Need help from math guru for acceleration info to beat ticket

[ez at nova]

The wife got a ticket in our Jeep Libery the other day for 43 in a 25. The ticket says he used a visual/stopwatch technique to clock her. The distance was 0.021 miles (110 feet) and this was beginning from a stop sign, so 0 mph. I am really finding it hard to believe that a Liberty (10.2 0 - 60 mph) could even get to 43 mph in 110 feet even if it was floored (and she has never once pressed the accelerator more than halfway, much less from a stop sign in a town).
The cop wrote actual speed of 43 mph, but the "ticket" speed was 30 mph, so I guess he knows he's full of it with this technique, giving a 13 mph leeway. But, I want to nail him in court and try to prove with math based facts that there was no way she could have hit 43 mph, so his 13mph "room for error" should not start at 43 mph, more like 35 mph therefore making the low end of the error range well within the speed limit.

Anyone care to take a crack at the theoretical top speed at 110 feet of the Liberty (no brake torquing )

 

[Craig Smith]

Timing a car over a certain distance can only give the average speed for that distance covered. That means that whatever he did came up with an average of 43 mph over this distance, meaning that she had to be WAY over 43 mph to get the average that high.

 

[XxDarkSidexX]

1.) YOU CAN FIGHT THIS AND U CAN WIN! (Ive done it)

2.) Measure the distance out on your street near home, and try it for yourself. MAKE SURE to wirte everything your testing down and record it like a run at the track. Mark out 110feet then from a dead stop floor it and see what mph u get to when you cross the 110 foot mark. It is highly unlike that your wife was flooring it in the Jeep anyways, so you could argue that as well.

3.) The cop said he used a visual/stopwatch???? WTF is that? Ether that means he had a stop watch and timed your wife. Or that is just what he calls, "I saw her driving fast so I gave her a ticket" Ether way I'm sure that the cop didn't show your wife a stopwatch or any other PHYSICAL PROF that he could show the court aginst you.

4.) If he doesn't have the prof you've got him by the nutz! If he trys to say that he can be the judge of speed by looking at something (in this case, he could look at your wife's driving and think he knows how fast she was going) just do this. Challange this by taking a coin out of your pocket and dropping it, and the ask him how fast the coin was going. This was the kicker in my case I ended up getting off with no fees or fines or ticket.

 

[87 WE2]

Jeff,

Tell her to go to court & fight the ticket.
She will win because there there is a chance for human error.

By that I mean if he clicked the stopwatch a second before or after she crossed the first line or the second line that he would have stoped the watch on it will have increased her speed, etc...

I have fought them and won. Got one once doing 67 in a 25 and got away with it, & I was speeding

Good Luck Come up to the Thomas Chevy cruise if you can Friday night its the last one of the season... Later, Ed

 

[1badTTA]

OK, I will give it a crack.

Saying 0-60=10sec, average acceleration is 6mph/sec

1m/h (1mph) =1.4667ft/sec

If she accelerated at 6mph/sec untill 25 and drove at 25 after that she would cover 109.95ft in 5.0seconds

If she would have continued accelerating after 25 she would be going 30 at 117.3ft in 5.0 seconds or 110ft in 4.86seconds.

Since the average human has a reaction time of .5 seconds (been to the drags lately?) then the .14 second difference is well within the margin of error the officer faced with a stopwatch.

Does that help?

 

[1badTTA]

Also, My TTA on direct scan usually shows an average of about 10mph/sec and will hit 60mph in about 4.2 seconds on the street again according to DS. See sig for times.

Also, how was the officer able to see both the "starting and finish lines" acurately from one spot since they were 110ft away from each other and be able to tell the precise moment that the same part of the vehicle crossed each?

 

[HurstGN]

Challange this by taking a coin out of your pocket and dropping it, and the ask him how fast the coin was going.

Well, best guess is it took about 1 second for the coin to hit the floor...I'd say about 22 mph.

 

[BLACK6PACK]

Hurst GN, I think your a little off. I got 21.8 mph if it took the coin 1 second to hit the floor! This is figuring a 32ft/sec/sec excelleration with no wind resistance!

 

[webleyaz]

I didn't bother breaking out any math. My 11.0 TTA took 660 ft. to get to 100MPH. Even if it got to half that speed in a little over 1/6 the distance I'd be suprised. I doubt the Liberty has sticky enough tires to do it.

 

[gn85]

If you want to figure out 'roughly' what the Liberty will do, why don't you just take it to the track on Friday Test and Tune. That way you will get a timed readout by supposedly accurate timing equipment. At the strip, you will of course get a 60 ft. time and other various timing marks, including an 1/8th and 1/4 with speeds. With all that information, and a little toying around with a spread sheet and an X, Y chart you can plot the acceleration and estimated speed at 110 ft.

Furthermore, there are more then a few police officers on this board who I'm sure could give you further advice on how to beat this rap.

 

[ez at nova]

Thanks for the info and support so far.

To be more specific, this is the device he wrote that was used. It said "Robic" http://www.omp1.com/robic.html

I'll have to pass all this good info on to her so she can represent herself properly. I wish I could do it for her but I bet the judge wouldn't appreciate that.

That is a good idea to go back to the spot and floor it and see what speed it hits at the 110 foot mark (I will also measure to see if where he says he was is actually 110 feet). In my WRX this morning, I gave it 3/4 throttle and barely hit 40 at the spot he got her from. This just makes me think even more that I'm right. Like I said, he obviously knows he's way off if he wrote 30 mph as the real ticket speed. But then, in my oppinion, it gets down to, why would a cop pull someone over for doing 5 over...that's a joke.

 

[webleyaz]

OK I was intrigued. Using the distance travelled versus time to obtain 43 MPH average from a dead stop she would be travelling 168MPH at the 110' mark. Not likely. At constant acceleration, The Liberty would require 215' to get to 43 MPH, 143' to get to 35, and would only get to 30 MPH in 110'. That's at full throttle.
I got problems.

 

[ez at nova]

Beautiful stuff webleyaz. Can you lay one of those examples out in a formula to show how you got your answer?

 

[AirBuick]

I think we're missing a key piece to this puzzle and unless I missed it, although she traveled 110 ft., we don't know how long it took her to do that! Did the cop not write down the total time to go 110 ft.? Since we're measuring distance in "feet" and time in "seconds", then velocity is "FEET per SECOND". For example, a constant velocity of 88 feet per second is 60 mph. (5280 feet divided by 60 seconds). To put it another way, if your cruising at 60 mph, you're travelling at 88 feet per second. You must have the "time" variable to compute velocity(speed) The other confusing factor here is since this ticket was clocked from a dead stop, you have to deal with "acceleration"(our favorite topic). Acceleration is defined as the rate of change of "velocity". During trial, ask the cop to provide these variables and compute her speed for the judge. I doubt he can do it. You need to apply a calculus formula to do it. Good luck.

 

[Buick From Hell]

Don't any of you guys have a timeslip hiding someplace? How fast is an 11 second car going at 60' under full throttle? (and do the timeslips show 100' speeds?)

 

[ez at nova]

AirBuick,
Yes, that is the same thing I was thinking...how we are seeing figures here with no known time. The cop obviously had the time...this is what his Robic timing device gave him, and he computed it. The formula/time was not written on the ticket, just the "answer" of 43 mph.

 

[ijames]

You need the time and need to know what acceleration assumptions he made. If he assumed she was accelerating at a constant rate (so many mph per second), then the average speed is half the instantaneous speed as she crossed the finish line. Did he claim that 43 mph was her top speed as the crossed the 110 ft line, or what? If that's the case, and if he assumed constant acceleration, then her average speed was 21.5 mph (14.66 feet per second) and she needed 7.50 seconds to travel the 110 feet. For comparison, using your 10.2 seconds to go 0-60 mph, that gives an average accleration of 5.88 mph/sec and in 7.5 seconds the highest speed the vehicle could have reached is 44.1 mph.

As for reaction time, that should be about the same at the start and stop so it should cancel out.

 

[DavzGN]

here's what i figured

At 30 mph, you travel 158400 ft/hr = 30mph * 5280ft/mile
So at 30mph you travel 44 ft/second = 158400Ft/sec divided by 3600 secs per hour

traveling 110ft at 44ft/sec would take 2.5 seconds 2.5 * 44 = 110

At 15 mph, you would be going 22ft/sec( half of 44ft/sec at 30mph). so in 5.0 seconds you will go 110 ft.
At 45mph you would go 66ft/sec. and it would take 1.667 seconds to go 110ft

The site http://www.car-videos.com/performance/view.asp?id1=32&id2=0 lists the 2002 liberty limited 4x4 as having 0-30mph time as 2.5 seconds, 0-40mph in 3.7 seconds, and 0 to 100ft in 3.3 seconds.

Based on this it would be possible to be going close to 40mph at 110 ft. If he was using radar, it might be possible to be clocked at 43mph. If he was timing a vehicle over a marked distance, it would be necessary to be going 70-80 mph at 110 ft to average 43 mph to over a measured 110 ft when starting from a stop. This would be pretty hard to do in a liberty, unless you dropped an LC2 in there.

David

 

[ez at nova]

So, is there a way that this Robic timing device could give him "top speed" or "speed at the finish line"? I understand how it could give average speed.

 

[bruce]

Originally posted by ez at nova

Anyone care to take a crack at the theoretical top speed at 110 feet of the Liberty

From some old software, as I recall a 2.0 60' time would put you at well over 30 MPH.
40 in a jeep at moderate throttle for 110' would sound easy.

Was she speeding?.
If so then why not admit to it?.