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Homework Help: Kinematic equations help

  1. Oct 4, 2011 #1
    Hey guys! New to the forum! :)
    Anyways, so in Physics class my teacher gave us these three equations on Kinematics. He says that "pretty much everything can be done with these three", and here they are:

    V(final)= V(0) + AT

    V(final)^2= V(initial)^2 + 2A(ΔX)

    X(final) - X(initial)= V(initial)T + 1/2AT^2

    So V= velocity

    A= Acceleration

    T= Time

    X= Distance

    So he makes us use these equations to do calculations and solve problems such as "Ball thrown up from ground at 23 m/s.. Where is it after 3.7 sec?" And, "Stone thrown down from high cliff at 12 m/s. How fast is it moving after 14 sec?"

    I've been wanting to know how I determine which equation to use for each problem, and how to properly plug in the data. Also, I politely request that you solve the above problems I listed, preferably in a step-by-step manner. Thanks!
  2. jcsd
  3. Oct 4, 2011 #2
    Hi there,

    I'm not going to solve those questions for you, you should think about how those equations given to you relate to the physical realm instead of just trying to figure out what variables you have. You should try to solve those problems and post your answers. Then there will be a whole flock of physics goons trying to help you understand.
  4. Oct 4, 2011 #3
    I'm not sure how to do that exactly. How do you suggest I get started?
  5. Oct 5, 2011 #4
    the equations given are supposed to be used in situations of constant acceleration.
    if you have learned any algebra , then you know that whatever is asked in the problem,you call it x. so in the problems given to you, first write down which quantities are given to you and which ones are asked to find. for example , in the first problem stated by you , you have been asked the difference in initial and final positions, or [itex]\Delta x [/itex], so what quantities are given there ?
  6. Oct 5, 2011 #5
    Thanks for the reply! So initially, I worked with the second equation because it had the "ΔX" symbol... But I had apparently flawed in doing so. I looked over how we did it in class, and my mistake was not in my math, but rather the equation that I used... My main issue is determining which equation to use. By the way, the answer that I calculated for the problem you referred to is X(final)= 18.019... Is this correct?

    I also attempted to work out the second problem. I got V(final)= 12m/s + (-9.8 m/s^2)(14)
    After I solved for V(final), I got 149.2 m/s
    Last edited: Oct 5, 2011
  7. Oct 5, 2011 #6
    yes very good. both are correct.....but in the second equation, you should have positive sign for the acceleration...otherwise your answer (which is correct) will not be what it is...... you should define the positive and negative directions before you start solving...
    if the downward direction is chosen as negative then the second equation should have been

    [tex]V_f=-12+(-9.8)(14)\quad \therefore V_f=-149.2 \;\mathrm{m}/\mathrm{s}[/tex]
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