# Homework Help: Kinematics Acceleration (27)

1. Jul 27, 2016

### Nanu Nana

1. The problem statement, all variables and given/known data
A train that runs 140 km / h requires about 700m to stop. Calculate the average acceleration of the train while braking.

2. Relevant equations
v: x/t

3. The attempt at a solution
v= x/t
so t= x/v= 700m/ (38.89m/s)=18 s
Now to find acceleration a= v/t
a= 38,89m/s / 18s =2,160 m/s^2
They're asking the average so 2,160 m/s2 is our initial acceleration
therefore
0-2,160 m/s^2 / (2) = -1,08024
Is it the correct formula to use ? or is it wrong ???

2. Jul 27, 2016

### Staff: Mentor

The equation you used for the velocity is correct only if the acceleration is zero (i.e., only if the velocity is constant).

3. Jul 27, 2016

### Nanu Nana

Oh I see. I think i've found out which kinematic equation to use .

I tried this one
initial velocity = 140 km/h (38,89 m/s)
Final velocity = 0 m/s because its going to stop
(vx)^2=(u)^2 + 2x a x700 m
(38,89)^2=1400 x a
a= -1512,34/ 1400
a= -1,0802

4. Jul 27, 2016

### PeroK

That's correct. Can you think of a way to confirm that answer is right?

5. Aug 3, 2016

### Nanu Nana

My workbook says so