that length measurements change as an object moves towards or away from us. The object moving towards us would have a shorter length measurement. This is answer (a.) shorter than.

    9.) I have no clue on this question. I guessed and had no idea were to start to figure this out.

    Thanks again for your time and patience.
  8. Nov 16, 2004 #7

    Doc Al

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    Staff: Mentor

    Incorrect. To keep moving with a constant velocity requires that the net force on it be zero. As long as you don't exert a horizontal force on the ball, it will keep the same horizontal velocity.

    An "inertial frame" is one in which Newton's first law--the law of inertia--is valid. That means things with no force on them keep moving at the same speed in a straight line. A non-inertial frame is one where this is not true. (Loosely speaking: an inertial frame moves with a constant velocity while a non-inertial frame is one that accelerates.) Think of yourself in a racecar going around a circular track: if you dropped a ball---would it just fall straight down? No... the ball would appear to move outward, away from the direction you are turning, as if some force were acting on it. (Even though nothing is pushing or pulling it.) In this problem, the accelerating train is a non-inertial frame. Drop something and it will appear to accelerate backwards.

    The two forces on the man are the force exerted by the elevator floor (which pushes up and is the force measured on the scale) and the weight (which pulls down). If the scale force equalled his weight, then the net force would be zero: no acceleration. Since we know he is accelerated upward, we know that the force exerted by the elevator floor (and thus the scale force) must be greater than his weight--1.5 times as much, in fact.


    Moving objects are measured to be shorter in the direction of motion. (Note that the problem says the meterstick moves by you, not towards you.)

    Hint: The stronger the gravitational field, the greater the acceleration due to gravity.
  9. Nov 17, 2004 #8
    Hey DocAl. I think I need to read these questions very slowly and several times before I attack. Question 3 - I change my answer to (b) the force acts backward. facts: the train is accelerating at a constant at a constant forward direction...the ball is dropped and will continue to move forward at the speed that the train was going when the ball was released...the train will continue to accelerate. answer: to an observer on the train the ball will appear to fall backward (as if acted upon by a backward horizontal force) as the train accelerates forward. Question 9 -Based upon you hint I am selecting (b) Earth...larger mass with greater acceleration due to gravity. What is confusing me here is the astronauts perspective. Wouldn't the ones on Earth actually acceleration faster see Mercury fading off and assume they were accelerating faster? Maybe that is illogical. The rest I am fine with. Thanks a million.
  10. Nov 17, 2004 #9

    Doc Al

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    Staff: Mentor

    A good plan!
    This is just an exercise to illustrate the equivalence of gravitation and acceleration (the equivalence principle).

    Without looking out the window, one can measure one's acceleration (in free space) by (for example) dropping an object inside the ship and measuring its apparent acceleration with respect to the ship. The more it accelerates, the greater you would presume the ship's acceleration to be. The occupants of the ship would feel a force pulling them back (just like gravity). Now, if the ship were parked on Earth (and you couldn't just look out the window!) how could you tell if you were accelerating or you were within a gravitational field? The point is: you can't! So which planet gives the greater acceleration? Earth. Thus, compared to a ship parked on Mercury, a ship parked on Earth would think it had a greater acceleration. Make some sense?
  11. Nov 18, 2004 #10
    Got it. Enjoy the weekend and talk to you soon. signed: Inertially "not-as-much" challenged.
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