# Force can only come from force, right? [closed]

Force is the interaction between objects, so force can only come from force, right?

1. Can force come from velocity?
2. Is the cause of one force another?
• This is sort of what Newton's 3rd law also says, yes. Dec 31, 2019 at 8:22
• Yes. However, one is not the effect of another, instead both the forces are supposed to be simultaneously there.
– user243267
Dec 31, 2019 at 8:27
• @enbinzheng Simply click the underlined hyperlink in my above comment to view the wikipedia page at en.wikipedia.org/wiki/Fundamental_interaction . Note that these are often interpreted as fundamental "forces" as well Dec 31, 2019 at 11:37
• @enbinzheng Forces can depend on velocity (like the magnetic force), but it is not quite correct to say that a force is due to or comes from speed or velocity. The reasons behind the forces, like questions such as why do two bodies attract each other?, do not and will not have a complete answer....(continued)
– user243267
Dec 31, 2019 at 12:47
• Even if you keep on creating new theories (like space-time fabric), you will again have to explain why those theories are correct and eventually you will have to admit that it's the way our universe works and accept it without reason. So the best answer to where do forces come from is that: No one knows, this is how our universe is and we have and had no power to decide how our universe is.
– user243267
Dec 31, 2019 at 12:49

Rate of change of momentum of the two bodies experience the forces this force occurs in pair, because there will be a change in the momentum of both bodies which is what newton third law says. So answer to your question is force come from momentum and momentum come from velocity. (keeping mass same).

Some forces do not arise from other forces, such as the gravitational and electric forces. To stick with the way OP put it, they "come from" mass and charge (at least in a manner appropriate for the level of this question). Similarly for the nuclear forces.

Some forces could be said to "come from other forces", such as friction really being a result of the electromagnetic force. When you boil it down, there are four fundamental forces known in the universe: Gravitational, Electromagnetic, Strong Nuclear, and Weak Nuclear. These are the forces that cannot be said to arise from other forces.

Note: Although mathematically speaking net force depends on acceleration/change in momentum, philosophically speaking it may be more right to say the net force causes the acceleration (the force does not "come from" the motion).

Force actually arises from the interaction of fields with objects but in a way, yes. Note that the forces are independent of each other and exist simultaneously such that net force is zero.

I don’t believe the origin of a force need be another force.

I have a gas in a cylinder fitted with a piston. The gas is in equilibrium with its surroundings and there is no net force on the piston. I now heat the gas which then expands resulting in a net force that pushes the piston and doing work on the surroundings.

I would argue that the source of the net force on the piston is not another force, but heat. And the interaction between the heat source and the gas in not a macroscopic force but heat transfer.

A force can come from velocity, but only from a change in velocity. For example if an object is moving at constant velocity with no force applied to it collides with another object that brings the first object to a stop, the second object applies a force to the first doing work to bring it to a stop according to the work energy theorem which states the net work done on an object equals its change in kinetic energy or

$$W_{net}=F_{ave}d=\frac{mv^2}{2}$$

Where $$F_{ave}$$ is the average stopping force, $$d$$ is the stopping distance and $$v$$ is the velocity of the object just prior to impact.

Hope this helps.

• Heat is also an interaction of microscopic particles, right? Jan 1, 2020 at 0:00
• @enbinzheng Yes. it is the transfer of kinetic energy from areas of high temperature (higher average kinetic energy particles) to areas of lower temperature (lower average kinetic energy particles). But those interactions are not normally counted as macroscopic mechanical work. Jan 1, 2020 at 0:30

Force is Mass times acceleration. So I believe as mass is not going anywhere, its acceleration,i.e., rate of change of velocity that gives rise to the presence of 'Force'. F = ma So in a way, Force does rise from the change of velocity of an object.