Why can't I get the answer to this problem?

[RESmonkey]

A car moving with an initial velocity of 3 mph accelerates at the rate of a(t) = 2.5t mph per second for 7 seconds. How far did it go in those 7 seconds?

A. 0.046 mi
B. 0.047 mi
C. 64.25 mi
D. 0.273 mi

I integrated acceleration, then used the initial velocity to find the velocity equation:

v(t) = 1.25t^2 + 3

I integrated that from 0 to 7, and got 163.92. WTF?

Then, I thoguht, I might have done 7 hours on accident, so I integrated from 0 to (7/60).
I got .351 mi. Again, WTF?

I went back, converted units (hey, maybe they need to be in Miles Per Seconds? ), replacing
3 mph with .05 mps and 2.5t with (1/24)t. Then integrated from 0 to 7.

I got 2.73 mi. That looks like choice D. But why is it not? Typo?

And why didn't I get 2.73 when I did it the second time around ( with integration from 0 to 7/60 )?



I got really pissed, and used an old physics formula, distance = InitialVelocity * time + (1/2)acceleration*timesquared.

Didn't get any of the answer choices, again.

Guidance Appreciated.


Cliffs:
Can't get simple problem.
I posted multiple ways I did it.

 

[imported_Tick]

We do not convert seconds to hours by dividing by 60. Try integrating from 0 to 7/3600.

Edit: or use X(t)=X(0)+V(0)t+(1/2)a(t^2).

 

[RESmonkey]

Thanks, but I'm still getting a wrong answer. And I LOLed when I saw that time mistake

 

[Ns1]

the only solution is to /yourself

 

[imported_Tick]

Oh, I see it. The accel is in mph/s. Mixxed units = bad.

 

[RESmonkey]

So only fix the 3 mph?

 

[Fenixgoon]

edit nvm

 

[RESmonkey]

I keep getting .0058 miles. This happened 3 times so far.

 

[RESmonkey]

I feel like plucking off my balls.

 

[videogames101]

oh, damn, it's seconds, not hours...

one sec

I got .026 miles

 

[imported_Tick]

A is not constant!

7
?(3/3600)t+(1/2)(2.5t/3600)(t^2)
0

 

[rezinn]

I got .0282

edit: its seven seconds, so it has to be a or b

 

[videogames101]

wait, i got .0262

got it by doing d=v*t

v, being average velocity (13.43 mph)


t, time being 7 seconds

 

[imported_Tick]

Originally posted by: videogames101
wait, i got .0262

got it by doing d=v*t

v, being average velocity (13.43 mph)


t, time being 7 seconds

a is a function of t.

 

[Cyco]

Originally posted by: RESmonkey
I feel like plucking off my balls.

How does one pluck off their balls...or better yet know the sensation to want to create said occurance?

 

[RESmonkey]

So no luck yet? hmmm...

And yes, I've known a is not constant. That's why I integrated 2.5t and not 2.5.

 

[naldo]

Eenie meenie myny moe ....

Thats what I would do.

 

[RESmonkey]

Grrrr....

 

[imported_Tick]

a=dv/dt
v=dx/dt

a=.000694t=dv/dt

v=.000347(t^2)+.000833=dx/dt

x=.000116(t^3)+.000833t|0,7 => .046

 

[Kyteland]

What's the problem? You have the answer right there. All you need to do is divide by the factor of 3600 like Tick mentioned.

163.92/3600 = 0.046 mi

 

[imported_Tick]

Originally posted by: Kyteland
What's the problem? You have the answer right there. All you need to do is divide by the factor of 3600 like Tick mentioned.

163.92/3600 = 0.046 mi

Now I feel stupid for working on this for so long... right in front of me.

Note to self, don't trust people when they say what you said to do didn't work. People are liars!

 

[RESmonkey]

Wouldn't dividing 163 by 3600 only give you some sort of average of 7 seconds in the 7 hours instead of the precise 7 second distance?

 

[imported_Tick]

Look at my post! It's correct!

 

[Eeezee]

Why would anyone ever assign a problem using mph and acceleration in mph/s... that's just stupid.

 

[imported_Tick]

Originally posted by: Eeezee
Why would anyone ever assign a problem using mph and acceleration in mph/s... that's just stupid.

Do you live in the US?