Two blocks in contact -- find the force between them

In this problem why do I need to use this formula? F – F12 = m1 a along the x-axis. What does F12=-F21 stand for? And I don't understand when they say to find the magnitude of the forces between the two blocks

lola9 said:

What does F12=-F21 stand for?

Newton's third law.

lola9 said:

In this problem why do I need to use this formula? F – F12 = m1 a along the x-axis.

Because it is Newton's second law applied to this problem.

lola9 said:

What does F12=-F21 stand for?

Newton's third law. (as already stated by @PeroK )

lola9 said:

And I don't understand when they say to find the magnitude of the forces between the two blocks

They are asking to find the size of the force exerted by one block on the other.

Now that you know what is being asked, please show your work. According to our rules you must show some effort before receiving homework help.

PeroK said:

Newton's third law.

but why do i need to use that in that problem ?

lola9 said:

but why do i need to use that in that problem ?

Because, presumably, you want to solve the problem. If you don't want to use Newton's laws, now is the time to drop physics.

The force balances on the two blocks separately are $$F-f=m_1a$$and $$f=m_2a$$where f is the magnitude of the contact force between the two blocks and a is the acceleration of the blocks. If we add these two force balances together, we obtain $$F=(m_1+m_2)a$$

PeroK said:

Because, presumably, you want to solve the problem. If you don't want to use Newton's laws, now is the time to drop physics.

sorry if I am wasting your time if you do not want to help me properly. Just don't reply. I know that I might be asking stupid questions, but I am making effort to understand the problem. If you do not want to help that is ok.

Did my answer in post 8 not make sense to you?

lola9 said:

sorry if I am wasting your time if you do not want to help me properly. Just don't reply. I know that I might be asking stupid questions, but I am making effort to understand the problem. If you do not want to help that is ok.

It can be difficult to understand what the blockage is that a student has.
Asking why you need to use an equation is a strange question unless you think you have another way to solve it. If an equation is valid, and using it allows the problem to be solved, and you can't find another way, then of course you need to use it.

Can you clarify your question? Is it that you do not understand what the equation says?

lola9 said:

I don't understand when they say to find the magnitude of the forces between the two blocks

By Newton III, action and reaction are equal and opposite. That means that if body A exerts a force ##\vec F_{AB}## on body B at some instant then, necessarily, body B exerts force ##-\vec F_{AB}## on body A at the same instant.
These two forces have the same magnitude; only their directions differ.

lola9 said:

sorry if I am wasting your time if you do not want to help me properly. Just don't reply. I know that I might be asking stupid questions, but I am making effort to understand the problem. If you do not want to help that is ok.

If you think that suggesting that you use Newton's Laws is NOT trying to help you learn how to solve the problem, then I have to agree with @PeroK, you need to find some other field of study.

We are NOT trying to be rude to you, we are pointing out that you don't seem to understand the fundamentals (Newton's Laws) that are used to solve such problems and that that is where you need to focus.

If you don't know and understand Newton's Laws, then forget about this problem --- go back to basics and learn them. THEN you'll be ready to solve this kind of problem.

lola9 said:

I know that I might be asking stupid questions, but I am making effort to understand the problem.

And your questions were answered. Making an effort is different from showing an effort.

phinds said:

If you think that suggesting that you use Newton's Laws is NOT trying to help you learn how to solve the problem, then I have to agree with @PeroK, you need to find some other field of study.

We are NOT trying to be rude to you, we are pointing out that you don't seem to understand the fundamentals (Newton's Laws) that are used to solve such problems and that that is where you need to focus.

If you don't know and understand Newton's Laws, then forget about this problem --- go back to basics and learn them. THEN you'll be ready to solve this kind of problem.

So you are telling me to abandon a field because I do not understand one problem? The problem I asked an explanation for is here to help me understand and apply Newton's law. So it is normal that I ask a question about the fundamental laws. Take into consideration that some people are slower than others. It takes time for me to understand some problems but that does not mean that I am incapable of doing physics or that I should find another field of study. I had a problem with that exercise because most of the time I use Newton's second law to solve that type of problem. I just did not understand why I needed to use the third law instead of the second law. That was my question

lola9 said:

So you are telling me to abandon a field because I do not understand one problem?

It was a light-hearted remark. The serious side is that, as far as classical mechanics is concerned, Newton's laws are the only game in town. If you don't use Newton's laws, you have nothing else.

lola9 said:

So you are telling me to abandon a field because I do not understand one problem?

No, was talking about your seeming refusal to see that Newton's laws are the way to solve the problem.

lola9 said:

The problem I asked an explanation for is here to help me understand and apply Newton's law.

No, that is most emphatically what you did NOT do. Even after it was pointed out that Newton's Laws were the way to attack the problem, you asked why they need to be used. The needed to be used because, as @PeroK just said, Newton's laws are the only game in town.

lola9 said:

I just did not understand why I needed to use the third law instead of the second law.

You use both of them. Draw well-separated free-body diagrams of each block. Use Law II to show that the net force on each block is nonzero. Use Law III to show that the forces that the blocks exert on each other are equal in magnitude but opposite in direction.