Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Meaning of: Indicate magnitude & direction of all vectors

  1. Jan 20, 2010 #1
    Meaning of: "Indicate magnitude & direction of all vectors"

    I don't actually need help solving this problem I am working on, as I already have the answers. What I need help with is understanding this phrase: "Include both magnitude & direction of all vectors" --- What does this mean?

    Here is the question and its answers:

    Question: A ball is thrown straight upward - it is traveling at 50 m/s at the moment it is thrown. Ignoring air resistance, and rounding the force of gravity to 10 m/s:

    (1) How long will it take for the ball to stop rising?
    (2) How far will the ball have traveled during this time?
    (3) What is the ball's acceleration during this time?

    Answers:

    (1) 2.5 seconds
    (2) 125 metres
    (3) -20 m/s2

    So, if someone could tell me what the "magnitude & direction of vectors" are, it would be greatly appreciated
     
  2. jcsd
  3. Jan 20, 2010 #2
    Re: Meaning of: "Indicate magnitude & direction of all vectors"

    Velocity and displacement for example are vectors. Vectors have both magnitude and direction opposed to scalers which have purely magnitude. You basically have got to say which direction it is acting in so for example if a ball is thrown upwards the force of gravity acts in the opposite direction so it would be negative against the direction of the ball.

    Also you need to check your answers i believe =]
    show your working so we can see where you have made a mistake.
     
  4. Jan 20, 2010 #3
    Re: Meaning of: "Indicate magnitude & direction of all vectors"


    Here is my work. (Remember that in the question it says, force of gravity may be rounded to 10m/s)

    DISTANCE:

    1/2mv2 = mgh [Mass cancels itself out]
    1/2v2 = gh
    --------------------------------------------------
    1/2(502) = 10h
    1/2(2500) = 10h
    1250 = 10h
    1250/10 = h
    125 = h


    TIME:

    t = d/v
    -----------
    t = 125/50
    t = 2.5


    ACCELERATION

    a = v2-v1/t
    ------------
    a = 0-50/2.5
    a = (-50)/2.5
    a = (-20)
     
  5. Jan 20, 2010 #4
    Re: Meaning of: "Indicate magnitude & direction of all vectors"

    Regarding the vectors:

    Is it enough then to write the answer for the distance question as "The ball has traveled 125 metres upward by the time it stopped rising" --- or am I missing something?
     
  6. Jan 20, 2010 #5
    Re: Meaning of: "Indicate magnitude & direction of all vectors"

    ahhh i see where u have made the mistake remember that when the ball is thrown upwards it is still under the influence of gravity so you cant use v=s/t because that assumes that the object isnt accelerating and it is in fact decelerating, gravity is slowing the ball down.For part C its the right method but the acceleration is still due to gravity so you dont actually have to do any calculations.
    With the vectors im sure that is fine. With Part C try and think of the ball working against gravity so its slowing the ball down. So the acceleration is acting in the opposite direction of movement
     
  7. Jan 20, 2010 #6
    Re: Meaning of: "Indicate magnitude & direction of all vectors"

    So are you saying that for Part C, the acceleration is simply (-10m/s2) since that is the force of gravity pushing down on the ball?
     
  8. Jan 20, 2010 #7
    Re: Meaning of: "Indicate magnitude & direction of all vectors"

    yes =]
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook