3 Balanced Forces2Forces on an object that are equal in size and opposite in direction.
4 Net Force 3 Sum of the forces that are acting on an object. Look at the figures. In fig.1, the car is to the right of the tree. In fig.2, after 2 seconds, the car is to the left of the tree. As the tree does not move, the car must have moved from one place to another. Therefore, here the tree is considered as the frame of reference.
5 Unbalanced Force4Forces that combine to produce a net force that is not equal to zero and cause the velocity of an object to change.
6 Friction5The force that opposes the relative motion between two surfaces.
7 Static Friction6The frictional force that prevents two surfaces in contact from sliding past each other.
8 Kinetic Friction7The friction between moving surfaces.
9 Sliding Friction8The force that acts in the opposite direction to the motion of a surface sliding on another surface.
10 Rolling Friction9The resistance that occurs when a round object rolls on a flat surface.
11 Air Resistance10AKA “drag.” The force that opposes the relative motion of an object moving through air.
12 Vacuum11A space entirely devoid “empty” of matter.
13 Terminal Velocity12The constant maximum velocity reached by a body falling through the atmosphere under the attraction of gravity.