Static friction

Question

Why is this wrong ? My teacher simply said that it is wrong and asked us to solve it by taking cases( The blocks move together or they move separately.),the first case being the blocks move together. This assumption came out to be true and the blocks do move together with an acceleration of 5/6 m/s². But why is this wrong ?

Edit- There is a force of 5 N to the 2 KG block.

It is not like I have made some calculation error and threw the solution to you to check in which line did I err. It is very much about understanding static friction, though I agree it will be of very less use to the community as a whole, but still about understanding static friction, I think. — Ali, Jul 07 '21 at 16:21
@Ali, what are the forces that cause acceleration? Your diagrams are not clear to me. — ACB, Jul 07 '21 at 16:21
I am very sorry, there is a force of 5 N in the right, in the question. — Ali, Jul 07 '21 at 16:22
@Ali has potential to be a fine question - just isolate the part you're wondering about and reformulate it so it's more general (and in your own words) — Señor O, Jul 07 '21 at 17:09
@ Señor O How can I make it more general ? I still have to show the FBD and my work and all ,and I don't know Latex, so I have to add a picture and people here don't like it.I will still get downvoted to abyss and it will be declared a check my work question. The truth is any high school physics question of more than 5 lines is not tolerated on this site. — Ali, Jul 07 '21 at 17:26
There are several things which would make your work much easier to follow. 1) Show the equations you are using; 2) Stay symbolic until you separate your unknown variable; 3) Show units on ALL numbers so people know what the numbers represent. — David White, Jul 07 '21 at 17:48
@Ali Here is a related example of a very similar highschool math question that was very well recieved and was made more general. https://physics.stackexchange.com/questions/51444/block-on-a-block-problem-with-friction?rq=1 — shai horowitz, Jul 07 '21 at 19:06
@ shai horowitz I think you are right. Now when I am thinking about it, the formatting of my question was too poor. No one would want to read it. — Ali, Jul 07 '21 at 19:17

score 1 · Accepted Answer · answered Jul 07 '21 at 18:28

1

When both blocks move together, the friction between the two blocks is static friction which is $f\leµN$ and not necessarily $f=µN$.
So, you can't use $f=µN$, because it is only valid for a case when the pulling force is the maximum force which the two blocks move together, it means in that case, if you increase the pulling force by $\epsilon $, the blocks move separately.

Now, how to solve this problem: 1- Calculate the acceleration for the system of two blocks (6 kg). So, static friction will not be taken into account since it is an internal force for this system.

2- Sketch free diagram for each block (now their acceleration is known) and write 2nd law of Newton for each of them. Just remember, static friction is just f (an unknown value, which can be obtained from the two equations you just wrote (Newton 2nd law).

answered Jul 07 '21 at 18:28

Ebi

1,106

Yes it is not necessarily f=uN. But how do you know that it will move together as I just can't assume that the friction would be limiting ? – Ali Jul 07 '21 at 18:40
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You calculate the required friction force for the two blocks to move together as I mentioned above. If your answer (f) is greater than $µN$ then the two blocks move separately, otherwise static friction (less than the max value of $µN$) makes them move together. – Ebi Jul 07 '21 at 18:44
Ok so why is it okay to assume they move together first and then find the value of static friction but it is not okay to assume they move separately and then find the value of static friction ? – Ali Jul 07 '21 at 18:50
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When you assume they move separately, f is already known $f=µN$. It is just a specific state of a more general $f\leµN$ – Ebi Jul 07 '21 at 19:01
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Thanks a lot! Now when I think about it the downvotes do make sense lol. – Ali Jul 07 '21 at 19:12

Static friction

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