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Homework Help: Newton's Laws and net force

  1. Sep 14, 2004 #1
    please help me check my answers, cause if i get one answer wrong, it's a big point deduction. i dont really like doing my hw online.

    here it goes....
    1.) An object cannot remain at rest unless which of the following holds?
    A.)The net force acting on it is zero.
    B.) The net force acting on it is constant
    C.) There are no forces at all acting on it
    D.) There is only one force acting on it.
    for #1, i got A

    2.) If a block is moving to the left at a constant velocity, what can one conclude?
    A.) There is exactly one force applied to the block.
    B.) The net force applied to the block is directed to the left.
    C.) The net force applied to the block is zero.
    D.) There must be no forces at all applied to the block.
    for #2, i got B

    3.) A block of mass 2kg is acted upon by two forces: 3N (directed to the left) and 4N (directed to the right). What can you say about the block's
    A.) It must be moving to the left.
    B.) It must be moving to the right.
    C.) It must be at rest.
    D.) It could be moving to the left or moving to the right or at rest.
    for #3, i think it's B cause it's moving more to the right

    4.) A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block must be __________.
    A.) continuously changing direction
    B.) moving at constant velocity
    C.) moving with a constant nonzero acceleration
    D.) moving with continuously increasing acceleration
    for #4, i got B. if it's a constant horizontal force, it would be constant velocity right?

    5.) Two forces, of magnitude 4N and 10N, are applied to an object. The relative direction of the forces is unknown. The net force acting on the object __________.
    A.) cannot be equal to 5N
    B.) cannot be equal to 10N
    C.) cannot be directed the same way as the force of 10N
    D.) must be greater than 10N
    Enter the letters of all the correct answers in alphabetical order. Do not use commas. For example, if you think only the last option is correct, enter D. (Only applies to question #5)

    for #5, i got D.

    so the summary for all my answers are:
    1.) A
    2.) B
    3.) B
    4.) B
    5.) D

    if someone would check my answers, it would be awesome. thanks
  2. jcsd
  3. Sep 14, 2004 #2
    The answer should be C. The block must be moving with a constant acceleration. Remember that F = ma, where F is the constant force, m is the constant mass and a is constant acceleration in this case.
  4. Sep 14, 2004 #3
    The answer should be A. The reasoning goes like this: If you drew a triangle with sides 4 and 10, the other side cannot have a length of less than or equal to 5. This is because of a law that states that the sum of two sides of a triangle always has to be more than the other side. But if your triangle had sides 5, 4 and 10, the sum of the length 5 and length 4 sides (which is 9) is less than 10.
  5. Sep 14, 2004 #4


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    Homework Helper

    Not all, but most of your answers are wrong.
    Read Newton's laws again. Don't forget that the forces add up as vectors. Try to make a triangle of those 10 N and 4 N forces. What can be the lenght of the third side?
    Remember that F =m*a. It follows that constant net force causes constant acceleration in the direction of the force.
    Velocity is not acceleration. The direction of motion is teh same as the direction of velocity. A body need not move in the direction of the net force acting on it.

    Try to answer the questions again.

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