# Beginner physics question

1Dim. Physics question

I'm in my 3rd weeks of physics, so I'm dealing with Kinematic Equations.

My question is: A train accelerates from an initial velocity of 25.0 km/hr to a final vel. of 65.0km/h in 8.5 seconds..Find its acceleration and the distance the train travels during this time.

for acceleration, I did A = Final Vel - Initial Vel / time and when put in m/s , it comes out to 1.31 meters per second squared...i checked the answered in the back of the book..it checks out..ok, i got that part...

now... for distance, I used X = 1/2(final vel + initial Vel)t no matter how many times I did it, my answer doesnt match up with the books correct answer of 106m

i also thought i might have used the wrong equation so i tried using:

X=Final Vel(time) + 1/2at squared

still doesnt come out right

what am i doing wrong? it's really frustrating, ...I need to get this and do real well because its part of my pre-med requirement, so any help would be appreciated..thanks

Last edited:

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X=Final Vel(time) + 1/2at squared
you didn't copy this equation correctly

sorry, I meant

x= initial vel*time +1/2at squared

ok, and you used this equation and it did not work?
can you write out the exact equation (with the numbers you used) to solve for the second part?

x= initial velocity(time) + 1/2at squared

x = 25(8.5) + 1/2(1.31)(8.5 squared)
x= 212.5 + .655(8.5squared)
x= 212.5 + 72.91
x= 285.41

yet, x is supposed to be 106 in the book

You should be using 25000m/3600s for your initial velocity, not 25. That'll get you 106. :)

Homework Helper
toddler said:
sorry, I meant

x= initial vel*time +1/2at squared
x = initial position + initial vel*time + 1/2at squared. Irrelevant in this problem, but somewhere else it just may not be. ahhh thank you everyone, really appreciate it

damn...i used Oksanav's advice and heres where im stuck

x= 1/2 (25,000m/3,600s + 65,000m/3,600s)(8.5s)

x= 1/2 (90,000m/7,200s) (8.5s)

now heres where im gettin stuck...im multiplying (90,000m/7,200s) by (8.5s) and getting 106.25 and then taking half of that because of the '1/2' at the beginning of the equation....the answer in the book is 106 though..so what did i do wrong

x= 1/2 (106.25) = 53.125??

nazzard
Gold Member
Hello toddler,

you are not applying the correct equation in your last post.

You've already found out in post #5 that:

$$x=v_{initial}t+\frac{1}{2}at^2$$

with a being

$$a=\frac{(v_{final}-v_{initial})}{t}$$

Regards,

nazzard