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Old 05-12-2015, 07:13 PM   #1
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Exclamation Remember your HS Physics?

Me neither. Didn't take it. But have come across something called 'kinetic energy' (energy of movement) that is quantified by the formula K.E. = 1/2 Mass times Velocity Squared. The question often comes up on RV forums about how fast people drive their rigs and disagreements ensue. Just saying . . . Do the Math.
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Old 05-12-2015, 07:31 PM   #2
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I remember this one well because I taught it and it's applications every year for 35 years.
"Energy can be neither created nor destroyed merely changed from one form to another."

Brakes are designed to change energy of motion into heat energy. If you hit a stationary object (concrete Bridge) at 80 MPH you will probably die and everybody else in your totaled vehicle. Or you can apply the brakes, convert that energy of motion (and there's a lot of it) into heat energy. Drive away, cool everything down and repeat over and over and over and over.

Then we complain about spending $100 for some decent pads, machine the rotors and they should always be machined when pads are changed. They work well and for a long, long time. Always buy the best parts and never cut corners because your life does depend on them.

Thanks, I love giving that lesson. It's so easy to understand, straight forward and very important.

TeJay
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Old 05-12-2015, 11:29 PM   #3
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Just hold the pedal to the floor, you'll get there faster...
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Old 05-13-2015, 09:35 AM   #4
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So, from the above, it can be understood that speed kills, and brakes save lives.
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Old 05-13-2015, 03:06 PM   #5
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Originally Posted by Threesticks View Post
So, from the above, it can be understood that speed kills, and brakes save lives.
And it also takes longer to get there...
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Old 05-13-2015, 05:47 PM   #6
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Just hold the pedal to the floor, you'll get there faster...
Many folks drive like they are in the Devil's own hurry to get where they are going.

Well.. I know where I'm going..(My funeral and grave) and what they do not i know is they are like destined (Their funeral and grave).

And yes.. You are 100% correct.. They will get there faster.
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Old 05-13-2015, 05:55 PM   #7
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I thought I posted this already.. but we had network issues.

Shortend.. once the wheels lock the size of the vehicle does not matter (NOTE:ABS brakes do affect this somewhat)

ON the standard "Reffernce" road (Something you will likely never find)

5.5 times (speed/10)^2

That means.. Divide your speed by 10,, Square the result and multiply by 5.5

That's how far you will skid

10 MPH 5.5 feet
20 MPH 22 feet
40 MPH 88 Feet
50 MPH 137.5 fet
60 MPH 198 feet

As you can see the distances are really climbing now

Time 3/4 of a second is 22 feet at 20 MPH (From Driver's ed many moons ago) so 27 1/2 feet in a second at 20 MPH
or 1.375 feet second = 1 MPH

At 60 mph you travel 82.5 feet in 2 seconds so if you are two seconds behind the vehile in front of you it will take you 66 feet (Normal reaction time) to hit the brakes and you will then slide 198 feet...

Consider expanding the gap.
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Old 05-13-2015, 05:58 PM   #8
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Actually, TeJay, you probably should have mentioned that "If you hit a stationary object (concrete Bridge) at 80 MPH," you are also transforming your kinetic energy into heat energy. The significant difference being that your brakes make the transformation just a wee bit more gradually than does the concrete bridge.

Joel
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Old 05-14-2015, 09:50 AM   #9
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Quote:
Originally Posted by wa8yxm View Post
.......

At 60 mph you travel 82.5 feet in 2 seconds so.........

wa8yxm, could you perhaps clarify? I'm pretty sure I missed a couple of points in your explanation, because 60 mph has been the equivalent of 88 ft/second for at least the past 67 or so years that I'm aware of, ;-)
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Old 05-14-2015, 09:54 AM   #10
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ALSO - from aerodynamcis, Drag goes up by by square of speed. If you double your speed, your aerodynamic drag increases by a factor of 4..
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Old 05-14-2015, 09:59 AM   #11
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Quote:
Originally Posted by wa8yxm View Post
I thought I posted this already.. but we had network issues.

Shortend.. once the wheels lock the size of the vehicle does not matter (NOTE:ABS brakes do affect this somewhat)

ON the standard "Reffernce" road (Something you will likely never find)

5.5 times (speed/10)^2

That means.. Divide your speed by 10,, Square the result and multiply by 5.5

That's how far you will skid

10 MPH 5.5 feet
20 MPH 22 feet
40 MPH 88 Feet
50 MPH 137.5 fet
60 MPH 198 feet

As you can see the distances are really climbing now

Time 3/4 of a second is 22 feet at 20 MPH (From Driver's ed many moons ago) so 27 1/2 feet in a second at 20 MPH
or 1.375 feet second = 1 MPH

At 60 mph you travel 82.5 feet in 2 seconds so if you are two seconds behind the vehile in front of you it will take you 66 feet (Normal reaction time) to hit the brakes and you will then slide 198 feet...

Consider expanding the gap.
Also to stop from 60 MPH the brakes are on longer and thus hotter.

I have experienced brake fade on the race bike as the pads got to thin. At the end of the straight going from 140 to 60 the level almost came back to the bar (I pulled off that lap)

Even the best brakes can fade. Fluid can boil even without water in it. And heat changes the coefficient of friction.

I love speed but never in the RV as I want to be around to go fast on other things.
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Old 05-14-2015, 10:26 AM   #12
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Quote:
Originally Posted by wa8yxm View Post
I thought I posted this already.. but we had network issues.

Shortend.. once the wheels lock the size of the vehicle does not matter (NOTE:ABS brakes do affect this somewhat)

10 MPH 5.5 feet
20 MPH 22 feet
40 MPH 88 Feet
50 MPH 137.5 fet
60 MPH 198 feet

As you can see the distances are really climbing now

Time 3/4 of a second is 22 feet at 20 MPH (From Driver's ed many moons ago) so 27 1/2 feet in a second at 20 MPH
or 1.375 feet second = 1 MPH

At 60 mph you travel 82.5 feet in 2 seconds so if you are two seconds behind the vehile in front of you it will take you 66 feet (Normal reaction time) to hit the brakes and you will then slide 198 feet...

Consider expanding the gap.
Thanks for posting this...some are nitpicking the numbers...but the point is I will skid about 5 vehicle lengths at 60MPH plus 2 lengths for reaction time.

Whatever I am following will also do some skidding/sliding/rolling, so I guess a good rule of thumb for a RV is at least 4 vehicle lengths. I would say six, but that would be impossible with the IDIOTS always cutting in

Dan
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Old 05-14-2015, 10:33 AM   #13
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Originally Posted by Waiter21 View Post
ALSO - from aerodynamcis, Drag goes up by by square of speed. If you double your speed, your aerodynamic drag increases by a factor of 4..
What's the Reynolds Number of your RV?
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Old 05-14-2015, 05:49 PM   #14
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When I posted this, I was simply trying to point out that the faster you drive the worse the crash is likely to be. This was prompted by the responses regarding the video of the DP that rolled following (presumably) a blow-out. The RV appeared to be traveling at a high speed, possibly in excess of 75. (Though not what I expected, I enjoyed the responses that it got!)
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