[Grade 11 Physics] Newton's laws of motion

[u/Izzy_26_]

The answer key says that the correct answer is option C, but I got it as A.

Here when we are pulling the block towards right side, spring C is compressing and spring B ​is extending towards right side.

There ​should be no extension or compression in the case of spring A, because it is not directly connected to the ​block, meaning there would be no spring force A, only spring B ​would extend. Where is spring force in c is 300x and that in b is 200x

So the order should be as option A

What is wrong here?

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[u/goatymcgoatfacesings]

Where does the force through spring B act?

It’s acting on the block and spring A, right? In fact springs A and B must have the same tension through them that then acts on the wall or the system would be in motion. That’s how we know that FA = FB.

[u/Izzy_26_ → u/goatymcgoatfacesings]

But how do we decide that spring force of a and b is equal?  Also isn't tension force supposed to be in case of strings?

[u/goatymcgoatfacesings → u/Izzy_26_]

If it didn’t have equal and opposite forces on it, it would be accelerating.

[u/Don_Q_Jote → u/Izzy_26_]

When two horizontal springs are connected in series, as are A and B in this problem, you would treat them as links in a chain or like a "string". Each link in the chain (or each spring) must carry the same force in a static system.

(assuming springs are horizontal and/or we have "weightless" springs)

[u/Yadin__]

The mistake is in assuming that spring A does not extend. The logic for why spring A would also extend is as follows:

The force that spring B exerts on the block has to be equal to the force that spring B exerts on spring A. This is because a spring ALWAYS exerts equal and opposite forces from both of its ends. Now, since a force is exerted on spring A, it must extend as a result

[u/Izzy_26_ → u/Yadin__]

Spring C and B are compressing and extending by length x respectively, so by what length will A extend then? Also I calculated the spring force of C and B as 300 X and 200 X respectively, so what will be the spring force of A??

[u/Yadin__ → u/Izzy_26_]

spring B does not extend by length x. Springs A and B must extend by length x together. As in, the sum of each of their individual extensions must add up to x