Benchmark 4F
The Physical Setting: Motion
Grades 6-8, page 90

An unbalanced force acting on an object changes its speed or path of motion, or both. If the force acts toward a single center, the object's path may curve into an orbit around the center.
 

NSES Content Standard B 
Physical Science: Motions and forces 
Grades 5-8, page 154 
An object that is not being subjected to a force will continue to move at a constant speed and in a straight line. 

NSES Content Standard B 
Physical Science: Motions and forces 
Grades 5-8, page 154 
If more than one force acts on an object along a straight line, then the forces will reinforce or cancel one another, depending on their direction and magnitude. Unbalanced forces will cause changes in the speed or direction of an object's motion.