# Resultant force and constant velocity

Let's say for example, Jack is pushing on a box with force $F$, and John is pushing from the opposite side with force $F$ as well. We say that there is no resultant force, and the box remains stable in its place, right? However, when a car is moving at a constant speed, the resistance is equal to the force that drives the car, so why is the car still moving? Isn't it supposed to be stable if there is no resultant force?

• Force is equal to mass time acceleration, not mass time velocity. This is known as Newton's second law. – DanielSank Feb 3 '16 at 3:05

You are using the word "resultant" in place of a more precise mathematical term - "net", or "sum". Add up all the forces on an object, and if they equal zero, the object experiences no acceleration. For your box, since the forces are in opposite directions, $$\sum F_i=F-F=0$$ For the car, the forces are actually quite complicated, but start with the fact that the car is "moving at a constant speed". That means it has no acceleration (assuming one-dimensional motion). No acceleration means no net force, so the car apparently has no net force on it. All the force "pushing it forward" is being counteracted by the force "pushing it backwards" (air friction, ball bearings, etc etc etc).
• because it had some initial velocity. Or, put another way, in order to get the car moving at all you can't have $\sum F=0$. To accelerate away from zero you have to apply more force in the forward direction then is opposing you in the opposite direction. Once it's moving, you can decrease the force you apply to match the opposition force, and move at a constant rate. – levitopher Feb 3 '16 at 17:38