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Gravity, Force and Mass Assignment, Can someone please check it?

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  1. Sep 25, 2014 #1
    1. The problem statement, all variables and given/known data

    Hello everyone!

    I have this physics assignment I'm working on. It's 10 questions and I've answered them to the best of my abilities but I wanted to get valuable input from you guys and I have a couple of questions. I would really appreciate any help! Thanks in advance. PS: My answers and reasoning are in bold and my questions are in red. Please ignore the (1.) beside all the choices....I couldn't get rid of it.

    1. A small car and a large truck collide head-on. During the collision, the small car experiences
    1. A much larger force than the truck.
      A much larger change in momentum than the truck.
      A much larger acceleration than the truck.
      All of the above.
      None of the above.
    (D) -All of the above- Because Force is related to mass, the larger truck would exert a larger force on
    the small car because the large truck has more mass. Because it is exerting more Force on the small
    car; the small car's motion would be affected more = change in momentum and because the car would
    be pushed back harder; I assume it would have a drastic change in direction (not to mention speed)
    which equals a change in acceleration.

    2. Imagine that you could magically move the moon closer to Earth, so that the distance from Earth to moon is cut in half. This would cause the force that Earth exerts on the moon to be

    1. halved.
      increased to four times its original value.
      reduced to 1/4 of its original value.
      doubled.
      Nonsense--Earth exerts no force on the moon.
    (B) -Increased to four times its original value- Because gravity is inversely proportional to the square
    of the distance between two objects. However, doesn't the mass of the objects also have an effect on
    gravity? So, the Earth being that much larger than the moon and the moon being closer to Earth than
    what it is already...wouldn't the gravitational pull of Earth be even greater than four times what it was
    originally?


    3. Suppose that the force of gravity between the sun and Earth suddenly stopped operating. What would happen?
    1. Earth would fall directly inward, into the sun.
      Earth would fly directly outward from the sun.
      Earth would continue orbiting the sun, but objects on Earth would have no weight and would float into space.
      Earth would stop in its orbit around the sun, and remain motionless at its present position.
      Earth would move straight ahead, in a straight line, rather than orbiting the sun.
    (E) -Earth would move straight ahead, in a straight line, rather than orbit the sun. Because the
    Earth was actually just traveling through space at a constant velocity and no other forces acting on it
    other than when it neared the sun; the sun's gravity curved it's path.


    4. You are in distant space. A giant boulder, many times larger than you, is at rest in front of you. You tap the boulder lightly with a small hammer. What does the boulder do?
    1. It accelerates during the tap, up to a slow speed [slower than walking speed], then keeps moving at that speed.
      It remains at rest.
      It accelerates during the tap, up to a high speed [faster than a fast-moving car], then keeps moving at that speed.
      It speeds up a little during the tap, but after the tap it soon [within a few minutes] slows down and comes to rest.
      It accelerates both during and after the tap.
    (A) -It accelerates during the tap, up to a slow speed [slower than walking speed], then keeps
    moving at that speed-
    Because there is no gravity, friction or air resistance in space; even a small tap
    would provoke some sort of acceleration in the boulder. However, because the boulder is large, a small
    hammer tap (a small Force) wouldn't cause it to accelerate much since force, acceleration and mass are
    related and you would need more force to move an object with more mass....then, it would never stop
    moving because there would be nothing to stop it.

    5. An auto weighing 3000 newtons has a forward drive force of 1000 newtons. Air resistance on the car is 100 newtons, and road resistance is 100 newtons. The auto moves along a straight level road. The force due to gravity acting on the auto is
    1. zero.
      3000 newtons downward.
      4200 newtons downward.
      800 newtons upward.
      3000 newtons upward.
    (B) -3000 Newton's downward- O.K, so to be completely honest, I'm not too sure how I reached that
    conclusion.
    What I did was pretty much a process of elimination. I figured that gravity only acts
    "downward" because it's like a pull towards the center of Earth, so, I got rid of answers (a), (d), and (e).
    Then I was left with (b) and (c) and I figured that you just don't add all the forces...because the direction of
    the force matter and none of the forces are acting in opposition to that of gravity...I figured that the force of
    gravity then just becomes it's weight.

    6. Regarding the relation between acceleration, force, and mass, an object's acceleration is
    1. proportional to the inverse of the height of the Empire State Building.
      inversely proportional to the force on it and proportional to the object's mass.
      proportional to the force on it and proportional to the object's mass.
      inversely proportional to the force on it and inversely proportional to the object's mass.
      proportional to the force on it and inversely proportional to the object's mass.
    (E) –Proportional to the force on it and inversely proportional to the object’s mass- Because, not
    just because my text book says so but also because it makes sense. The more force you apply on an
    object the faster it will go. Also, the larger or denser the object the more force required to accelerate it just
    as much as a lighter object.

    7. During this century, we have found that Newton's physics does not apply to
    1. high speed objects.
      very strong gravitational forces.
      very small objects.
      All of the above.
      Nonsense--Newton's physics always applies.
    (E) –Newton’s physics always applies- I don’t know…just going by what my book says.

    8. Would you be richer if you had a hunk of gold whose weight is 1 newton on the moon, or one whose weight is 1 newton on Earth, and why?
    1. 1 newton on Earth, because then the gold's weight on Earth would be larger.
      1 newton on Earth, because then the gold's mass would be larger.
      1 newton on the moon, because then the gold's mass would be larger.
      1 newton on the moon, because then the gold's weight on the moon would be larger.
      It wouldn't make any difference, because the amount of actual gold would be the same in both cases.
    (D) -1 N on the moon, because then the gold’s weight on the moon would be larger- So, I’m not
    sure if the question is just ambiguous or if I’m not understanding it correctly. If I look at it strictly from the
    mass point of view then answer (e) is correct: an objects mass is constant no matter where it is in space
    because its number of atoms doesn’t change. BUT, if we’re talking just about the “weight” (which is the
    effect or pull of gravity on the mass) then wouldn’t 1 N of anything in the moon have to be much larger
    (about 6x more) than on Earth in order to weight the same (1 N) because the pull of gravity is that much
    less?


    9. Astronauts in orbit around Earth feel weightless because

    1. they must lose weight for the space program.
      they are in "free fall" around Earth.
      there is no gravity out there.
      they are moving with constant velocity.
      no forces at all are acting on them.
    (E) –No forces at all are acting on them- Again, I’m not sure. I know that gravity still exists within the
    orbit of Earth (actually it’s still quite strong) so it makes my answer (e) kind of contradictory but they “feel”
    weightless because there is nothing “pushing up on them” (no contact forces) so they can’t “feel” gravity
    though it’s still there….? If that reasoning is not correct, then I would go with (b) just because the rest of
    the answers don’t make sense to me either…


    10. If you somehow increased Earth's radius, without changing its mass, would this affect your weight?
    1. Yes, your weight would increase.
      No, because the force of gravity on your body depends only on your mass and on Earth's total mass--it doesn't depend on Earth's size.
      Yes, your weight would decrease.
      No, because your weight always remains the same regardless of how you change the external surroundings.
    (C) –Yes, your weight would decrease – Though, I am tempted to pick (b)…I decided to go with (c)
    because if the Earth expanded its radius (the Earth uniformly expanded its radius –so really it’s the Earth diameter) then you’d be further away from the center and thus the pull of gravity would be less, which is what affects your weight not your mass…?
     
  2. jcsd
  3. Sep 25, 2014 #2

    Doc Al

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    Staff: Mentor

    Careful! What does Newton's 3rd law have to say about this?

    Does the mass of the objects affect the gravitational force? Of course. But here the masses have not changed. The only thing that changes is the distance.

    (Note: In general, it's not a good idea to cram too many questions into the same post!)
     
  4. Sep 25, 2014 #3
    2. I agree with your answer: "Increased to four times the original value."
    Increase in mass does increase the gravity of the object, so yes the mass does have an effect on gravity. However when the moon is brought closer to Earth, the mass does not increase (it is constant). Therefore, only the distance is effecting the amount of gravity, not the mass.
    Look back at the formula, see what is and is not changing.

    7. Well look at what kind of physics has been developing lately, they talk about it all the time on Discovery channel. Where do the "traditional" laws of physics break down?

    9. Think for one second on this one, which force is always acting on an object, no matter where you are in space? If you are on in an airplane and it suddenly goes downward accelerating at 9.81m/s^2, do you "feel" gravity or weightlessness?
     
    Last edited: Sep 25, 2014
  5. Sep 25, 2014 #4
    Thank you! Yes, I received an email about not putting more than one question on a thread, ooops, sorry.
     
  6. Sep 25, 2014 #5

    Doc Al

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    Staff: Mentor

    Good!

    Good!

    Yes, the weight is the force of gravity. They tell you that up front! (You have to know that it acts downward, of course.)
     
  7. Sep 25, 2014 #6
    Thank you! Yes that makes sense! :)
     
  8. Sep 25, 2014 #7
    THANK-YOU! Yes, it makes sense.
     
  9. Sep 25, 2014 #8
    Thank you for taking the time to look at my work. I reviewed Newton's third law and it states that for every action there is an equal and opposite reaction. Does that mean then, that both the truck and car experience the same force? But, that seems counter-intuitive (not that intuition is correct).
     
  10. Sep 25, 2014 #9

    Doc Al

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    Staff: Mentor

    Right answer, but careful with your reasoning. Size or density doesn't matter, only the mass and the net force.

    Better check your book! Newton's physics has been show to break down in certain areas. (See: Quantum mechanics and Relativity.)

    Right answer. Of course you want the most mass of gold. So a given hunk of gold, whose mass doesn't change going from moon to earth, will weigh less on the moon. That means it takes more gold (more mass) to weight 1 N on the moon than it does on earth.
     
  11. Sep 25, 2014 #10

    Doc Al

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    Staff: Mentor

    Yes! When car and truck collide, they exert equal and opposite forces on each other. Now, that same force will have a different effect on the small car versus the huge truck, but that's a different question.
     
  12. Sep 25, 2014 #11

    O.k...I see! So, based on the effects that one would have on the other...wouldn't the small car still experience a greater acceleration and change in momentum compared to the large truck? ...making the answer still "all of the above"?
     
  13. Sep 25, 2014 #12
    I agree with your answer.
     
  14. Sep 25, 2014 #13

    Doc Al

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    Staff: Mentor

    Answer E is incorrect, as you suspect. Astronauts in orbit still feel gravity--it's what is holding them in orbit. (The force of gravity is a bit smaller because they are further away, but still large.)

    They feel "weightless" because they are in free fall. Nothing's pushing them up. Example: If you stood on a platform, it supports your weight. If that platform suddenly started falling (in free fall), you'd feel "weightless" as it would stop supporting you.

    Good. C is correct.
     
  15. Sep 25, 2014 #14

    Doc Al

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    Staff: Mentor

    The small car will experience a greater acceleration. But not a greater change in momentum. In a collision, momentum is conserved. Both car and truck will have equal and opposite changes in momentum, with the net change being zero.
     
  16. Sep 25, 2014 #15
    So, I'm thinking that Newton's physics is having some trouble reconciling with quantum mechanics and how wavelike sub-atomic particles behave. I'm going with (c) -very small objects.
     
  17. Sep 25, 2014 #16
    I just reviewed this in my book and although I didn't see it at first...I get what your saying and it makes sense. Goes to show how "feeling" that something is right can be completely deceiving. Honestly, thank-you so much for taking the time to look this over and helping me out! :)
     
  18. Sep 25, 2014 #17
    Thanks so much for looking this over!
     
  19. Sep 25, 2014 #18

    Doc Al

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    Staff: Mentor

    Careful! The realm of the very small is just one area where Newtonian physics breaks down.
     
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