Engineering Science

[david7]

If anyone can help with my last assignment question please.

A car accelerates along a straight road with a uniform acceleration when it passes A it has a speed of 12m/s and when it passes B it has a speed of 32m/s. points A and B are 1100m apart. Determine the time taken to move from A to B.

Thanks in advance

David

[HowardKennitby]

110 seconds

[HowardKennitby]

no it isn't

[Zacs-Master]

Roughly 60 seconds.

[HowardKennitby]

figure it out from here

http://en.wikipedia.o...i/Equations_of_motion

[nescio]

using the equations
v^2 = u^2 + 2as and v = u + at, where u=initial speed, v=final speed, a=acceleration, s = distance travelled and t=time and rearranging,
you get
t = 2s / (v+u)
=2200/44
= 50 m

[nescio]

oh bugger !

50 s

[wildwood]

Is it; at an average speed of 22m/s between A and B we divide 1100 by 22 ???

[New Judge]

The average speed between the two points is (32-12) =20m/s. 1100m at 20m/s takes 55 seconds.

Or is that too simple?

[wildwood]

^^ I though it was too simple too, but wouldn't the average speed be 22m/s? I suspect it is one of those trick questions that are meant to appear simple but have an hidden agenda. Where are the math experts??

[factor-fiction]

Hi New Judge- did you mean to add rather than subtract
i.e. (32+12)/2 =22
for the average speed

[New Judge]

Hi Factor. No, I didn’t mean to do that, but I did make a balls up (too much electric soup!).

Although I get the same answer as you I like to calculate it by suggesting that the average speed must be the mid point between the starting and ending rate. I would calculate that by halving the difference between the two and adding it to the lower. (32-12 =20; 20/2=10; 12+10=22). So the time needed to cover the 1100m must be 1100/22=50 seconds.

Thanks for shaking me out of my stupour. Think I’ll have another large one!

[HowardKennitby]

would someone care to explain 'electric soup'

[HowardKennitby]

NJ

are you referring to the Urban Dictionary definition?

[nescio]

OK,
I admit I had had a fair bit of wine when I worked out my answer last night (hence my putting 50m instead of 50s),
but the answer is fairly obviously found by dividing the distance by the average speed, as wildwood and factor30 rightly pointed out. I arrived at the same answer by a rather more circuitous route...

[jomifl]

To put it into plain English
The average speed between A an B is(12 +32)/2=22
1100/22=50(seconds)

[jomifl]

of course plain English would use the word 'and' instead of 'an' :-)

[New Judge]

My “electric soup” is not the Scottish version of milk and gas. I use the term to describe any strong-ish drinks (above 20% or thereabouts). So, not wine or beer, probably not sherry or port. To give a bit of detail which led to my brain being fuddled last night, I had drunk a fair noggin of seven star “Metaxa” which I had brought back from a recent visit to Greece. It’s a lovely drink and very moreish. So I had some more, and was incabale of dealing with the simplest of problems!

[Ignoblius]

110 seconds.

Plot speed V as a function of time T. You get an up-sloping line (slope=acceleration) from (TA,12m/s) to (TB,32m/s).

The area beneath this line consists of a right triangle atop a rectangle. The rectangular area represents the distance covered up to point A, which is of no concern. The triangular area represents the distance from A to B, which is 1100m.

Area of triangle=1100m=(TB-TA)(VB-VA)/2

VB-VA=32m/s-12m/s=20m/s

DeltaT=TB-TA=(2*1100m)/(20m/s)=110s

[vascop]

Ignoblius you are wrong when you say: "The rectangular area represents the distance covered up to point A, which is of no concern.".
The rectangle represents the distance that would be travelled if the velocity remained at 12 m/s, and the rectangle the distance travelled due to the increase in velocity over the time interval.
so Area of triangle+area of rectangle=1100=(TB-TA)(VB_VA)/2+(TB-TA)VA
VB-VA=20/m/s
So DeltaT=TB-TA=1100/(20/2+12)=1100/22=50