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Homework Help: Simple projectile motion - problem with units

  1. Sep 24, 2010 #1
    1. The problem statement, all variables and given/known data

    (a) A ball of mass 0.4 kg, initially at rest, is kicked directly toward a fence from a point 20 m away, as shown below.
    The velocity of the ball as it leaves the kicker’s foot is 17 m/s at angle of 42◦ above the horizontal. The top of the fence is 5 m high. The ball hits nothing while in flight and air resistance is negligible.
    The acceleration due to gravity is 9.8 m/s2.
    Determine the time it takes for the ball to reach the plane of the fence.
    Answer in units of N.

    (b) How far above the top of fence will the ball pass? Consider the diameter of the ball to be
    negligible. Answer in units of s.

    2. Relevant equations

    irrelevant for question

    3. The attempt at a solution

    This problem seems quite easy. The problem is that its asking for the answer in units of N. How can I express time in newtons?

    The same problem occurs in question b. I know how to calculate the answer, but how can I express height in seconds?

    I have tried inputting the answers as if they are supposed to be in seconds for number one and feet for number two (assuming the "answer in units of N" and "answer in units of s" are typos) but I'm greeted with the message, "sorry that answer is incorrect, try again."

    What am I missing here?
    Last edited: Sep 24, 2010
  2. jcsd
  3. Sep 24, 2010 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to Physics Forums.

    It really does appear that those units are typos, in both (a) and (b). (And hopefully your answer for (b) was in meters, not feet?)

    If you're willing to post your work, we (real humans) could at least check your answers.
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