# Homework Help: Classical physics: Motion question

1. Oct 2, 2012

### d3nat

I just want to know if I'm thinking about these the correct way.

Something is put on a track of length d where it slides down and at a certain point, the time is taken given t. Acceleration is constant.

How long do you need to make the track for the time to be 2t.

Initial velocity is zero because it starts from rest.

do I need to do something like a = final velocity / 2*time, find how much that increases my final velocity, then use it in (new final velocity) = d (new distance) / t

and solve for d?

Thanks

2. Oct 2, 2012

### voko

Why do you care about the final velocity? It is not anywhere in the problem. You have d, t and some constant acceleration. Can you not relate these?

3. Oct 2, 2012

### d3nat

Hmmm, acceleration is distance over time squared. Could I just relate it that way? Seems too simple.

4. Oct 2, 2012

### voko

That's exactly what it is. Given this, solve the problem.

5. Oct 2, 2012

### d3nat

Thanks so much for the advice! Guess I was just over thinking things.

6. Oct 3, 2012

### Anan275

Hey Guys sorry, but do you know if this is right?

Question: A car has a final velocity of 26 m/s [N 120 W] after accelerating at 2.5 m/s2 [W] for 4.0 s. Find the initial velocity of the car.

Answer: v1 = =aΔt + v2
v1 = -(2.5 X 4) + 26
v1 = -10 + 26
v1 = 16

7. Oct 3, 2012

### voko

Your velocities have different directions. You must use vector addition to solve the problem.

8. Oct 3, 2012

### Anan275

Can you show me?

9. Nov 22, 2012

### Simon Bridge

@Anan275: your question only seems a little like the original one. How about putting new questions in their own threads?