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!

Homework Help: Projectile Motion Kinematics Problem

  1. Mar 7, 2010 #1
    1. The problem statement, all variables and given/known data

    Will a ball, kicked at 14.0 m/s vertically and 9.0 m/s horizontally, clear a bar 3.0 m high and 20 m away from the kicker? Solve.

    2. Relevant equations

    Kinematics Equations.

    3. The attempt at a solution

    Haven't encountered this type of problem, didn't know where to start.
  2. jcsd
  3. Mar 7, 2010 #2


    User Avatar
    Homework Helper

    Horizontal motion , and vertical motion, are independent ... linked by TIME .

    re-word the question to make TIME explicit :
    "will the ball be at least 3m high WHEN it gets to the bar at x=20m ?

    After you figure out what time that happens, you can find its height.
  4. Mar 8, 2010 #3
    ok, so im trying to learn these ones too. here is what i know. 14m/s vertical vector component of velocity. how high will the ball travel? well, this a kinematic equation problem.
    Vo = initial velocity = 14m/s. (says in the question)
    V = final velocity (when the ball is at the top of its flight path, this is) = 0
    a = acceleration = -9.8m/s^-2 (only force acting on this object)
    t = time = ?
    x = change in x, or change in displacement, or change in distance vertically = ?

    but with three knowns, we can find a fourth. in this case it would be good to see how high the ball is going to go, so we want to find 'change in x'

    we know that
    V^2 = Vo^2 + 2*a*x
    this is on of the five kinematic equations derived from the ideas that velocity = change in distance/ change in time, and, acceleration = change in velocity/ change in time

    so, rearanging mathematically this formula, to isolate x, we get
    ((V^2) - (Vo^2)) /2*a = x
    now we can substitiute in the values we know to solve for x
    ((0-14^2))/2*-9.8 = x
    10 = x is what i get. so this tells us the ball will get to a maximum height of 10m at some point. this is the first thing we need to know
  5. Mar 8, 2010 #4
    so then, to find the time it takes for the ball to get to the maximum height
    use the formula (V-Vo)/a = t
    this gives a time of 1.43s

    to find how far horizontally this point is from where the ball is kicked, now look at the horizontal component. 9m/s. the ball is travelling with a velocity of 9m for 1.43 s,
    using the formula v = change in distance/change in time, rearange this to get
    velocity*time = change in distance
    9*1.43= 12.87m. so it has not yet reached the bar 20m away.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook