1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Understanding Equations

  1. Mar 8, 2010 #1
    I have been in Physics for a month now and have been plagued by one simple detail, why in most Kinematic equations is acceleration given the coefficient of one-half? I have been researching lately and have not found any understand reasoning, in most explanations I just see the one-half appear as it had always been there.

    Granted we do not have to understand how to derive one equation into another, I cannot have something like this on my conscience. I guess I like to understand rather than follow blindly. If anyone has explanation for the phenomenon I will greatly appreciate it. My math background goes as far as Calculus 1 which stopped at Integrals so hopefully these equations do not require anything beyond that.

    Thank you. I love the forums, have been an abundance of help with any questions I come across in my efforts to do homework.
  2. jcsd
  3. Mar 8, 2010 #2


    User Avatar

    Staff: Mentor

    It comes from integration and differentiation. Are you familiar with basic calculus?
  4. Mar 8, 2010 #3
    Some "Equations" conform to meet the "reality". Nature is not interested in conforming to "nice looking" equations.
  5. Mar 8, 2010 #4
    @berkeman: Yes, I am familiar with basic Calculus. I know the majority of Physics deals with Calculus and honestly I am upset I am not allowed to take Physics 207 which is a Engineering physics course that revolves around Calculus.

    @jmatejka: You are quite right, the equation is not "nice looking." I am more curious at why it is that way. I have a feeling my stay in Physics this semester will be much more enjoyable if I understand rather than memorize.
  6. Mar 8, 2010 #5
    Well I probably didn't understand what you want but if I did, it comes from the fact that position = average speed * time, and average speed = (initial_speed + final_speed)/2. As final_speed is acceleration * time, after multiplying you get that a*t/2 in the formula...
    Sorry if I said something stupid, anyway...
  7. Mar 8, 2010 #6


    User Avatar

    Staff: Mentor

    So start with a constant acceleration a.

    To get velocity, you intetrate acceleration:

    [tex]v(t) = v_0 + \int a dt = v_0 + at[/tex]

    To get position, you integrate velocity:

    [tex]y(t) = y_0 + \int (v_0 + at) dt = y_0 + v_0 t + \frac{1}{2} a t^2[/tex]

    And in the case where a = -g (pointing down in the -y direction), you can see the equations that you normally use for projectile motion.

    Does that help?
  8. Mar 8, 2010 #7
    Yes, that was basically all I wanted to find out. I had no clue what the first step was, I knew how to get to certain parts but a lot of steps are skipped when people explain things. By taking the integral it certainly makes more sense now.
  9. Mar 9, 2010 #8
    If an object starts from rest and accelerates, then a plot of its velocity over time is a line, starting at zero with a slope equal to the acceleration.

    The distance traveled is the area under the velocity vs. time graph. That area under the sloped line is a triangle. The base of the triangle is the time, the height is the final velocity, or the acceleration*time, and so the area is

    [itex] a = \frac{1}{2}base*height = \frac{1}{2}t*at = \frac{1}{2} at^2[/itex]
  10. Mar 9, 2010 #9


    User Avatar
    Homework Helper

    If acceleration is constant, then you can derive the formula using algebra

    Δt = t1 - t0
    v1 = v0 + a Δt

    average velocity (if acceleration is constant):
    vavg = 1/2 (v0 + v1)

    d1 = d0 + vavg Δt
    d1 = d0 + 1/2 (v0 + v1) Δt
    d1 = d0 + 1/2 (v0 + (v0 + a Δt)) Δt
    d1 = d0 + 1/2 (2 v0 + a Δt)) Δt
    d1 = d0 + v0 Δt + 1/2 a Δt2
    Last edited: Mar 9, 2010
  11. Mar 9, 2010 #10
    "If acceleration is constant, then you can derive this using algebra"

    To be a derivation starting from the fact of constant acceleration, you'd have to prove the equation about average velocity rather than taking it for granted.
  12. Mar 9, 2010 #11


    User Avatar
    Homework Helper

    This can be done geometrically using a graph, time on the x-axis, velocity on the y-axis.

    define delta t and midpoints:
    Δt = t1 - t0
    tmid = 1/2 (t0 + t1)
    vmid = 1/2 (v0 + v1)

    Assume zero acceleration, constant velocity. Draw the graph from {t0, vc} to {t1, vc}. Then area_under_line = vc x Δt = velocity x time = distance.

    For constant acceleration, the midpoint of the line {t0, v0} to {t1, v1} occurs at {tmid, vmid}. The geometric area under the line {t0, v0} to {t1, v1}, can be rerranged by moving the triangle shaped area above the horizontal line vmid to the triangle shaped gap under below the horizontal line vmid, to create a rectangle of area = vmid x Δt = distance. Since average velocity = distance / time, then average velocity = vmid = 1/2 (v0 + v1).
    Last edited: Mar 9, 2010
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook