Trying to understand the relationships between Forces, Mass and Inertia

[ida10]

Homework Statement

Its not really a homework question but something i cant really figure out myself:

An object in motion wants to stay in his motion. This is the reason, why for example, when i sit in a car and it accelerates i press against the seat due to my body wanting to stay at rest. But now what i dont really understand is following:

When applying a force, it like pushing against something right? But what exactly am i pushing against. I get that the object is again pressing against me due to its inertia as it wants to stay in its initial motion. Is this pressing against me due to inertia what i am pushing against? Is this the resistance that is written about in all the articles? But that "pressing" (i dont know how else to call it but i hope its clear what i mean) is the object getting pressed against me due to wanting to keep its motion.

Relevant Equations

I dont want an explaination for the formula F=ma
As i can use it, but i want the theory behind it

I hope i made clear what i mean and i really hope i can get my answers here
Thank you very much for answering already

 

[PhDeezNutz]

$\vec{F}_{net} = m \vec{a}$ is a postulate (or law if you will) there is no deeper theory behind it. It is an assumption that produces valid experimental results.

You're pushing against the car seat due to Newton's 3rd law $\vec{F}_{\text{seat on you}} = - \vec{F}_{\text{you on seat}}$ which is also a postulate (or law if you will) there is also no deeper theory behind it. It is also an assumption that produces valid experimental results.

This might be a quibble but the seat isn't pushing against you due to its inertia. You're being pushed by the car seat due to your inertia. And in turn you're pushing against the car seat because of Newton's 3rd Law.

 

[ida10]

Thank you very much for the reply,
But i mean is there really no answer to my questions?
I mean i must be pushing against something right? How else could you explain the resistance...
I would love I could get a little more detail on my question (if there is :) )

Thank you:)

 

[ida10]

So when i want to change the velocity, the object still wants to go on with its initial motion... now as i want to change its velocity, the mass of the object is still "pushing" the pbject forward... is that what causes the resistance?

I would be really happy if someone helped me understand... what is meant by this resistance thats written everywhere but never really what it means... i get that i am pushing against something...meaning something must be there to resist... but what is it what resists?

 

[haruspex]

The resistance you feel is the force the object exerts on you (tending to accelerate you the other way). One of Newton’s great insights is that forces always come in equal and opposite pairs, so the force you exert on the object is equal and opposite to the resistance you feel.

the mass of the object is still "pushing" the pbject forward...

No pushing is needed to keep the object going forward. It is only needed to change the speed or direction of that motion.

As to what causes the resistance to such a change, I guess that is the Higgs field, but that is a very advanced topic.
https://en.wikipedia.org/wiki/Higgs_boson

 

[ida10]

Yes ok i get that the resistance to going forward that i feel is the force coming from the object on me...
And that this force comes from how much the car is pressed against me, due to its inertia wanting to stay in its initial motion... and that i have to offer exactly same and opposite force on the car to accelerate it...

But is this force on me what is meant with the resistence due to inertia?

 

[jbriggs444]

Yes ok i get that the resistance to going forward that i feel is the force coming from the object on me...
And that this force comes from how much the car is pressed against me, due to its inertia wanting to stay in its initial motion...

Unfortunately, this is wrong. The force from the car on you has nothing to do with the inertia of the car.

If you press on a feather with a force of 50 pounds, the force from the feather on your hand will also be 50 pounds. If you press on a car with a force of 50 pounds, the force from the car on you will also be 50 pounds.

The fact that the feather is accelerating away under the force of your hand does not mean that the force from the feather on your hand is less than the force of your hand on the feather. Those two forces are exactly equal at all times.

The acceleration of the feather has nothing to do with some supposed imbalance in Newton's third law. It has everything to do with Newton's second law. There is a net force on the feather. So it accelerates.

Newton's third law is a way of stating the law of conservation of momentum. Momentum is conserved. If you transfer momentum to a feather, that momentum must come from you. A "force" is a transfer of momentum. The numeric value of the force is the rate at which momentum is being transferred. The rate out from you has to be identical to the rate into the feather. Otherwise, momentum would not be conserved.

 

[PeroK]

Yes ok i get that the resistance to going forward that i feel is the force coming from the object on me...
And that this force comes from how much the car is pressed against me, due to its inertia wanting to stay in its initial motion... and that i have to offer exactly same and opposite force on the car to accelerate it...

But is this force on me what is meant with the resistence due to inertia?

It's possible to learn and understand physics without the concept of inertia. There are simply forces, masses and acceleration. Also, I'd use resistance to mean forces that act against motion, like friction, air resistance and rolling resistance. Mass itself provides no resistance to motion.

If we imagine every object as hypothetically made up of a number of identical elementary objects, then it seems reasonable that the amount of force needed to achieve a given acceleration is proportional to the number of these elementary objects. With this model the mass of an object is simply the number of elementary particles of which it is made.

It would seem absurb to imagine that the laws of physics ought to allow someone to throw an elephant the same as they can throw a ball.

 

[Lnewqban]

Mass itself provides no resistance to motion.

I am sorry to say that this statement sounds confusing to me.
Would you mind explaining that idea a little further for me and for the OP?

 

[jbriggs444]

I am sorry to say that this statement sounds confusing to me.
Would you mind explaining that idea a little further for me and for the OP?

If anything, mass preserves motion rather than resisting it.

Mass could be seen as resisting changes in motion (i.e. acceleration), though "resist" is not exactly the right word since @PeroK is thinking of resistance in the sense of forces doing the resistance.

 

[PeroK]

I am sorry to say that this statement sounds confusing to me.
Would you mind explaining that idea a little further for me and for the OP?

It's all relative. Any mass, no matter how large, will accelerate under any force, no matter how small. It cannot resist acceleration per se. However, the more the mass the smaller the acceleration. I accept that is something of a semantic point, but that's the way I look at it.

A large mass, in a sense, accelerates as fast as possible given the force applied!

This whole idea of "not wanting to move" seems phony to me.

As an analogy imagine getting willing people onto a bus. No one resists, but it takes more time the more people you have to get on board.