Energy as a Function of Speed - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 15 (permalink) Old 09-14-2010, 09:08 PM Thread Starter
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Energy as a Function of Speed

I was reading that cutting your speed by only 10 MPH reduced the amount of energy released in a crash greatly. Since kinetic energy is proportional to the velocity squared, it makes sense. (KE=1/2*M*V^2) So I figured I would make an Excel spreadsheet and see for myself........

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post #2 of 15 (permalink) Old 09-14-2010, 09:27 PM
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yes back when I was doing construction we learned a 200lb man in a 6 ft fall his tie off point will recieve almost 5000lbs force thus the new shock absorbing lanyards and tieing off in 2 places.


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post #3 of 15 (permalink) Old 09-18-2010, 09:17 PM
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And that is why one should slow down before entering an intersection.


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post #4 of 15 (permalink) Old 09-18-2010, 11:55 PM
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It's an even bigger deal than that. Stopping distance is ALSO affected as a second-order function of the speed at which you start...Every MPH you slow down, it requires less distance to slow the next MPH.

In the distance it takes to slow from 100 MPH to 95 MPH, a vehicle traveling 31 MPH could be stopped.

In the distance it takes to slow from 100 MPH to 90 MPH, a vehicle traveling 43 MPH could be stopped.

In the distance it takes to slow from 100 MPH to 70 MPH, a vehicle traveling 71 MPH could be stopped.

It works the same way at city speeds, too:

In the distance it takes to slow from 40 MPH to 30 MPH, a vehicle traveling 26 MPH could be stopped.
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post #5 of 15 (permalink) Old 09-20-2010, 05:22 PM
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And all this data is fine , but really means nothing unless you are trying to make some point about impact energy. Speed is of course relitive...relitive to what is the question here...lol.

Simply, do not go fast unless you have the time and space to slow/stop if something calls for it. Going 150 mph does not kill you. But don't try going that fast unless you not only have a safe/secure bike...but also a remote and empty stretch void of other traffic.

Our poor bodies can only absorb so much energy before parts fail. An impact to a helmeted head at 25 mph may only cause a minor concussion, but the same impact at 30 could kill you.....so realizing the needed stopping distances you need for a particular speed is valuable info to have.

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post #6 of 15 (permalink) Old 09-20-2010, 09:29 PM Thread Starter
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Quote:
Originally Posted by IMBoring25 View Post
It's an even bigger deal than that. Stopping distance is ALSO affected as a second-order function of the speed at which you start...Every MPH you slow down, it requires less distance to slow the next MPH.
How is distance a second order function of speed? I think you mean acceleration.......

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Originally Posted by IMBoring25 View Post
In the distance it takes to slow from 100 MPH to 95 MPH, a vehicle traveling 31 MPH could be stopped.

In the distance it takes to slow from 100 MPH to 90 MPH, a vehicle traveling 43 MPH could be stopped.

In the distance it takes to slow from 100 MPH to 70 MPH, a vehicle traveling 71 MPH could be stopped.

It works the same way at city speeds, too:

In the distance it takes to slow from 40 MPH to 30 MPH, a vehicle traveling 26 MPH could be stopped.
These numbers look right....( I'm curious where you got them?) mathmatically anyway. ( V(stop)=sqrt(Vi^2-Vf^2) ) But this assumes the same acceleration, and that just ain't gonna happen ( reaction time distance, not to mention the friction coefficient is different at different speeds, and so on, blah, blah...... ) Of course your point is right on..... Stopping distance is increases with speed, hence the two second rule for minimum following distance. ( as per MSF guidelines)
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post #7 of 15 (permalink) Old 09-20-2010, 09:41 PM Thread Starter
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And all this data is fine , but really means nothing unless you are trying to make some point about impact energy. Speed is of course relitive...relitive to what is the question here...lol.
That was my point.... impact energy increases as a function of the speed^2, therefore speed is 'relitive' to how much energy is released in a collision

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Originally Posted by Knifemaker View Post
Simply, do not go fast unless you have the time and space to slow/stop if something calls for it......
Yes, and the point of the story I was reading was to learn and practice emergency stopping. The quicker you can decelerate makes a huge difference in how much damage the resulting collision is going to have.
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post #8 of 15 (permalink) Old 09-20-2010, 10:33 PM Thread Starter
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Sorry, but when I see numbers I have to figure out where they came from......
Below is a photo of how I checked out ImBoring's data. It was a fun math exercise

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post #9 of 15 (permalink) Old 09-20-2010, 11:50 PM
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I guess I don't need to show the second-order nature of stopping distance vs. initial velocity any more, since you just did it, complete with some underlying calculus...Nice.

I find that very poignant. In the worst-case, no-time-to-react scenario, the energy released in a crash decays with the square of a reduction in speed, which was how you started the thread.

However, if you have time to react, the benefit of the slower starting speed is also seen in the amount of speed you can scrub off, so the speed at impact (If any impact even still occurs) is reduced by MORE than the initial reduction of speed at the start of the emergency braking event, and it is that speed at impact which is squared to determine the proportionality of the energy released, so you see what might be called a snowball effect.
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post #10 of 15 (permalink) Old 09-21-2010, 12:48 PM
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Post Avoiding the Thump

Spot on Guys.

When I first started riding it seemed cars were pulling out on me, when in actuality my over the speed limit driving caused the drivers to misjudge my actual closing speed. I now ride speed limit or less in town, added a Head Lamp Modulator, and LED Brake-Flash Lights to license plate.

That being said, Emergency Stopping was one of the biggest learning experiences I had in the safety course. Keeping my balance with all of that energy being absorbed into the suspension system was a challenge. (learning to stand on the pegs and balance the bike helped a-lot)

Braking fast and looking for "away out" of the collision is a combination to think about and consider.

Do you Emergency Brake, then slowly point for that ditch to avoid a truck that is crossing your path? After-all, that ditch may be a bit softer than the side of a truck.

Each situation is unique and each rider's bike and ability is different.

I find myself "looking for away out" even when there is no clear emergency.
I do this to cut reaction/decision time down. I also keep my distance from other motorist, because let's face it; no matter who is at fault, in the end the rider is going to loose some skin.


Last edited by w4nmh; 09-21-2010 at 12:51 PM.
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