# Kinematics in 2D

1. Oct 12, 2007

### circuscircus

1. The problem statement, all variables and given/known data
A particle moves in the xy plane with constant acceleration. The particle is located at $$\vec{r} = 2\hat{i} + 4\hat{j}$$ m at t=0s. At t=3s it is at $$\vec{r} = 8\hat{i} - 2\hat{j}$$ m and has a velocity $$\vec{r} = 5\hat{i} - 5\hat{j}$$ m/s

What is the particle's acceleration vector a?
2. Relevant equations

3. The attempt at a solution

Here's what I did
$$\vec{a} = (\hat{r_{1}} - \hat{r_{2}}) / \Delta t - \vec{r} = 5\hat{i} - 5\hat{j}$$

The answer is just that without the $$\vec{r} = 5\hat{i} - 5\hat{j}$$

Could someone tell me why?

2. Oct 12, 2007