physics
posted by nikki .
A soccer ball with mass 0.410 kg is initially moving with speed 4.30 m/s. A soccer player kicks the ball, exerting a constant force of magnitude 44.0 N in the same direction as the ball's motion. Over what distance must the player's foot be in contact with the ball to increase the ball's speed to 6.70 m/s?

force(time)=change in momentum=mass(VfVi)

force(time)=change in momentum=mass(VfVi)
That gives you the time of contact. The average velocity is Vf+Vi /2, so distance=avg velocity*time. 
i still don't understand

A sled with a mass of 9.50 kg moves in a straight line on a frictionless horizontal surface. At one point in its path, its speed is 2.80 m/s; after it has traveled 2.50 m beyond this point, its speed is 6.50 m/s. Use the workenergy theorem to find the force acting on the sled, assuming that this force is constant and that it acts in the direction of the sled's motion.