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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You’re thinking of the wrong thing. F=MA is about the force currently applied to an object, not how much it is “generating”. As such, a speeding car where the gas and brake are not being used will have little force applied to it.
What you are thinking of is the amount of kinetic energy it has, which is related to how much force needs to be applied to stop it. This is (1/2 * mass * velocity ^ 2) and does increase with speed.
[https://www2.chem.wisc.edu/deptfiles/genchem/netorial/modules/thermodynamics/energy/energy2.htm](https://www2.chem.wisc.edu/deptfiles/genchem/netorial/modules/thermodynamics/energy/energy2.htm)
Because things that move tend to keep moving.
Think about a car in space – it has a speed and a mass, but it doesn’t take any force to move it. It’s just going to float forever. If you wanted to slow it down or speed it up, you’d need to apply force to make that change.
The car has momentum, but no force is involved. If the car were to hit an asteroid and shove it, then it would be applying that momentum as force – given by the asteroid’s mass multiplied by the change in its speed.
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.
For the car, it has to constantly push against the air so it needs force just to keep at the same speed (the air is applying the same force back against the car).
So let’s imagine things in space instead:
If you push a rock floating in space it will speed up. If you stop pushing on the rock it will keep going at the same speed even though you’re not applying any force to it.
It still has energy from its speed of course, and if hits something it converts that speed into a force on the other object.
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
The usual statement statement is confusing in my opinion. No acceleration means not net force, not necessarily no force at all. In your car example, the car is generating force on the ground with the tires. It just so happens that it is exactly equal to the force the air is applying in the opposite direction (drag). If one of those 2 is stronger, the car will either accelerate or slow down. The car as a whole has not net force applied to it, but is still feeling more stress on the components as you get to higher constant speed, because both forces, while still equal, are stronger (more drag and more torque from the motor). Also, energy on the other end depends on speed, not acceleration. Energy can be visualized as the amount of damage in a car crash, as material deformation absorbs energy.
> If a car moves consistently at 80 kmh and it weighs at 600kg, is it not generating any force?
Correct.
When you’re in a car, you can’t feel it moving if the speed is constant. You can only feel it changing its velocity or direction.
Or perhaps a better example is a plane. You can feel forces during take off, landing, and turbulence, but do you feel anything when the plane is in constant motion? No it feels no different than when it’s sitting there on the runway.
This is a core principle of relativity–and not just special relativity, but Galilean Relativity, which Galileo worked out hundreds of years ago. He claimed that if you were below deck on a boat, and the sea was smooth, there is no experiment you can do that could determine if the boat is in motion or not. He was right–pretty clever for a guy who had never actually been on a boat.
And that was important, because one of the main arguments against his claim that the Earth revolved around the Sun was that surely if the Earth was moving we’d be able to feel it.
If you could feel force from just velocity, then you’d feel the force of Earth rotating at 460 metres per second right now. Instead, it feels like you’re sitting still.
Also, an important note: in physics, “acceleration” doesn’t just mean “getting faster”. Any change of velocity is acceleration–including slowing down or changing direction. Changing direction without changing speed is still acceleration, that’s why you feel a force when your car turns around a corner.
It is generating force, but less force than if it were accelerating. The SUM of all forces on an object equals mass times acceleration. In your example, the car tires are putting enough traction force on the ground to equal the air resistance and rolling friction that are pushing against the car.
A good way to visualize it is to ride a bike on flat ground and stop pedaling it at a low speed (say 10 mph). Air resistance and friction are very low at that speed and the force you are contributing is zero. Your MOMENTUM( weight * 10 mph) is still signficant.
Newton’s first law states that, unless influenced by something external, an object does not change the way it is moving. Something that is still remains still, and something that is moving remains moving.
If an object does change the way it is moving (i.e. if it accelerates), then there must be an external influence on it. We refer to influences that change an object’s motion as “forces”, and the amount of force acting on an object we *define* to be equal to its mass times the rate at which its motion changes*. This is Newton’s second law.
So, yes. If an object moves at constant speed, that means either A) there is no force acting on it or B) there are forces acting on it, but their combined effect is the same as if there were no forces acting. In either case, we would say that the net force on the object is zero.
*technically, force is defined as the rate of change of an object’s momentum, which is only equal to ma for objects with unchanging mass.
Force is defined as the rate of change of momentum. Momentum is the product of mass and velocity. Mass is a constant for a given object at this level. So force is the product of the mass and the rate of change of velocity – and rate of change of velocity is known as acceleration.
All of this stuff gets much easier when you can use calculus: a lot of the formulas to memorise in mechanics are just special cases of standard results from calculus.
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