wiki Word: Acceleration Due to Unbalanced Forces 

Definition: Acceleration is the rate of cange of velocity. Unbalanced forces always cause a change in motion. They are not equal and opposite.

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If you were to run and stop runing with out unbalanced forces, and all the forces were balaced, you would go on for ever but if an unbalanced force were to act on you, you would accelerate(slow down, speed up, or change direction). If the force is acting with you, you will speed up. If the force is acting against you, you will slow down. If the force was acting on you on an angle, you will change direction. 

 In this picture  In this picture

the forces are balanced.            the forces are unbalanced. 

In this picture there are many unbalanced forces like the firction and the wieght of the box.

 In one of our class labs we made a car out of Styrofoam and attached a balloon to the back, blew it up with air and let it go. There unbalanced forces working on this such as when the air pushes the car there is friction on the car and the force of the push which are unbalanced.