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

Question

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

Answer 1

Force multiplied by velocity is momentum. Newton tells us that an object with no force resultant force acting on it will have a constant momentum. For an object of fixed mass, this means the velocity does not change. Newton also tells us that any resultant force will change the momentum according to the relationship that force is proportional to the rate of change of momentum. For a body of fixed mass, this makes force proportional to rate of change of velocity, that we call acceleration.

Resultant force is important here. The resultant force is the sum of all of the forces acting on the body (a vector sum, so direction is important as well as size of the force). If we consider the car travelling on a road at 80 km/h, there will be lots of forces acting on it, for example it has weight due to gravity, there will be forces pushing up on the tyres from the road, there will be aerodynamic drag, rolling resistance, and the engine will be providing some driving force through the wheels. If the car is moving at a constant speed, what it means is there is no resultant force, that is to say all of these forces exactly cancel one another, and momentum remains constant (ignoring things like fuel burn and air entering the engine and leaving the exhaust pipe to keep things simple).

In the real world it is essentially impossible to get something that has zero forces acting on it, because we are sitting on a big planet with mass and hence gravity, so any object with mass will have a force acting on it due to gravity (weight). We are also sitting in an atmosphere, so there is air pressure pushing on everything, and while that pressure is almost constant all around, there is a slight pressure gradient that causes buoyancy (hence balloons can fly). If anything moves, or the air moves (wind, hence flags and kits fly and sailing boats move), there will be aerodynamic forces.