# Homework Help: Parametric equations, find speed and direction

1. Aug 6, 2012

### Mushroom79

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

An object moves so it's coordinates at the time t is given by the relationships

x = 25t
y = 20t-5t^2

What is the object's speed and direction at 3 sec?

t = 3 sec

2. Relevant equations

v = √(dy/dt)^2 / (dx/dt)^2

Pythagoras theorem

3. The attempt at a solution

dx=25
dy=20-10t

I'm not sure how I should use this by combining the two formulas above.

2. Aug 6, 2012

### Redbelly98

Staff Emeritus
What you are calling dx and dy are really
$$\frac{dx}{dt} \text{ and } \frac{dy}{dt}$$
which is the same as vx and vy. So you have found the two components of the velocity vector v -- or at least you'll have them once you plug the time value into your expression. So as a start, figure out what the values of vx and vy are, using the expressions you got.

3. Aug 7, 2012

### Mushroom79

Ah, but that is the part I get stuck on,

√25^2+(20-10t)^2