1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finding Final Velocity & Displacement

  1. Sep 20, 2013 #1
    1. The problem statement, all variables and given/known data
    A man shoots an arrow 145m/s at 61o from a height of 15m where the ground below is the origin.

    A.) Where will the arrow be in 1.5 seconds (X,Y)?
    B.) What will be the displacement when the arrow lands in regards to where it was shot from



    2. Relevant equations
    I believe the kinematic equation Displacement=Vit+(1/2a)t2

    X=ViCosTheta
    Y=ViSinTheta
    Theta=Tan-1(Y/X)

    as well as Vf=Vi-at

    where
    Vi = Initial Velocity
    Vf = Final Velocity
    a = Acceleration
    t = Time



    3. The attempt at a solution

    I got 105.4m for x and 190 - 1.5(9.83) for y

    for my location of where the arrow landed I got (1050m,-15m), because -15 is (0,0) and in regards to where it was shot it is 15m down and 1050=145*12.8+(1/2 -9.83)*12.82

    for the Vf I got 19.1m/s because Vf=145-9.83*12.8

    Note: I honestly am lost how to solve the problems
     
  2. jcsd
  3. Sep 20, 2013 #2

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    I get same answer for x.

    The answer for y is wrong. It looks like you got the vertical component of the initial velocity right (but the last bit is wrong). The SUVAT equation you used I know as...

    S = ut + 0.5at2

    S = displacement
    u = Initial Velocity
    a = acceleration
    t = time

    = 145Sin(61)1.5 + 0.5(-9.8)(1.5)2
    = 190.2 - 11.04

    That gives you the vertical displacement from the launching point. However that's not the "y" value they ask for. Read the question again. Where is the origin?

    For part b...

    I haven't checked x but the y co-ordinate isn't going to be -15.
     
  4. Sep 20, 2013 #3
    hm...so is the y will have plus 15? and as for b i thought if the displacement was in regards to the place fired from and the ground 15m below is the origin wouldn't the displacement be -15? and I still don't really get how to get the answer to them especially factoring in ark and gravity
     
  5. Sep 21, 2013 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes. yi = +15.
    Yes, the vertical displacement is -15m, but I think it's clear that b is asking for the horizontal displacement.
     
  6. Sep 22, 2013 #5

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Oops sorry I misread part (b) of the question. It says "displacement when the arrow lands in regards to where it was shot from" so yes the y co-ordinate will be -15.
     
  7. Sep 22, 2013 #6
    Okay time for an update:
    A.) 120.5m(105.50 + 15m height),178m
    B.)_m,-15m
    C.)
    I have deduced to answer the last two I need how long the flight was...and I do not know how to do that.
     
  8. Sep 22, 2013 #7

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Golbez22, it would be much easier to help you if you were to post all your working.
    You have the equation yf = yi + vi t + 1/2 a t2 for the vertical. You know yi, vi, yf and a. Solve the quadratic.
     
  9. Sep 23, 2013 #8

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Nope. Which is the horizontal and which is the vertical co-ordinate? Which one do you need to add 15m to ?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted