# Forces: In what manner do they act on objects?

Newton's laws explain how the application of a force affect other objects. But, in what manner is this force applied? I know about contact forces and action at a distance forces, so my question is not related to them.

My question is this: When a force is applied on a object, does the entirety of the force act upon the object straight away or does the force gradually increase into becoming the said force?

Eg - If I exert a force of 5N, will it act on the object right away or will it gradually increase into being 5N?

It is a rather simple question really. I came to this question through my study of Newton's third law. When we apply a force of 50N on an object and the object can only withstand up to 30N of force, I am told that the force would only act up to 30 N, as that is the maximum reaction force that the object can provide. This brought me to the idea of a gradually growing force.( The force increases up to 30N and stops as it is the maximum force that the object can withstand) Am I wrong here?

If this is indeed correct, does the remaining 20N of force act as an unbalanced force on the parts of the object (now broken) and create an acceleration?

Pls point out any misconceptions I have, if you so find any. Also, pls keep the answers simple, I am still an amateur.

• Your question (v1) is not related to contact forces or action-at-a-distance forces. What else is there? – rob Aug 1 '16 at 19:17
• @rob I believe you mean "what other types of forces are there, except contact and action-at-a-distance forces?" , by "what else is there?" Well, I am not asking about types of forces in my question, rather I am asking about the manner in which forces are being applied. (Pls read paragraph 2 & the example) – SNB Aug 1 '16 at 19:31
• Force is mass by acceleration, I can't imagine the full acceleration being applied immediately because of the inertia of what it is acting on. Just an amateur guess. – user108787 Aug 1 '16 at 19:43
• The problem with Newton's second law is that it defines forces that homogeneously accelerate a body. Now, all real bodies in classical mechanics have some finite size, they have a shape, composition, elasticity etc.., i.e. the assumption that they accelerate homogeneously is an approximation. In practice we can use it if the forces are adiabatically (slowly) turned on, so that no internal degrees of freedom (vibration, heat) are being excited. If that is not the case, then we have to use the formulation of continuum mechanics, which expresses the same laws but allows for deformation. – CuriousOne Aug 1 '16 at 19:56
• I up-voted your comment, but the problem isn't with Newton's laws, rather the problem is applying it to a model that's too simple to address these questions. After all, continuum mechanics is Newton's laws applied to a composite system. – garyp Aug 1 '16 at 20:03

It is commonly accepted that any non-ideal change of state cannot happen instantaneously. If you are transitioning from one state to another, you have to move through every state in between (we're sticking to classical physics here).

That is why many of the equations that we use in physics involve taking the integral over some period of time or distance. It is because the change of state needs either time or distance to take effect.

Another way to think about this, is to think of what would happen if you very gradually increased the force you put on an object. Taking your 50N load applied to an object with a 30N max load, imagine increasing the load very slowly from 0 to 50N. In this case, it is very easy to imagine that the object will break when you reach 30N.

Now speed this process up until it happens very fast (or at whatever speed you wish), the object will respond the same way as it did when the process was slow because the initial and final states are the same.

• Nice answer, I just have one query: when I am increasing the force up to 50N the object shall break when it reaches 30N. Will this completely end the force application process or will the remaining 20N act on the broken parts of the object, providing an acceleration? – SNB Aug 2 '16 at 2:40
• That depends on the material properties of the object. Glass might shatter, steel might bend, paper might rip. I suppose the most general thing that one might say is that for the area over which the effects of the force can be felt, the acceleration of the force will be applied. Remember that the whole object doesn't change state instantaneously. But also, not all parts of the object change state instantaneously. Depending on how the force is applied, it will be felt as a wave traveling out from the initial point of load. – Miniucnchew Aug 2 '16 at 14:29