Acceleration question

[petal5]

I've been attempting the following question:

A 120gram hockey puck sent sliding over ice with an initial speed of 5.5m/s is stopped in 15m by the frictional force on it from the ice.What is the acceleration experienced by the puck?

I'm getting an answer of 2.0157m/s^2.I'm wondering if this is correct or not?
Thanks.

[Hootenanny]

If you assume the acceleration is constant then your answer is incorrect. Perhaps if you show your working we could correct you. Also note that the acceleration should be negative since the puck's velocity is decreasing.

[petal5]

i used the formula vf-vi/tf-ti to get that answer

[Hootenanny]

i used the formula vf-vi/tf-ti to get that answer
And how did you calculate the time?

[petal5]

i divided 15m by 5.5m.however,i now realise this is wrong as the 5.5m. is INITIAL speed

[Hootenanny]

i divided 15m by 5.5m.however,i now realise this is wrong as the 5.5m. is INITIAL speed
Indeed, so if we assume the acceleration to be uniform how does the displacement, initial and final velocity relate to the acceleration?

[petal5]

sorry,i've no idea.

[Hootenanny]

sorry,i've no idea.
No problem, you may want to take a look at these kinematic equations (http://www.physicsforums.com/showpost.php?p=905663&postcount=2) courtesy of Doc Al.

[petal5]

would it be this equation:v^2=vo^2+2a(x-xo)

[Hootenanny]

would it be this equation:v^2=vo^2+2a(x-xo)
Indeed it would, all that is left to do is some algebraic manipulation and to substitute in your values.

[petal5]

thanks for all your help.i'm now getting -1.01m/s^2

[Hootenanny]

thanks for all your help.i'm now getting -1.01m/s^2
Looks about right to me. My pleasure.

[petal5]

Thanks so much,you're a great teacher!

[Hootenanny]

Thanks so much,you're a great teacher!
Thank you very much :smile: I do my best.