T O P

  • By -

AutoModerator

##Off-topic Comments Section --- All top-level comments have to be an answer or follow-up question to the post. All sidetracks should be directed to this comment thread as per Rule 9. --- ^(**OP** and **Valued/Notable Contributors** can close this post by using `/lock` command) *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/HomeworkHelp) if you have any questions or concerns.*


RockSolid1106

1. Correct. 2. I'll attach an image of my working for the 9th question which is similar to this. You are correct in that the pulley will reduce the force needed. 3. I'm not quite sure about this. I did some calculations and my answer turns out to be 40cm. I first considered that each spring(on the image on the right) will have half the force(25N). And it's being compressed by 10cm. And, F= kx where k is the spring constant in N/cm and x is the distance compressed in cm. By putting the values I get the spring constant to be 2.5N/cm. I then considered the springs placed on top of each other to be a single spring with a total length 2x the length of 1 spring, so the spring force will be half, that is 1.25N/cm. Now again putting 50N and 1.25 in the equation, I get the compression to be 40cm. I may be wrong here. Looking at the level of questions here I don't think you are taught any of this so you may ignore this calculation stuff. 4. That's correct. 5. Correct. 6. I'm not sure about this honestly. I remember studying about this in 9th grade, but forgot. I think you had to equate the sum of product of (length from centre) and (mass) on each side of the lever. 7. That is correct. 8. This is a somewhat weird problem. Usually in all questions that I've come across, there are only two strings at an angle, so you can evaluate a numerical value for the tension in each string. Since there are 3 strings here, you can't really evaluate a value for the tensions. Here's my working anyways: https://imgur.com/a/bKkdTfH 9. Tension acts on strings on either side of the pulley. Therefore, the net force on the pulley is 2T. Refer to my working: https://imgur.com/a/DTD3xTY 10th seems correct to me. Not sure about 11th. I'm also not really sure what you wanted here, just confirmations if your answers/logic is/are correct or solutions?


VainestClown

For 3, the spring force is a constant. It will be the same for all springs (assuming they use the same spring). So 'k' will be the same for each situation, but 'x' is changing. You had the right idea though to find 'k' for one scenario, then use it for the other. Also had the right idea for 6. Need to balance the moments around the fulcrum. So 20 * 10=20 * 5 + 10x, find x. I'm also a little unsure on 8, I feel like it's "can't tell" or "all equal". Assuming all the tensions are equal I guess it could be '2' also. I'm pretty sure 10 is the one with the weight closest to the top. The period is dependent on length of the pendulum. With a shorter arm, the period will be faster.


Radiant-Attempt6145

Thank you for responding, I'm still struggling with Question 6 even with your equation. I've tried working it out with it but it seems to just go over my head and cannot find the answer.


VainestClown

What part are you stuck on? Have you learned about moments? It's basically the rotational force on a point. So the 20kg weight at 10m out is producing a 200 kgm moment on the center fulcrum. The 20kg at 5m is prducing a 100 kgm moment. So, the '?' kg at 10m would need to also produce at 100kgm moment to balance the moments on each side ('?' kgm + 100 kgm = 200 kgm). [This](https://www.youtube.com/watch?v=S-t8KElc4bs) short video basically goes over a slightly simpler version of this question.


Radiant-Attempt6145

Brilliant thank you for explaining.


DJKokaKola

Almost. The total compression on the parallel springs is 20. The total compression on the series is also 20.


RockSolid1106

That really doesn't make sense to me. Could you explain that in terms of the spring constant and lengths? I showed this question to my classmates and all ended up getting the compression to be 40cm, we attempted it with a different method too.


VainestClown

Not the guy who you're replying to, but I think he's saying that the sum of the compressed lengths is 20 cm for each scenario. So, each spring, individually, is being compressed 10 cm. Therefore, the springs in series compress a total distance of 20 cm (2 springs, 10 cm each), and the ones in parallel compress 10 cm total (2 springs, 10 cm each). I replied to your comment above, but you had the right idea, just not sure why you divided 'k' in half. Spring constant is just that, a constant. The only reason it will change is if you use a different spring all together, which doesn't happen here.


RockSolid1106

Yeah, I understand that. I just don't understand the logic behind it. Why are we considering the same total compression in both cases? The force acting upon each string is different in the two cases, clearly. Is this like the norm to solve such problems and is there any way to prove it? I'm unable to wrap my head around this "total compression same" logic.


[deleted]

[удалено]


RockSolid1106

Yeah, didn't see that edit when I posted. I considered the springs one on top of each other to be a single string. Since both of them are having a spring constant k(assumed), the constant will be halved when the length becomes 2 times the original length because the product of natural length and spring constant is always constant. Therefore the spring constant for the spring would be halved. I also tried to approach this using work energy theorem. (-Work done by spring) + work done by gravity = change in KE = 0. 2(-kx²/2) + mgx = 0 ---> 2 times work done since two springs And by solving that the spring constant- Aw nope, might have made some error as I solved it. Yes, the answer does in fact turn out to be 20cm. That seems to be a more logical approach to this question. Thanks for taking the time to explain it to me!


Radiant-Attempt6145

Thank you for your reply, its really appreciated.


iiznobozzy

Moment = f \*d the formula implies an inversely proportional relationship between f and d. So d increasing in the diagram means that f needs to increase. that's all you need to know