You want to slide a 0.39 kg book across a table. if the coefficient of kinetic friction is .21, what force is required to move the book with an acceleration of 0.18 m/s^2?

Find the force that would be required in the absence of friction first, then calculate the force of friction and add them together.  This is done because the friction force is going to have to be compensated for.  We will need that much more force than we otherwise would to achieve the desired acceleration:



The friction force will be given by the normal force times the coefficient of friction.  Here the normal force is just its weight, mg



Now the total force required is:

0.0702N+0.803N=0.873N

If there was no gravity then their would be no downword force on the football to have it return the the grownd, and if there was no friction (mainly air friction) then there would be no opposing force the stop the ball from moving through the air allowing the ball to travle ∞.