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Calculate speed of object given g-force

  1. Jul 31, 2015 #1
    I am new to this website and am not looking for homework help. In fact I have no background in physics whatsoever.

    I am looking for the speed necessary to cause a specific g-force rating of 5.22 on an object weighing in at 11, 245 lbs and the distance covered would be approximately 1 foot to where the speed would equal zero. The object was traveling in a straight line at the time the impact occurred.

    Thank you for any help you can give me with this. Also, if you could include the equations used to calculate this with a brief explanation I would appreciate it.

    I apologize in advance for my ignorance if I have left anything out.
  2. jcsd
  3. Jul 31, 2015 #2


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    "g-force" is NOT caused by speed, it is caused by acceleration: F= ma. The fact that you then say "the distance covered would be approximately 1 foot to where the speed would equal zero" makes it clear that you are talking about a constant acceleration, not speed.

    At a constant acceleration, a, assuming starting with speed v0, an object will have speed v= v0+ at time t and would have traveled distance s= (1/2)at^2+ v0t. If I am reading this correctly, the "speed would equal 0" when v= v0+ at= 0 so a= -v0/t. In that time, it will have gone a distance which you can calculate. Then solve for t, and determine a.
    Last edited by a moderator: Jul 31, 2015
  4. Jul 31, 2015 #3


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    The object would be decelerating, for which we use the the symbol a, from an initial speed, vi , to zero - the final speed, vf , in the distance of 1 foot, s.
    A constant acceleration formula of value here would probably be vf 2 = vi 2 + 2as
    This requires an force acting on the object to change its speed from v to zero.
    This force comes from the object that it is colliding with.
    The force can be calculated form the object's mass, m, and its deceleration, a, according to Newton's 2nd law
    F = ma
  5. Jul 31, 2015 #4
    best to look at the SUVAT equations

    Find the one that has one unknown (initial speed) and knowns (your inputs) and away you go

    you can google SUVAT
  6. Jul 31, 2015 #5


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    when you say 'g-force rating of 5.22', I assume that means that the object decelerated during impact at an average rate of 5.22g's, where g is the acceleration of gravity, which on planet earth is equal to 32.2 ft per sec per sec (32.2 ft/sec^2). So the object decelerated from its initial speed to zero speed in a 1 foot distance at a = (5.22)(32.2) = 168 ft/sec^2. Use therefore a = 168 ft/sec^2 and s = 1 ft in your v^2 = 2as equation to solve for the initial speed in ft/sec. (You should note as an aside that the average horizontal force of the impact is 5.22 times the objects weight).
  7. Aug 11, 2015 #6
    I want to thank each and every one of you who replied to this thread. I do appreciate that.
    The issue here is I have no understanding of physics so most of this is Greek to me. The reason I posted this crude question is due to a work related issue.
    The object in question would be a forklift with the weight of the forklift, the battery invaded within, and the driver. It was traveling across a cement floor and hit a ramp going into a trailer. There was no load on the forks at the time. Our employer has an instrument attached to each forklift that, when you hit an object, supposedly measures the g-force of said impact. Each forklift has a governor installed to limit maximum speed to 9.2 mph. I am wondering if the is any scenario in which an object weighing that much and traveling at 9.2 mph could produce a g-force "rating" of 5.22. And if not, how fast would the forklift have to be traveling to register that.
    The debate is whether or not these instruments attached are giving a valid measure of impact or if they are being misused by management for disciplinary reasons.
    Not knowing what other information you would need to give an approximate speed I don't know what else to add to the initial question.
    I'm only looking for an approximation. Which I'm sure isn't a very popular term used in physics. Once again, I have almost no working knowledge of physics and the math is Greek to me.
    Thank you for listening to this rant. :)
  8. Aug 11, 2015 #7
    In case anyone needs to know the company who manufactures the attachments named their items "ShockWatch"
  9. Aug 11, 2015 #8
    Also, battery was not "invaded" within it was stationed.
  10. Aug 11, 2015 #9


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    It is possible. The mass of the forklift is not relevant.
    There are scenarios where, if the wheels are rigid enough, driving over a small crack could create acceleration in excess of 5.22g, the speed as read on the speedo doesn't have to even change. Hell, if someone flicked the 'shockwatch' with their finger the reading may exceed 5.22g.
    g force isn't a great measure of driver incompetence, sustained high g force indicates your driver is fast and productive :wink: Low g drivers have probably been fired as too slow.
    If people may get fired if they exceed some arbitrary acceleration value with no evidence of property damage or dangerous driving then productivity will likely suffer. A high peak g force value could indicate ramming the bosses BMW or driving up a ramp lip that was sitting proud due to someone else's error.
    You can see something as innocuous as hopping is in the same ball park as rolling an f16:

  11. Aug 11, 2015 #10


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    The physics can only give a rough approximation by idealizing the impact...in actuality, there are a lot of variables and unknowns. I am not sure how you arrived at that one foot stopping distance. But assuming the forklift stopped in 1 foot and assuming it was traveling at 9.2 mph just before impact, then it's average deceleration would be about 3 g. If however the stopping distance was a foot and the avg deceleration was measured as 5.22 g, them it's initial speed just before impact would have been around 12 mph. Stopping distance is key...hit a wall of marshmallows and distance stopped is greater and g force a lot less than if you hit a steel wall!
  12. Aug 12, 2015 #11


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    Did the forklift battery and the driver "invade" the trailer as a result of the collision with the ramp?
    Assuming that the ShockWatch is on the forklift it would need to come to halt in about 6.5 inches
    to produce such an deceleration.
    Last edited: Aug 12, 2015
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