 Force—a push or a pull that may cause an object’s motion to change. › Units: newtons (N) in the SI, pounds (lb) in English › Force is a vector quantity

 Contact Forces —require objects to be touching each other, such as friction, normal force or tension forces.  Field Forces —do not require objects to be touching each other. Examples are gravity, magnetic force and electrostatic force.

 Balanced Forces —occur when all of the forces acting on an object cancel each other out. When the forces on an object are balanced, the object’s motion (its velocity) will remain constant (i.e. it will not speed up, slow down or change direction) › Examples —a tug of war that is tied, a book sitting on a desk, a train moving at a constant velocity.

 Unbalanced Forces —occur when the forces acting on an object do not cancel each other out. When the forces on an object are not balanced, the object’s motion (its velocity) will change (i.e. it will speed up, slow down or change direction) › Examples —a tug of war in which one team is pulling harder than the other team, a book falling to the floor, a train that is going around a curve in the track.

 Net Force —the force that results when many forces are added together—like “net pay” is the amount of $ you get to take home and spend from your paycheck. A net force always causes an object’s velocity to change. › The net force on an object can be found by adding together all of the individual forces that are acting on the object. F net = ΣF