The makers of a certain automobile advertise that it will accelerate from 15 to 50\hr.in 13sec.whats the acceleration in ft\sec. squared

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It's given in your problem, it's '15\hr' though I have no idea what you meant there :rolleyes:

acceleration = a = (v2 - v1) / t;
In other words, acceleration is the rate of change of velocity, per unit of time.
v2 = 50 mph = 50 mi/hr
v1 = 15 mph = 15 mi/hr
t = 13 sec
You must first convert v2 and v1 from mi/hr to ft/s.
v2 = (50 mi/hr)*(5280 ft/3600s) = 73.33 ft/s
v1 = (15 mi/hr)*(5280 ft/36002) = 22.0 ft/s
a= (v2 - v1) / t = {(73.33 - 22.0) ft/s} / 13s
a= (51.33 ft/s) / 13s = 3.948 ft/ s^2

RPVega, I know you mean good, but since you're around for quite some time now, the helpers here try not to do the homework/work/assignment of the students who come here. What we usually look for is an interactive session with the asker and try make himself/herself 'see' the way to get the answer.

For one, students can't be given questions without the proper introduction to the topic. They should be able to solve the problem and that's why we go little by little where possible.

I know that here, the question is to find the acceleration, but the fact that the OP asked for the initial velocity in the title suggests that (s)he is confused about something and through my post, I wanted him/her to see that (s)he has all the values required to find the acceleration.

I hope you understand that. There's a Chinese proverb that says, "Fish for someone hungry and he won't be hungry today, teach him to fish and he never will be hungry." Or something around those lines, if you see what I mean :)