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09-13-2009, 05:33 PM
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#1
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Join Date: Aug 2009
Location: South Dennis
Posts: 48
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High School Physics
The driver of a car going 88.0 km/h suddenly sees the lights of a barrier 38.0 m ahead. It takes the driver 0.75 s to apply the brakes, and the average acceleration during braking is -10.0 m/s2.
He will hit the wall.
What is the maximum speed at which the car could be moving and not hit the barrier 38.0 m ahead? Assume that the acceleration doesn't change.
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09-13-2009, 05:50 PM
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#2
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Power Member
Join Date: Oct 2007
Location: Milford NJ
Posts: 1,526
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threve
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09-13-2009, 05:58 PM
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#3
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Join Date: Aug 2009
Location: South Dennis
Posts: 48
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Quote:
Originally Posted by //<86TA>\\
threve
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...what?
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09-13-2009, 06:01 PM
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#4
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MIR
Join Date: Jul 2005
Posts: 9,692
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57.
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09-13-2009, 06:04 PM
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#5
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Join Date: Aug 2009
Location: South Dennis
Posts: 48
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Quote:
Originally Posted by Tsar
57.
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No...
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09-13-2009, 06:09 PM
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#6
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MIR
Join Date: Jul 2005
Posts: 9,692
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Yes
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09-13-2009, 06:13 PM
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#7
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Join Date: Aug 2009
Location: South Dennis
Posts: 48
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Quote:
Originally Posted by Tsar
Yes
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The website rejected the answer. So, I guess not.
I hate WebAssign 
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09-13-2009, 06:14 PM
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#8
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Join Date: Dec 2008
Location: NNJ
Posts: 1,489
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wait does he hit the brakes with 38.0m to go or is 38.0m then he hits the brakes?
__________________
1997 Camaro Z28 - 355 LT1, T56, CC503, LTs, ORY, Bald Eagle-back exhaust, !emissions, Lingenfelter CAI, EWP, !CAGS, BMR LCAs, UMI PHB, Koni Str.t/Koni SA, C5 Z06 front / LS1 F-body rear brake swap, factory hurst with short stick 
2014 2LT Cruze 2.0 Turbo Diesel
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09-13-2009, 06:15 PM
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#9
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Co-Founder / Site Admin
Join Date: Jul 2004
Location: Ewing, NJ
Posts: 22,476
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Quote:
Originally Posted by 97camaro
wait does he hit the brakes with 38.0m to go or is 38.0m then he hits the brakes?
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When he sees the barrier he is 38m away, then .75 seconds later he actually applies the brakes.
- Justin
__________________
1999 Camry - Beigemobile DD
2002 Suburban - Wife's DD
2004 Grand Cherokee - Not running / Project / Selling?
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09-13-2009, 06:16 PM
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#10
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MIR
Join Date: Jul 2005
Posts: 9,692
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Ok try 58.
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09-13-2009, 06:17 PM
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#11
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Join Date: Dec 2008
Location: NNJ
Posts: 1,489
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Quote:
Originally Posted by Tru2Chevy
When he sees the barrier he is 38m away, then .75 seconds later he actually applies the brakes.
- Justin
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ok thought soo
__________________
1997 Camaro Z28 - 355 LT1, T56, CC503, LTs, ORY, Bald Eagle-back exhaust, !emissions, Lingenfelter CAI, EWP, !CAGS, BMR LCAs, UMI PHB, Koni Str.t/Koni SA, C5 Z06 front / LS1 F-body rear brake swap, factory hurst with short stick
2014 2LT Cruze 2.0 Turbo Diesel
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09-13-2009, 06:19 PM
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#12
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Join Date: Aug 2009
Location: South Dennis
Posts: 48
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Quote:
Originally Posted by Tsar
Ok try 58.
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Not that either...
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09-13-2009, 06:19 PM
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#13
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MIR
Join Date: Jul 2005
Posts: 9,692
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59?
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09-13-2009, 06:21 PM
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#14
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Join Date: Aug 2009
Location: South Dennis
Posts: 48
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Quote:
Originally Posted by Tsar
59?
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...are you just guessing?
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09-13-2009, 06:22 PM
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#15
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Join Date: Jan 2007
Posts: 5,623
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idk tsar...I came up with 59.4km/h
__________________
2000 Tahoe Z71 5.7
1982 Z28 Pacecar 305
1999 S10 
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09-13-2009, 06:23 PM
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#16
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MIR
Join Date: Jul 2005
Posts: 9,692
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Quote:
Originally Posted by ljstella
...are you just guessing?
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Well the answers are being beamed down to me from the Space Station via the satellite, so the numbers come in a little bit distorted.
Just trying to help.
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09-13-2009, 06:24 PM
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#17
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Join Date: Dec 2008
Location: NNJ
Posts: 1,489
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w00t
__________________
1997 Camaro Z28 - 355 LT1, T56, CC503, LTs, ORY, Bald Eagle-back exhaust, !emissions, Lingenfelter CAI, EWP, !CAGS, BMR LCAs, UMI PHB, Koni Str.t/Koni SA, C5 Z06 front / LS1 F-body rear brake swap, factory hurst with short stick
2014 2LT Cruze 2.0 Turbo Diesel
Last edited by FlyingDutchman; 09-13-2009 at 06:47 PM.
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09-13-2009, 06:24 PM
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#18
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Join Date: Jan 2007
Posts: 5,623
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at least it didn't come in as 75 
__________________
2000 Tahoe Z71 5.7
1982 Z28 Pacecar 305
1999 S10
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09-13-2009, 06:25 PM
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#19
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Join Date: Aug 2009
Location: South Dennis
Posts: 48
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Quote:
Originally Posted by Tsar
Well the answers are being beamed down to me from the Space Station via the satellite, so the numbers come in a little bit distorted.
Just trying to help.
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No its okay, its just that I have a limited number of tries...
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09-13-2009, 06:27 PM
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#20
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MIR
Join Date: Jul 2005
Posts: 9,692
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How many more do you have left?
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09-13-2009, 06:28 PM
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#21
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Join Date: Dec 2008
Location: NNJ
Posts: 1,489
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Quote:
Originally Posted by Tsar
How many more do you have left?
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59...
__________________
1997 Camaro Z28 - 355 LT1, T56, CC503, LTs, ORY, Bald Eagle-back exhaust, !emissions, Lingenfelter CAI, EWP, !CAGS, BMR LCAs, UMI PHB, Koni Str.t/Koni SA, C5 Z06 front / LS1 F-body rear brake swap, factory hurst with short stick
2014 2LT Cruze 2.0 Turbo Diesel
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09-13-2009, 06:34 PM
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#22
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MIR
Join Date: Jul 2005
Posts: 9,692
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Quote:
Originally Posted by 97camaro
59...
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No, you lie.
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09-13-2009, 06:36 PM
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#23
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Join Date: Dec 2008
Location: NNJ
Posts: 1,489
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wait i think i got it... the maximum speed he can be going and stop in time is 19.84 m/s or 71.4 km/h
try that i misunderstood the first time lol
using
d = (v)(t) + (1/2)(a)(t)^2
thats how i came up with my number . . .
v = 88 km/h = 24.4 m/s reaction = 0.75s distance before brakes = d = (24.4 m/s)(0.75)s d = 18.3m to hit brakes total 38.0m - 18.3m leaves 19.7m to stop
t = sqrt (d)(2)/(a) t = sqrt (19.7)(2)/(10) t = 1.98s to stop in time
maximum Velocity he can go using d = (v)(t) + (1/2)(a)(t)^2
19.7 = (v)(1.98s)+(0.5)(-10m/s^2)(1.98s)^2
19.7 = 1.98v - 19.602 39.302 = 1.98v v = 19.84 m/s
__________________
1997 Camaro Z28 - 355 LT1, T56, CC503, LTs, ORY, Bald Eagle-back exhaust, !emissions, Lingenfelter CAI, EWP, !CAGS, BMR LCAs, UMI PHB, Koni Str.t/Koni SA, C5 Z06 front / LS1 F-body rear brake swap, factory hurst with short stick
2014 2LT Cruze 2.0 Turbo Diesel
Last edited by FlyingDutchman; 09-13-2009 at 06:46 PM.
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09-13-2009, 06:39 PM
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#24
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MIR
Join Date: Jul 2005
Posts: 9,692
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Quote:
Originally Posted by 97camaro
wait i think i got it... the maximum speed he can be going and stop in time is 19.84 m/s
try that i misunderstood the first time lol
using
d = (v)(t) + (1/2)(a)(t)^2
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That's not correct.
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09-13-2009, 06:47 PM
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#25
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Join Date: Dec 2008
Location: NNJ
Posts: 1,489
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Quote:
Originally Posted by Tsar
That's not correct.
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how so?
__________________
1997 Camaro Z28 - 355 LT1, T56, CC503, LTs, ORY, Bald Eagle-back exhaust, !emissions, Lingenfelter CAI, EWP, !CAGS, BMR LCAs, UMI PHB, Koni Str.t/Koni SA, C5 Z06 front / LS1 F-body rear brake swap, factory hurst with short stick
2014 2LT Cruze 2.0 Turbo Diesel
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