# Homework Help: Electron - Vector Problem

1. Sep 12, 2004

### 0aNoMaLi7

One part of this problem has me confused....id appreciate any guidance. I have part (a) and (b) but (c) and (d) are TOTALLY losing me. i don't even know where to begin. THANKS

"An electron's position is given by r=3.00t i - 5.00t^2 j + 3.00 k, with t in seconds and r in meters"

(a) In unit-vector notation, what is the electron's velocity v(t)?
My answer: 3.00 i - 10.0t j+ 0.00 k

(b) What is v in unit-vector notation at t=6.00s?
My answer: 3.00 i - 60.0 j+ 0.00 k

(c) What is the magnitude of v at t = 6.00 s?

(d) What angle does v make with the positive direction of the x axis at t = 6.00 s?

Thank you.

2. Sep 12, 2004

### Tide

To find the magnitude of a vector just square each component, add them up and find the square root.

You can find the angle between two vectors using the "dot product:"

$$\vec A \cdot \vec B = A B \cos \phi$$

where A and B are the magnitudes of the vectors and $\phi$ is the angle between them.

3. Sep 12, 2004

### 0aNoMaLi7

thanks.... solved it :-)