# Eli5 Why is force equal to mass multiplied by acceleration and not speed ?

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If a car moves consistently at 80 kmh and it weighs at 600kg, is it not generating any force? since you know, a=0, am I stupid or is Newton ?

In: 13 Because speed is constant if no force is being applied. A force of zero doesn’t make a thing suddenly stop… And if you think it does, you should consider “stopped, relative to what?“ No, it is not generating any force. It has a lot of energy, but only outputs force when it is slowed down either by hitting the brakes or a wall. It has momentum which is what you are describing. Because of friction you need to apply a small force to keep the car at constant speed. Because force is calculated through *CHANGE* in rate and direction of the velocity (speed with direction) and not through the velocity the object already has.

What you are thinking if is momentum where P(power)= M(mass) x V(velocity).

Edit: Also as you asked: “is the car not generating any force”, force cannot be generated out of thin air if there is no other transfer of energy in the system. Meaning that if there is no combustion, propulsion, friction, or any other way for energy to transfer, there is no force acting on the car which means the car is staying a constant velocity. One more thing, it is incorrect to say force is generated because force cannot be generated but it can act on a body from another source that transfers it’s energy like I said above.

You may already know this but just in case, force can also act in a body to slow it down, for instance when you step on the breaks in a car, the breaks are apply a force to the car by causing friction which transfers kinetic energy to thermal energy(heat). What I’m saying is you can have negative forces as well which means that it’s a force acting in the opposite direction based on your perspective. What you are missing out of your reasoning is the force of friction caused by the air, tyres and mechanics of the car. This is forces which slow you down and do act proportional to the speed. If you account for friction then in order to move at a constant speed you need to generate as much force as the friction which is a constant multiplied by the speed. If this is not the case then the car is going to accelerate depending on the difference in force between the friction and the force you are generating.