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 ?
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Force is always an interaction between a thing and something else.
If you consider a simplified car which just weights 600kg and it somehow finds itself going 80kmph, at that moment in it’s time, no, it is not experiencing any force.
How did it to get to go 80kmph in the first place is a different question there were forces acting on it which transferred energy into it which made it gain speed (accelerate).
Now the car will apply force and experience force when it does interact with train going 100kmph relative to it , but even that only happens when the car transfers energy into the train and slows down or bounces in another direction.
There is no mathematically rigorous proof to what Newton told us. But you and I experience this phenomenon and can tell that it is close to being true. And of course experiments have empirically proven that Newton’s force law holds when speeds are not a significant fraction of speed of causality.
EDIT: Newton law is a simplification for a simple point mass object. A realistic car is a smorgasbord of interactions all ending up accelerating it using an engine at a pathetic rate of efficiency. Each such interaction is either trying to move the car or trying to slow it down. The sum total of these individual is either more than zero or less than zero
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