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Stopping distance

  1. Jan 22, 2008 #1
    Hello all, I am new to the forum and to learning physics. I was involved in a BAD motorcycle accident on November 10th, 2006 that resulted the amputation of my left foot. The accident sparked my interest in physics. I am trying to figure out exactly how fast I was going when I impacted the moron who pulled out in front of me. All of the formulas that I have found are for total stopping distance and since I did not come to a stop on my own I believe that they will not work for what I am after.
    I would like to thank everyone for their help in advance, and if I posted this in the wrong section I apologize.
     
  2. jcsd
  3. Jan 22, 2008 #2
    Its a simple equation but you need some info. Particularily if you dont know how fast you were going, how fast was the other person going? Was the person stationary? If so, how far did that persons car slide backwards? Whats the mass of the other persons vehicle? You will be able to find your speed if you find the distance the other car drifted if stationary, or the exact speed that person was traveling at the point of impact with its mass. Simple algebra imo. I am not very effective in physics yet so I am sure there are more ways to find this, and probably much more efficient ways since you dont know how fast you were going.
     
  4. Jan 22, 2008 #3

    russ_watters

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    I assume you are thinking of how long it took you to slide to a stop? All you need is your coefficient of friction and stopping distance, but coefficient of friction is not a simple thing to figure out. In real life, it is usually found by experiment. Also, even if you find this answer, it doesn't help too much if the impact slowed you down (unless you flew over the handlebars).

    Sorry about your accident.
     
  5. Jan 22, 2008 #4
    I was traveling 50 MPH when she pulled in front of me. Her car did not move at all, and my skid mark was 77 feet, and by my calculations I was approximately 150ft from her when she started out in front of me. (Using one of the formulas I found on here with the average reaction time and the speed I was traveling)
     
  6. Jan 22, 2008 #5

    Stingray

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    If you were skidding on a dry road, your deceleration was probably around 0.7 g. Starting at 50 mph and decelerating for 77 feet gives you a final speed of about 30 mph.

    The main error in this calculation comes from the initial speed. Being off by 5 mph changes the final speed by 9 mph. Uncertainties from the deceleration rate and skid distance are comparatively small.
     
    Last edited: Jan 22, 2008
  7. Jan 22, 2008 #6
    Stingray: Thank you, I didn't realize that the initial speed changed the final by that much. So I guess I was going between 30 and 40 when I hit her.
    How does the weight of the bike factor in the equation, and does the fact that the rear wheel was skidding while the front wheel was not make much of a difference.
     
  8. Jan 23, 2008 #7

    Stingray

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    The weight of the bike has very little effect. Your maximum rate of deceleration is mainly determined by your tires (and occurs at a point just before skidding). It does make some difference if both tires or just one locked, but I don't think it's going to change the answer very much. It's actually a lot harder to estimate real-world braking performance on a bike versus a car. Cars tend to be much more consistent.

    Anyway, are you sure it was the rear tire that was locked up? It seems like you'd immediately spin out of control if that happened.

    Also, I did assume that the 50 mph starting point was when you started skidding. I don't know if you were braking less agressively before then. If so, the answer I gave won't be accurate at all. For reference, you can derive from the usual kinematics equations that decelerating at a rate [itex]a[/itex] over a distance [itex]x[/itex] gives you a final speed of [itex]\sqrt{v_0^2 - 2 a x}[/itex].
     
  9. Jan 23, 2008 #8

    rcgldr

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    Since the center of mass is relatively high on a motorcycle, most of the weight transfers to the front tire, (all of it in the case of some sport motorcycles that can do "stoppies"), and the rear tire doesn't help braking that much. Assuming normal motorcycle tires, your rate of deceleration was probably between .7 and 1.0g's, depending on how hard you applied the front brake, and what type of motorcycle you were riding (sport bike versus cruiser).

    Since the impact speed is unknown, the initial speed at the start of the 77 foot skidmark would be difficult to estimate. As posted, an increase in the initial speed will result in a much bigger increase in the impact speed.
     
    Last edited: Jan 23, 2008
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