Simple acceleration problem

[rasen58]

A car accelerates uniformly from rest, covering a distance of 125 m in 5 s. What is the car's acceleration in m/s^2?

 

I thought the velocity would just be d/t = 125/5 = 25 m/s

And then I thought the acceleration would be v/t = 25/5 = 5 m/s^2

But apparently the answer is 10 m/s^2

 

Why?

[Behrus58]

Hi.

You can't use d/t for a movement that has acceleration.do you want to find the velocity of the start of the movement ?

you don't need that.

From v=at+v0 , we know that : x = 1/2 (a)(t)^2 + (v0)(t) + x0

so just put these numbers : x = 125 , t = 5 and you'll have a = 10 m/s^s

The reason you can't use d/t is that , the idea v = d/t is only for movements with no acceleration , which have a constant velocity.

[rasen58]

Oh wow, I didn't know that. Thanks, that helps.

[swansont]

A car accelerates uniformly from rest, covering a distance of 125 m in 5 s. What is the car's acceleration in m/s^2?

 

I thought the velocity would just be d/t = 125/5 = 25 m/s

 

 

That would be the average velocity, but the final velocity has to be higher. Since a is constant and you start from v=0, the final speed is actually twice the average.