Can't Identify Error in Calculation

[RBF]

[Mentor's note: This thread was moved to the homework section from General Physics, so it doesn't use the template.]

Going over a basic kinematics problem with the prompt stating vi=0, x=1000m and Δt=5 and vf and a need to be solved using average v and a. Calculated vf=200ms and a=40m/s/s. But then I also calculated for the same variables using x=.5(Δv)t and all turned wonky and I can't figure out why. Solving for vf=(2)(1000m)(1/5) I get 400m/s. Any insight into what error(s) Iam making?

 

[phyzguy]

Your first calculation looks to be wrong. Show more work on how you got to vf=200 m/s and a = 40m/s^2 and we will figure out where your error is.

 

[RBF]

I used Δx/Δt to get vf with my initial time and displacement set as 0. 1000m/5s=200m/s. I calculated average acceleration using a=(vf-vi)/(tf-ti) initals values both at 0 (200m/s)/5s to get a=40m/s/s.

 

[phyzguy]

I used Δx/Δt to get vf with my initial time and displacement set as 0. 1000m/5s=200m/s. I calculated average acceleration using a=(vf-vi)/(tf-ti) initals values both at 0 (200m/s)/5s to get a=40m/s/s.

Using Δx/Δt will give you the average velocity, not the final velocity. If the initial velocity is zero, the average velocity is 200 m/s, and the acceleration is constant, what is the final velocity?

 

[RBF]

Not given, which made me rethink my approach. Should have used x=vi(t)+(1/2) at² to solve for acceleration and then use that value for vf=vi+at.

 

[phyzguy]

Not given, which made me rethink my approach.

It doesn't need to be given. You should be able to figure it out given:
(1) Initial velocity = 0
(2) Average velocity = 200 m/s
(3) Acceleration is constant

Should have used x=vi(t)+(1/2) at² to solve for acceleration and then use that value for vf=vi+at.

That's how I would have done it.