# Homework Help: Particle in Positive Direction

1. Apr 10, 2012

### Painguy

1. The problem statement, all variables and given/known data
d) When is the particle moving in the positive direction?
$f(t) = cos(πt/4), t ≤ 10$

2. Relevant equations
$f '(t) = -(π/4)sin(π(t)/4)$

3. The attempt at a solution
$0 < -(π/4)sin(π(t)/4)$

$(π(t)/4)>πn$

$t>4n 0<=n<=2$

$t>4 t>8$

im probably skipping a simple step. wut do?

2. Apr 11, 2012

### Staff: Mentor

So after multiplying both sides by -4/π, you get sin(πt/4) < 0.
Sketch a graph of y = sin(πt/4) and you'll see that 4 < t < 8 is indeed the interval.

I don't get what you did to arrive at the inequality below.