Question

When Jackson throws a tennis ball in a straight path, what two forces cause the ball to eventually stop moving?

Answer 1

After Jackson lets go of the ball, there are two forces on it
that influence what it's going to do.  They are gravity and
air resistance.

Air resistance is just plain friction.  As long as the ball continues
to move through air, it loses kinetic energy to the air and gradually
slows down.

While the ball is in the air, there are no other forces on it that cause
it to stop moving. 

If you absolutely must have another one, it doesn't occur until the
ball hits the ground.  But that isn't even another force.  It's just more
friction, from scraping against concrete and grass.

Gravity pulls the ball down to the ground.  But gravity doesn't make
the ball stop moving.  In fact, gravity tries to make it move faster and
faster, and gravity would succeed if it weren't for all the friction that
the ball encounters.

Answer 2

Well, the force that originally caused it to start moving was inertia, but the  forces that cause it to stop are friction and gravity. Gravity pulls the object towards the center of the Earth, causing it to slow and eventually stop, and friction is the resistance between the ball and the ground (or whatever).

Hope this helps!