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Some true and false questions

  1. Jan 27, 2008 #1
    Hi, I have some true and false questions for my homework and I just want to double check my answers. The statements are either always true or can be false.

    1. A bicycle initially moving at a constant velocity will slow down unless a small net force is applied. FALSE (an object in motion will stay in motion unless force applied)

    2. If two objects have the same acceleration they are under the influence of equal forces. FALSE (not always true, but I'm not sure)

    3. If a net force acts on an object, the object's velocity will change. (TRUE; force affects acceleration, which should affect velocity)

    4. During the collision of a car with a large truck, the truck exerts equal size force on the car as the car exerts on the truck. (I'm not sure; I think this has something to do with Newton's 3rd law. I know though that forces come in pairs, but of equal size?)

    5. The net force which acts on an object which remains at rest is zero. TRUE

    6. IF an object's speed does not change, no net force is acting on the object. TRUE (same as number 1)

    Thank you!
     
  2. jcsd
  3. Jan 27, 2008 #2
    It looks like you have the right idea.
     
  4. Jan 27, 2008 #3
    I just tried these answers and one or some of these are wrong though..
     
  5. Jan 27, 2008 #4

    rock.freak667

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    Homework Helper

    First one said constant velocity. Speed and velocity are two different things.

    Consider an object moving in a circular path. It's speed stays constant but because its direction is changing it is accelerating and hence a force is being applied on it.
     
  6. Jan 27, 2008 #5
    You are definitely right about #1

    #2) You are right

    #3) If this question writer is an a$$hole, he may count a net force of zero

    #4) You are right about this one. The forces would be equal

    #5) You are right

    #6) Ah, speed is used instead of velocity. Velocity is a vector, while speed is a scalar quantity. Direction can change while speed remains constant, and velocity will change
     
    Last edited: Jan 27, 2008
  7. Jan 27, 2008 #6

    jambaugh

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    Gold Member

    Compare number six and number three. What is the difference between speed and velocity?

    [BTW greeniquana00's statements are not all true!!!]
     
    Last edited: Jan 27, 2008
  8. Jan 27, 2008 #7
    :D Already made the edit
     
  9. Jan 27, 2008 #8
    1. Correct.
    2. Corerct, because they can have different masse: F = m*a.
    3. Not neccesarily, it could change the direction instead (but he may be meaning that it is the velocity vector, in which case it would be true).
    4. True. Newtons third law says the both forces will be equal in size. Action = Reaction.
    5. True.
    6. True.
     
  10. Jan 27, 2008 #9
    It's because of F=ma. Depends on mass of the object, a1 = a2 so... F1/m1 = F2/m2.

    The forces can be different, so long as the masses of the objects balance the equation.
     
  11. Jan 27, 2008 #10
    All objects, regardless of mass, exert equal and opposite forces.

    So yes, it's Newton's 3rd Law.
     
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