# Homework Help: Help With finding displacement in uniform acceleration

1. Sep 27, 2013

### mithilsheth

1. The problem statement, all variables and given/known data

A subway train starting from rest leaves a station with a constant acceleration. At the
end of 7.8 s, it is moving at 11.778 m/s. What is the train’s displacement in the ﬁrst 5.6394 s of motion? Answer in units of m

2. Relevant equations

a= velocity final - velocity initial / time

delta x = velocity initial(time) + 1/2 (a)(t)^2

3. The attempt at a solution

delta x = 0 (7.8)+ 1/2 (1.51)(7.8)^2
delta x = 1/2 91.8684
delta x = 45.9342
that is delta x for 7.8 seconds. how do i find delta x for 5.6394 seconds?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 27, 2013

### Staff: Mentor

Use the same formula, but with the different time! (Why did you solve for x at t = 7.8 second?)

3. Sep 27, 2013

### mithilsheth

i also did substitute 5.6394 s for the time rather than 7.8 seconds, and i got 66.4208532002 for delta x. but this was wrong. what did i do wrong?

4. Sep 27, 2013

### Staff: Mentor

Show your calculation. (You know you must have done something wrong. This displacement is bigger than the one you found for the longer time. That can't be right.)

5. Sep 27, 2013

### mithilsheth

delta x = 1/2 (1.51)(5.6394)^2
delta x = 24.0111384

that is what i got this time.....

6. Sep 27, 2013

### Staff: Mentor

That's more like it.