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: Kinematics Problem

  1. Jul 10, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    s = ((v + u)*t)/2 - an equation of motion for constant acceleration, which is applicable in this situation.

    3. The attempt at a solution
    Let u be the velocity at which the projectile was launched.

    It is given that sx = symax. So:

    sx = symax = u*cos(θ)*t


    symax/t = u*cos(θ)

    Now, in the y-direction, the projectile was given initial velocity u*sin(θ), and at the top of its trajectory (where it has maximum height symax), it has final velocity 0 m/s. Thus:

    symax = ((0 + u*sin(θ))*t)/2

    , which means:

    symax/t = u*sin(θ)/2

    However, since symax/t = u*cos(θ) :

    u*cos(θ) = u*sin(θ)/2


    cos(θ) = sin(θ)/2

    2*cos(θ) = sin(θ)

    2 = sin(θ)/cos(θ)

    2 = tan(θ)

    θ = tan-1(2) = 63.4349°

    However, this was marked wrong.

    Can somebody see what could have gone wrong?

    Thank you.
  2. jcsd
  3. Jul 10, 2015 #2


    User Avatar
    Homework Helper

    The time to reach the highest point is not equal to the whole time of flight.
  4. Jul 10, 2015 #3
    Ahh... I got it. It takes only half the total travel time to reach the top of the trajectory. Thank you.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted