Physics Question

[ladyskydiver 0]

Ok...I've completed all but 1 of my physics homework problems (not that the professor collects homework but still....), and I can not figure this bloody problem out. HELP!!



If a car travels 1 mile in time t at speed s and the next mile accelerates to speed (s+5) and time is (t-11), what was the original speed of the car?

Life is short! Break the rules! Forgive quickly! Kiss slowly! Love truly, Laugh uncontrollably. And never regret anything that made you smile.

[sinker 0]

you'll have to ask the state trooper who clocked him on the radar gun.

-the artist formerly known as sinker

[VivaHeadDown 0]

I'll guess s = (5(t-11))/11

Don't Confuse Me With My Own Words

[waltappel 1]

Quote

Ok...I've completed all but 1 of my physics homework problems (not that the professor collects homework but still....), and I can not figure this bloody problem out. HELP!!



If a car travels 1 mile in time t at speed s and the next mile accelerates to speed (s+5) and time is (t-11), what was the original speed of the car?

The trivial answer is 1mi./t

You need to define the problem a little more clearly.

Do you assume uniform acceleration over the second mile?

Also, don't you mean t+11?

edited to add:

d = total distance = 2 mi.
t= time to traverse first mile
a = acceleration during second mile
s0 = speed during first mile
s1 = speed during second mile

d = (s0)(t) + (1/2)(a)(t-11)**2

2 = 1 + (1/2)((s1 - s0) / (t-11))(t-11)**2
1 = (1/2)(5)(t-11)
2/5 = t-11
57/5 = t

Since original speed = 1/t then s = 5/57

Walt

[diverdriver 6]

Quote

Ok...I've completed all but 1 of my physics homework problems (not that the professor collects homework but still....), and I can not figure this bloody problem out. HELP!!



If a car travels 1 mile in time t at speed s and the next mile accelerates to speed (s+5) and time is (t-11), what was the original speed of the car?

Might be set up something like this? (and I'm not responsible if you get it wrong...mK?)

2=(ts)+(s+5)(t-11)

1=ts
t=1/s

2=(1/s)s+(s+5)((1/s)-11)
2=1+(1-11s+(5/s)-55)
2=-53-11s+(5/s)
55=-11s+(5/s)


Do you think I did it right up to this point?
Can you solve for S from here?

[jmpnkramer 0]

Quote

Also, don't you mean t+11?

I am not sure what universe you come from but usually when a person drives faster it takes less time to get from A to B instead of more time.

.

The REAL KRAAAAAAAAAAAAAAAAAMER!

"HESITATION CAUSES DEATH!!!"
"Be Slow to Fall into Friendship; but when Thou Art in, Continue Firm & Constant." - SOCRATES

[waltappel 1]

Quote

Quote

Also, don't you mean t+11?

I am not sure what universe you come from but usually when a person drives faster it takes less time to get from A to B instead of more time.

.

Gotcha--the wording made it confusing to me.

Walt

[GeordieSkydiver 0]

Quote

I am not sure what universe you come from but usually when a person drives faster it takes less time to get from A to B instead of more time.

Unless they are driving my car.

(my engine just blew up)

Lee _______________________________

In a world full of people, only some want to fly, is that not crazy?
http://www.ukskydiver.co.uk

[riddler 0]

Quote

If a car travels 1 mile in time t at speed s and the next mile accelerates to speed (s+5) and time is (t-11), what was the original speed of the car? Crazy

Edit - just realized you are probably looking for a number Gotta start thinking less abstract. See a few posts above.

Trapped on the surface of a sphere. XKCD

[kallend 2,106]

I agree the wording is ambiguous. It does not say if the acceleration is constant over the second interval, or if the car "instantly" accelerates and then holds constant speed, or something else altogether.

Until this information is given, you have an infinite number of possible answers, all correct.

...

The only sure way to survive a canopy collision is not to have one.

[justinb138 0]

Quote

Quote

Also, don't you mean t+11?

I am not sure what universe you come from but usually when a person drives faster it takes less time to get from A to B instead of more time.

.

Remember the physics definiton of acceleration?

Quote

the rate of change of velocity with respect to time; broadly : change of velocity

Doesn't mean the speed increased, just that it changed.

[diverdriver 6]

I'm thinking (and yes I'm guessing) that the question was simplified to just assume "instant" acceleration. Or it could be that these are not two consecutive miles. Just two miles being compared and the acceleration was done in between the two miles in question.

Chris Schindler
www.diverdriver.com
ATP/D-19012
FB #4125

[FrEaK_aCcIdEnT 0]

im not smart enough for this thread

ExPeCt ThE uNeXpEcTeD!
DoNt MiNd ThE tYpOs, Im LaZy On CoRrEcTiOnS!

[ladyskydiver 0]

Sorry about that...it was late and my brain had fried when I posted it. Here's the exact wording of the problem.


Driving along a crowded freeway, you notice that it takes time t to go from one mile marker to the next. When you increase your speed by 5.0 mi/h, the time to go one mile decreases by 11 s. What was your original speed?

Life is short! Break the rules! Forgive quickly! Kiss slowly! Love truly, Laugh uncontrollably. And never regret anything that made you smile.

[azureriders 0]

Damn ladyskydiver, you have reminded me of two things. 1) how much I like such problems, and 2) how rusty I have become at them. I am a residential contractor and rambel on and on about the math behind a the length of a rafter or the slope of a hip or valley, but I just dont use enough physics to stay in tuned. I did however get the answer. S=0.08931 and T=11.19649014. I was doing ok until I reached a binomial equation which I had no idea of how to solve. I set up a spread sheet to derive the answer from there. I have inclosed both a copy of my blackboard work and the spread sheet. hope this helps, but I probably would not say anything to the professor about the spread sheet, proabably not the way he had in mind of solving it.

[ladyskydiver 0]

Note to self....include Azureriders in panic physics questions.



I know what the answer is supposed to be. According to the book, it is 38 mph. However, I don't know how to get 38 mph from what the problem gave me, and I refuse to just put down the answer without understanding how to get from point A to point B.

Life is short! Break the rules! Forgive quickly! Kiss slowly! Love truly, Laugh uncontrollably. And never regret anything that made you smile.

[azureriders 0]

your rewording of the problem changed everything, working on it, give me a few

[ladyskydiver 0]

Quote

your rewording of the problem changed everything, working on it, give me a few

Whoops! Sorry about that! And thanks for the help! It's appreciated.

Life is short! Break the rules! Forgive quickly! Kiss slowly! Love truly, Laugh uncontrollably. And never regret anything that made you smile.

[azureriders 0]

There are 3600 sec in any given hour. there for if take any speed given in miles per hour and divide that number ino 3600, the result is the number of seconds to travel one mile. EX. if you are traveling at 60mph then most people know that is should take one min, or 60 sec, to get from one mile marker to the next, this has been a standard of checking your speedometer that truckers have used for years. so 3600/60=60. So if you divide 3600 by S the result would be T, or if you divide 3600 by S + 5 the result would be T - 11. By substituting T in terms of S you get and equations that can be solved for S. However my equation solving skills are very rusty. I am sure yours are fresh and keen so see what you can do with this and please let me know how it worked

[azureriders 0]

plugged 38 into the equation and it works as the correct answer, if I only knew how to slove for it

[diverdriver 6]

Didn't I do that?

I guess not.

[azureriders 0]

after thinking about it for awhile I got one more aproach. Same result though. 38 still checks out as the correct answer (38.02917 to be more exact) but still unable to solve for x. I got it down to a quadratic, but it does not factor, given the correct answer is not 38 evenly.

[Everon 0]

This is a "trick" question. The original speed of the car is clearly "s," as is given. I used to give these kind of questions to my calculus students - and being a little scientifically naive, always sought a complicated solution to an "obviously" easy question. Again, the initial speed is "s.".

Ooops, sorry, did not see your qualifications.....

[azureriders 0]

Quote

Driving along a crowded freeway, you notice that it takes time t to go from one mile marker to the next. When you increase your speed by 5.0 mi/h, the time to go one mile decreases by 11 s. What was your original speed?

The answer S may have been the answer I would have given if I had been takeing a test with the original question on it, but if you read her repost of the real problem, you find that the professor does not give the Term S and is indeed looking for a real number

[ladyskydiver 0]

Unfortunately, the problem is expecting a numerical answer. Which of course is driving me nuts as I haven't been able to solve for it, yet.

Thanks for your help everyone!

Life is short! Break the rules! Forgive quickly! Kiss slowly! Love truly, Laugh uncontrollably. And never regret anything that made you smile.