The photo shows two groups, each pulling in a different direction on a rope. When the combination of forces on each side equal, the forces on either side are said to be balanced. The ribbon tied in the middle of the rope indicates motion as well as the direction of the motion.
If one team pulls with more force than the opposing team, the ribbon as well as the opposing team move in the direction of the greater force. The force that causes the motion is unbalanced. Any time an object at rest moves, it is said to be accelerating, which means to change speed.
Things that free fall accelerate due to the force of gravity. The force of gravity causes objects of different masses to have the same acceleration. For information about this, see PHYSICS:NEWTON’S SECOND LAW OF MOTION.