I don't understand the ''definition/law'' aspect of Newton's laws

[Delta2]

When you put many heavy books on a plastic table, you will see that the table deforms. There is force between the table and the books, but no acceleration.

yes but the deformation happened because some of the points of the plastic table (the points under the books where the deformation happened) were accelerated abit and then they stop abit lower.

 

[stevendaryl]

yes but the deformation happened because some of the points of the plastic table (the points under the books where the deformation happened) were accelerated abit and then they stop abit lower.

Well, the forces present NOW shouldn't depend on what happened in the past. Right now, there are no accelerations. So if force is defined in terms of acceleration, then it would seem to imply that there are no forces right now.

I think that logically it makes more sense to let "force" be primitive, and to view "force cause accelerations" as a property of forces.

 

[DrStupid]

In the case of balancing forces, the TOTAL force is zero. But gravity still exerts a downward force, and the desk still exerts an upward force.

According to the second law there are corresponding changes of momentum that are balanced too.

So force is not defined in terms of accelerations.

Acceleration is involved in the qualitative definition of force only.

 

[stevendaryl]

According to the second law there are corresponding changes of momentum that are balanced too.

Well, those component accelerations are not observable. If the point of defining F as ma is to define forces as things that are observable, then having unobservable accelerations defeats the purpose, I would think.

I suppose you could define force counterfactually as something that would cause mass $m$ to accelerate at rate $a$, if there were no other forces at work.