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!

Need help with velocity, acceleration, time and distance

  1. Jan 15, 2016 #1
    1. The problem statement, all variables and given/known data

    You need to calculate the acceleration (m/s 2 ) of a jet fighter which is launched from
    an aircraft-carrier by catapult, given the jet’s required takeoff speed (km/hr) and the
    distance available on deck to reach that speed (metres) as the catapult accelerates the
    jet from rest to takeoff. Assume constant acceleration. You are also to calculate the
    time (secs) for the jet to be accelerated to takeoff speed. You are also required to
    report the acceleration in terms of its g-force where g = 9.80665m/s 2 .

    The analysis part of this lab is based around 2 simultaneous equations, with unknowns
    for acceleration and time given velocity and distance. Solve these equations so that
    you can calculate both acceleration and time in terms of distance and velocity. The
    easiest way to do this is to solve the first equation for a (acceleration) and then to
    substitute that expression into the second equation and then solve it for t (time).


    2. Relevant equations

    Relevant formulae are (simple Newtonian motion with initial velocity of 0):
    v = at
    s = ½at 2
    where v = velocity, a = acceleration, t = time, s = distance.

    3. The attempt at a solution

    Hello
    I dont understand the question, could someone please explain it to me?
    I know the input or given data is velocity and distance
    but how to solve everything? Thank you!
     
  2. jcsd
  3. Jan 15, 2016 #2

    Suraj M

    User Avatar
    Gold Member

    Which part didn't you understand?
    You're given distance and final velocity, is there a way you can calculate the acceleration? Any equation you know?
     
  4. Jan 15, 2016 #3
    Is this the correct equation? but then how to find the time and where do we get the vf and vi?
    Thanks
    http://media.wiley.com/Lux/18/329818.image1.png [Broken]
     
    Last edited by a moderator: May 7, 2017
  5. Jan 15, 2016 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The first part of that, ##a=\frac{\Delta v}{\Delta t}##, is correct since you are told the acceleration is constant and the initial velocity is zero. But you cannot use velocity=distance/time since that only gives an average velocity. Instead, the second of your two Relevant Equations is appropriate. That is valid for constant acceleration and an initial velocity of zero.
    I recommend that you remember the more general form of it, ##s=v_it+\frac 12at^2##. That will cover the cases where the initial velocity is not zero.
    For the present problem, you know the initial speed, the final speed and the distance; not time. So this equation is not sufficient either.
    However, you can combine it with your v=at equation. That gives you a pair of equations each containing unknowns a and t, so you can solve.
     
    Last edited by a moderator: May 7, 2017
  6. Jan 16, 2016 #5
    Hi, how to combine it? thank you.
    Can you give me step by step equation which one I should do it first? thank you
     
  7. Jan 16, 2016 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Have you never solved simultaneous equations?
     
  8. Jan 16, 2016 #7
    Yes,
    Please help me ><
     
  9. Jan 16, 2016 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Take the two equations you quoted in the very first post on this thread. Use one of them to express t in terms of other variables, then use that to substitute for t in the other equation. This is how we solve simultaneous equations, isn't it? Please show more effort.
     
  10. Jan 16, 2016 #9
    thank you but "then use that to substitute for t in the other equation"
    i dont understand that part
    also how do I solve both equation since I dont know the t?
    T = distance/velocity right?
     
  11. Jan 16, 2016 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    No, that is not one of your two equations in your original post. I already told you you cannot use velocity=distance/time here because it is not constant velocity.
    The equation you do have is t=v/a. This means that wherever you see t in the other equation you can replace it by v/a. That is what is meant by substitution. It is the standard way of solving simultaneous equations.
     
  12. Jan 16, 2016 #11
    Thanks, last question. How do you know that t=v/a ?
     
  13. Jan 16, 2016 #12

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    It's a simple rearrangement of the v=at equation you quoted in post #1.
    As I mentioned in post #4, it should really be ##\Delta v= at##, i.e. the increase in velocity is acceleration times time. But in this case we know the initial velocity is zero.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted