Find the braking force required to allow a bike to move down a frictionless slope, inclined at $\theta$ radians above horizontal, with a constant velocity of v m/s.
I'm confused because Newton's 2nd Law of motion implies that the sum of forces on the bike must be zero in order for velocity to be constant, meaning that the braking force required is simply the bike's weight, resolved long the direction of the slope, in the opposite direction. But this only makes sure that velocity is constant.
How is it possible to apply a force to make the bike move with a constant velocity, of exactly v? My guess is that it has something to do with work, energy and power, but I haven't found a way yet.