# Homework Help: Direction and Average Acceleration

1. Sep 7, 2015

### Manh

You and your little brother are rolling toy cars back and forth to each other across the floor. He is sitting at x = 0, and you are at x = 4.0 m. You roll a car toward him, giving it an initial speed of 2.2 m/s . It stops just as it reaches him in 3.0 s.

In what direction is the car's acceleration?
A. The acceleration is in the +x direction.
B. The acceleration is in the −x direction.
I picked B but it was wrong. Can anybody explain it to me?

What is the car's average acceleration?

2. Sep 7, 2015

### Staff: Mentor

What is its initial velocity (note: this has a sign), what is its final velocity? Did the number increase or decrease?

How did you get that number?
With that acceleration, what is the velocity change in 3 seconds? Can that be right?

3. Sep 7, 2015

### Manh

I think its initial velocity is 2.2 m/s which probably has a positive sign. The final velocity would be 0 m/s. Since it moves from 4 m to 0 m, I consider it moves in negative direction.

4. Sep 7, 2015

### Staff: Mentor

If it has a positive sign, the x-coordinate increases. That is not true.

5. Sep 7, 2015

### Manh

I found final velocity is 4/3 m/s then delta velocity 4/3 m/s - 2.2 m/s and divide that by 3 s to get the answer.

6. Sep 7, 2015

### Staff: Mentor

The car stops. What is its final velocity?

7. Sep 7, 2015

0 m/s ?

8. Sep 7, 2015

### Manh

So acceleration is simply 2.2 m/s divided by 3s ?

9. Sep 8, 2015

### haruspex

Yes.

10. Sep 8, 2015

### Manh

Thank you very much!