1. Limited time only! Sign up for a free 30min personal 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!

Throwing a ball against a wall for projectile motion

  1. Apr 26, 2014 #1
    1. The problem statement, all variables and given/known data
    A woman throws a ball at a vertical wall d = 3.4 m away. The ball is h = 1.7 m above ground when it leaves the woman's hand with an initial velocity of 11 m/s at 45°. When the ball hits the wall, the horizontal component of its velocity is reversed; the vertical component remains unchanged. (Ignore any effects due to air resistance.)
    (a) Where does the ball hit the ground? meters (from the wall)
    (b) How long was the ball in the air before it hit the wall? seconds
    (c) Where did the ball hit the wall? meters (above the ground)
    (d) How long was the ball in the air after it left the wall? seconds
    ScreenShot2014-04-26at45348PM.png


    2. Relevant equations

    gravity=-9.8m/s2
    Time= 0=v0T-½gT2 T<0
    t= (v0y+√(v0y2-2gy))/g
    distance/range x=x0+V0xt
    x/y velocity- v0x=Vcos(θ); v0y=vsin(θ)


    3. The attempt at a solution

    I did this for a practice problem with different values, and got the correct answers. With these values it says i've gotten all incorrect answers… If someone could please help and verify what i've done wrong!

    *I made the person's hand the origin
    We're not dealing with forces, and therefor we don't lose velocity due to impact.

    V0y= 11sin(45°)=7.778
    Solving for total time t: ½(9.81)t2-7.778t-1.7 (h=1.7)
    t=+1.324s

    (a) I solved for total distance as regular, and subtracted (d=3.4)
    v0x=11cos(45)=7.778
    X=0+7.778(1.324)-3.4
    x=10.29-3.4= 6.898m

    (b) i made the x value 3.4 (distance)
    3.4=0+7.778t
    t=.539s

    (c ) I plugged in the time it took to get to the wall to the vertical formula, and added h 1.7
    y=7.778(5.39)-½(9.81)(5.39t2)+1.7
    y=4.47m

    (d)
    total time 1.324 - ans(b) = .784s
     
    Last edited: Apr 27, 2014
  2. jcsd
  3. Apr 27, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Do you mean: $$\frac{1}{2}gT^2-vT\sin\theta-h=0$$ ... it can really help to keep the variables around as long as possible.

    Did you realize that ##\sin(45)=\cos(45)=\frac{1}{\sqrt{2}}##?

    The idea is to solve:
    ##x=\frac{1}{\sqrt{2}}vT - d## ... where x is the distance from the wall.

    Where T is given by:
    $$T=\frac{-v + \sqrt{v^2 +4gh}}{g\sqrt{2}}$$
    ... it's a + sign in there because we don't want to accept T<0.

    So $$x=\frac{-v^2 + v\sqrt{v^2 +4gh}}{2g} -d$$

    OK: if this looks like what you did, and you got it wrong, then the only thing I can think of is that you rounded something off somewhere or your calculator threw up a bogus value or something like that.

    Check the arithmetic.

    Also always possible that the model answer is incorrect.
     
  4. Apr 27, 2014 #3
    I actually found that I did my arithmetic wrong solving for time (subtracted instead of addition).

    Thanks for the help!
     
  5. Apr 27, 2014 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Happens to the best of us :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Throwing a ball against a wall for projectile motion
Loading...