u/Aokayz_

Hey guys :) I'm a co-founder of Project ReWear, a student-led initiative dedicated to reducing waste. We do this by collecting unused clothes and redistributing them within schools, universities, or communities. Incase you'd like to know, I'm a Year 12 student in Dubai (the workload is insane).

We want to find student ambassadors like us to expand.

In our first drive, we collected around 200 kg of clothes, preventing approximately 5,000 kg of CO2 emissions. We managed to do this in our school in just 2 weeks. After speaking to our principal, We’re now looking to expand across Dubai (and down the line, internationally) by recruiting ambassadors at other schools, universities, or communities.

As an ambassador, we need you to build your own team at your community, and run a clothing collection drive. Feel free to even leverage our proven results when pitching to your administration if you need to! You won’t be starting from scratch.

If you want push to for a greener earth, start where you are. We really need people who want to make real impact.

Check us out on instagram @proj.ectrewear If you or anyone you know would be interested!

Connecting two charged plates to a power supply, if I let some uncharged conductor touch one of the plates, will it immediately gain the same amount of potential as the plate? Or will it take some non-negligible amount of time? Why so/not?

I'm asking in context of a mechanical mouse trap. To set the trap, I need to pull down the lever 90 degrees, stretching a spring, before locking it in place.

If I want to figure out how much work is done on the spring, then I can model it so that the force F (which is exerted perpendicularly on the lever) I apply on the lever * the arc length that force is applied for is the work done on the spring.

However, since I am also doing work on the lever to tilt it down, is it a more accurate answer to take into account the GPE (gravitational potential energy) that the lever lost?

To do this, I can use the change in height of the lever's center of gravity * the weight of the lever, then add that to the work I had calculated earlier. This way, I can take into account the work done by the force I applied and the work done by gravity to stretch this spring.

My question though is if this is valid reasoning? Something doesn't sit right with me taking into account the GPE of the lever, even if its change was non-negligible.

By the gradient theorem, I know that the line integral of the gradient of some scalar function U is equal to U + C. This also means that if the line integral is equal to U + C then the integrand F must be the gradient of U, assuming the curl of F is 0 (and the other topological conditions I fail to remember hold).

In this case, I wondered if it meant that not all line integrals can be equal to any scalar function at all. Here is my reasoning of why (based on intuition than rigor):

For visual purposes, imagine that U is a function of x and y, so we can graph it in a 3D space. Clearly, we notice that any scalar function is inherently independent of the "path" taken to get to a certain point, because U only takes in two inputs, the x-y coordinates of that point.

In contrast, the line integral of F from some constant points p to (x,y) will have different values depending on the path taken. Hence, it can never be equal to a scalar function.

My doubts are:

1. Is this kind of reasoning actually wrong?
2. Is it actually possible for a scalar function to somehow take into account the path of a line integral as an input? What would such a function look like if it existed?

Consider a light metal sphere with capacitance Cs suspended from an insulating string. The sphere is between two parallel metal plates with a large potential difference V between them due to being connected by an arbitrary power supply.

If the sphere is positioned at rest such that it touches one of the charged metal plates, it will gain some charge Qs.

We know that

Cs = Qs/Vs =>
Qs = Cs * Vs

where Vs is the potential difference applied to the sphere.

Apparently, the sphere is applied a potential difference equal to V, which is the potential difference between the plates. So

Qs = Cs * V

But how is that possible? For one, I can't seem to even grapple with the idea that an object not a part of some circuit can have a potential difference: what difference in potential are they talking about? The sphere and at infinity? Two opposite ends of the sphere?

Secondly, why Vs equal to V? Why is the change in electric potential energy of the sphere per unit charge the same as that of the potential difference between the plates?

I'm sorry if I'm not making sense and for the long post. This is all just very confusing to me.

Consider a flyboard with mass M, hovering at a constant height by exerting a downward force F on a jet of water. The rate of flow of the mass of water is m/t and the water has negligible velocity before being ejected out with a velocity v

The resultant force on the flyboard is

-Mg + F = 0

and on the water,

resultant force = -mg - F

By Newton's second law

-mg - F = mv/t

and so

v = (-mg - F) * (t/m)

but from the question, they seem to completely neglect -mg. Instead,

v = (-F) * (t/m)

I believe the force in the impulse equation must be the resultant force by Newton's second law, but it seems any force is okay enough? So why did they only use -F?

Sources:

- Question 11 from (https://qualifications.pearson.com/content/dam/pdf/International%20Advanced%20Level/Physics/2018/Exam-materials/wph14-01-que-20220601.pdf)

- Answer for Question 11 (https://qualifications.pearson.com/content/dam/pdf/International%20Advanced%20Level/Physics/2018/Exam-materials/wph14-01-rms-20220818.pdf)

Edit: I'm sorry i think i miscommunicated my question. To clarify, v is the velocity of the water being ejected out. It seems people are addressing the impulse on the flyboard (why is 0 vertically because it's at constant height) but I'm looking to understand the impulse on the water and how that can be used to find its velocity v downwards.

According to my syllabus, A Many-to-Many relationship between two tables violates 3NF (Third Normalization Form). When I think of why, it would be because a many-to-many relationship means that for at least one of the tables, a single record can have multiple FK (Foreign Key) fields, and multiple records can have the same FK field.

As a result, for at least one of the tables, you'd either have to create a non-atomic field in one table to store the multiple FK values, or have to make a composite PK out of the original PK of the table and the FK. Ignoring the former, because that "clearly" violates 1NF, the latter would violate 2NF, since it doesn't guarantee that all the fields would fully depend on both the original PK AND the FK.

Is this the reasoning that leads to the idea that in 3NF, the tables should not have a direct many-to-many relationship? And therefore, does a many-to-many relationship violate 2NF?

According to my high school syllabus (Cambridge International AS Level Computer Science), when code is partially compiled, it is "optimized for the CPU as the machine code is generated at run time".

But to my knowledge, this doesn't make sense. Taking Java as an example, the JVM (Java Virtual Machine) is the software that interprets the byte code into machine code, not the CPU. In fact, isn't partial compilation for the purpose of translating and executing machine-independent code?

So, What do they mean? How is the CPU involved in partial compilation in a way that it becomes "optimized" too?