AP Physics B help

[Q]

A car slows down from 23 m/s to rest in a distance of 85 km. what was its acceleration , assumed constant?

i don't understand it and none of my friends over AIM are answering :gloom:. If you can, please tell me answer and how you got it. thanks appreciate the help :awesome:

[Chillax]

Don't you use the formula (Vf)^2=(V0)^2+2ad and just plug in to find a?

0^2=23^2+2(a)(85000)
a=0.0031 m/s^2

I think this is the right answer, but don't hold me to it.

[Sean]

The car's Vi (initial velocity) is 23m/s, and the question tells you it stops, so therefore Vf (final velocity) is 0m/s, Vave (average velocity) is then (Vf - Vi) / 2, giving you (23-0)/2 = 11.5m/s on average. In total, the car travels 85 000 meters. Vave = displacement/time, aka time = displacement/Vave. Time = 85 000m / 11.5m/s = 7 391.3 seconds

Acceleration = ΔV / Δtime : Acceleration = 23m/s / 7391.3s = 3.1x10^-3 m/s^2, same answer as Chillax. Method is different but thought it might help you understand better if seen from another perspective ^^

[Lan]

...Oh god I'm doomed :T_T:

[Cannibal]

I hated physics...

[Chillax]

...Oh god I'm doomed :T_T:

It's not really that bad compared to Organic Chemistry.

Although it worked for this question, doing averages of the speed can get you into the habit of doing that for questions where the speed is not averaged, which is why I prefer just plugging all the information into the formulas or reworking the formulas that I'm given.

[Lan]

Chemistry and Biology I can do. Physics...not so much.

[Shanghai]

...Oh god I'm doomed :T_T:

This.

[Phunkie]

Pick all B's on the multiple choices.

You have a 25% of getting them all right each time!!

[Phunkie]

Pick all B's on the multiple choices.

You have a 25% of getting them all right each time!!

[Infamy]

More than once has my physics teacher put all the correct answers on E.