High School Physics

[ljstella]

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.

[//<86TA>\\]

threve

[ljstella]

threve

...what?

[Tsar]

57.

[ljstella]

57.

No...

[Tsar]

Yes

[ljstella]

Yes

The website rejected the answer. So, I guess not.

I hate WebAssign :(

[FlyingDutchman]

wait does he hit the brakes with 38.0m to go or is 38.0m then he hits the brakes?

[Tru2Chevy]

wait does he hit the brakes with 38.0m to go or is 38.0m then he hits the brakes?

When he sees the barrier he is 38m away, then .75 seconds later he actually applies the brakes.

- Justin

[Tsar]

Ok try 58.

[FlyingDutchman]

When he sees the barrier he is 38m away, then .75 seconds later he actually applies the brakes.

- Justin

ok thought soo

[ljstella]

Ok try 58.

Not that either... :(

[Tsar]

59?

[ljstella]

59?

...are you just guessing?

[bubba428]

idk tsar...I came up with 59.4km/h

[Tsar]

...are you just guessing?
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.

[FlyingDutchman]

w00t

[bubba428]

at least it didn't come in as 75 :lol:

[ljstella]

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.

No its okay, its just that I have a limited number of tries...

[Tsar]

How many more do you have left?

[FlyingDutchman]

How many more do you have left?

59...

[Tsar]

59...
No, you lie.

[FlyingDutchman]

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

[Tsar]

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
That's not correct.

[FlyingDutchman]

That's not correct.

how so?

[ljstella]

Well, I just got locked out, just before I tried something I came up with... I just emailed the teacher to see if I can get another few tries... ugh :( thanks anyway

[FlyingDutchman]

Well, I just got locked out, just before I tried something I came up with... I just emailed the teacher to see if I can get another few tries... ugh :( thanks anyway

whatd you try?

[ljstella]

whatd you try?

I tried using Velocity Final squared is equal to Velocity Inital Squared plus two times the acceleration times displacement.

I got 0= V initial squared + 2(-10.0)(34.04)
V initial squared = 680.6881
V initial = 26.09
It didn't work... or maybe I'm doing it wrong...

[FlyingDutchman]

I tried using Velocity Final squared is equal to Velocity Inital Squared plus two times the acceleration times displacement.

I got 0= V initial squared + 2(-10.0)(34.04)
V initial squared = 680.6881
V initial = 26.09
It didn't work... or maybe I'm doing it wrong...

hmm well your lookin for the max velocity he can go right?

88 km/h = 24.444444444 m/s

so first you figure out how much distance his reaction takes (0.75s)

d = (v)(t) -> d = (24.4)(0.75) = 18.3m til he hits the brakes leaving 19.7m to stop yes?

[Knipps]

Why are you guys doing this kid's HW?

[FlyingDutchman]

Why are you guys doing this kid's HW?

i ... dont ... know .... lol but now i wanna know the right answer

[ljstella]

I came up with 88km/h as something else in m/s...

I multiplied 88 by 60 to get km/s and then divided by 1000 (that's my error... I should have multiplied by 1000 to get m/s) but then I get something ridiculously huge... if I divide, I get 5.28 m/s and then multiply that by .75 and subtract the product from 38.0 to get 34.04m as the distance from when he starts to brake...

[FlyingDutchman]

I came up with 88km/h as something else in m/s...

I multiplied 88 by 60 to get km/s and then divided by 1000 (that's my error... I should have multiplied by 1000 to get m/s) but then I get something ridiculously huge... if I divide, I get 5.28 m/s and then multiply that by .75 and subtract the product from 38.0 to get 34.04m as the distance from when he starts to brake...

km/h -> m/s

x1000 then / 3600

[LTb1ow]

So this proved you all suck at physics.

[ljstella]

km/h -> m/s

x1000 then / 3600

why 3600?

[WildBillyT]

6.023 x 10^23

[FlyingDutchman]

So this proved you all suck at physics.

why?

[ljstella]

6.023 x 10^23

Physics, not chemistry! Avogadro's number isn't going to help me!!

[madness410]

he would end up in jail. speed limit is only 65.

[jims69camaro]

So this proved you all suck at physics.

:nod:

why?

because you haven't come up with the right formula yet, which means if you do hit the answer (by some means, perhaps the russian's beaming method) it will be by pure luck, not because you have applied the right formula.

that's all physics is unless you are among the elite - then you get to try your own formulas and spend years proving/disproving them, supported, of course, by educational grants from the government and wealthy privateers... applying the right formula.

[WildBillyT]

So this proved you all suck at physics.

No, the answer is 19.67 m/s. I posted Avegadro's number because I felt like busting balls- as he should really do his own homework so he doesn't get boned on a test.

I got it the same way 97Camaro did. 18.33m covered until the brakes are applied, 19.67m of distance left, equation will zero out when the initial velocity is ~19.83m/s. The only "ball buster" is how far they want the car to be from the wall to be considered "not hit". 0 is not really right IMO, because they would be exactly touching.

[Tsar]

No, the answer is 19.67 m/s. I posted Avegadro's number because I felt like busting balls- as he should really do his own homework so he doesn't get boned on a test.

I got it the same way 97Camaro did. 18.33m covered until the brakes are applied, 19.67m of distance left, equation will zero out when the initial velocity is ~19.83m/s. The only "ball buster" is how far they want the car to be from the wall to be considered "not hit". 0 is not really right IMO, because they would be exactly touching.So 59 is NOT the answer? :?:

[WildBillyT]

So 59 is NOT the answer? :?:

No?

[FlyingDutchman]

because you haven't come up with the right formula yet, which means if you do hit the answer (by some means, perhaps the russian's beaming method) it will be by pure luck, not because you have applied the right formula.

that's all physics is unless you are among the elite - then you get to try your own formulas and spend years proving/disproving them, supported, of course, by educational grants from the government and wealthy privateers... applying the right formula.

orly? i used my cheat sheet from physics last year with all the kinematic (http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L6a.html) equations and those are the 'right' formulas

[Tsar]

No?
Oh man, I'm so disappointed!

[WildBillyT]

orly? i used my cheat sheet from physics last year with all the kinematic (http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L6a.html) equations and those are the 'right' formulas

LOL. I used the exact same website. Couldn't remember most of them.

[LTb1ow]

Did not figure in speed of light, fail.


:wink:

[WildBillyT]

Did not figure in speed of light, fail.


:wink:

If you want we can help you determine the electrical resistance of a dead fuel pump.

[LTb1ow]

Prob pretty high. I would guess more than enough to melt those puny wires that feed it.

[sweetbmxrider]

wait, what motor?

[LTb1ow]

http://www.capeconcrete.com/products/images/new_jersey_barrier_3000.jpg

Jersey barrier. Gotta love it.

[ljstella]

Just figured I'd let you know, I figured out how to solve it :)

[sweetbmxrider]

Jersey barrier. Gotta love it.

ya, good for wallrides and quarter pipe stuff 8-)

[FlyingDutchman]

and crashing smart cars (http://www.youtube.com/watch?v=ju6t-yyoU8s)

[ljstella]

So, I had to use the quadratic equation, which I didn't think about, and the answer ended up being 75.852 km/h

[LTb1ow]

Wootz for complex numbers!

[MyFirstZ]

what kind of brakes was he using. . . baer, brembo, stock? # of pistons in the caliper? friction material, rotor size (friction surface). . . i need things that matter here, number wont help

[jims69camaro]

orly? i used my cheat sheet from physics last year with all the kinematic (http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L6a.html) equations and those are the 'right' formulas

thank you so much for that website. i have my high school textbook around here somewhere, but i have been unable to find it. that website kicks ass.