For beginners like me , how do I develop the intuition to manipulate the equation so p exists in that KE equation ?
Also , if KE becomes 2KE , p becomes 2p as well ? How to prove this ?



All right , noted . Thank you .

Here's a question from an SPM Olympiad Fizik book :



This post has been edited by maximR: Aug 14 2013, 06:24 PM

The answer is B?

According to the guy who provided the question , yes .

If so, you may experience it when the car/bus where you sit brakes.

Solution provided is about pressure , artificial gravity , inertia ...

That's complicated.
My reason :
When you are sitting in a moving bus, you are moving as well. When the bus stops, your inertia causes you to continue to move.
Same to the question.
The initial state of the objects is static. When you push the box, the inertia will cause the object to try to stay in the initial state, that is not moving.

You might use Newton's Third Law to think about it.

This post has been edited by kingkingyyk: Aug 14 2013, 06:18 PM

Erratum: Please take note that there was a typo in the last line of the previous image. Last line “√(2p)” should be “√2*p”. Thanks Krevaki for pointing it out. It has been corrected.

Basically, it is very similar to proving Trigonometric Identities in SPM Add Maths. When you have the knowledge of momentum p = m*v, you will think of ways to express KE in terms of momentum and at the same time, conserving the dimensions of the original KE equation.

By the way, care to tell me about the reception of your Science Fair project?

The steel (?) ball and the helium balloon are attached to the box by strings. The connections are not rigid and when you move the box, you would expect their inertia to hold them in their original position.

Alamak, why mention my name? Anyway, you're welcome.

Under normal circumstances, your daily experience, common sense plus Newton’s 1st Law generally would tell you the “rational” answer seems to be B. However, if B were the intended answer, why would the person who set the question bother to add the floating helium-filled balloon in the boxcar? Is it a tricky question?

There are 2 clues provided:
(1) If the helium-filled balloon wasn't tied to the floor of the boxcar, it would float all the way up to the ceiling.
(2) In physics, Impulse of Force is used to refer to a fast-acting force that is applied briefly: I = F*δt = m*δv.

Lastly, logical thinking is not just using our “common sense.” After all, “common sense” tells us the earth is flat. But even when “common sense” is correct, it may tell us, for example, that gravity cannot be pulling a thing down and pushing up the other at the same time. Therefore, critical thinking goes well beyond just using such basic principles.

Perhaps Krevaki and work_tgr can offer some clues as well.

The answer would be A then.
What I meant by "logical thinking" here is think based on law's logic.

Admittedly, no. Not quite. I was paying more attention to the application rather than the origin of the ideas which was fine at SPM level. But now at A level I realize it would be imprecise to define or explain force using F=ma instead of F=(mv-mu)/t. That could pose some problems when answering structure questions at A level.

Of all posts, why quote this one?

Didn't manage to finish before the deadline due to many reasons but mostly because of my incompetence. Might participate again next year if possible.

Hmm....I think A levels would serve a better purpose for this because for AS physics mechanics/Mathematics Mechanics 1 the knowledge from SPM can be extrapolated quite easily to fit the needs of the question. If you do exercises from STPM however, watch out for pre requisite knowledge.

Learn to relate different quantities together by the use of formula you already know. A good exercise would be proving formulas that you know, such as differentiation by 1st principle, proving E=mgh and E=0.5mv^2 from W=FS, proving P=h(rho)g from P=F/A and other formulas and so on. Proving trigonometric identities as mentioned by Critical_Fallacy is good too.

I couldn't stress how important this skill is if you are planning to study Physics/Mathematics/Further mathematics at A level/STPM.

Perhaps watching the YouTube video below would give you new insights into the physics of helium balloon.

P(X >= 1) = 1 - P(X < 1) = 1 - P(X = 0) = 0.8

Yup, binomial distribution.

Technically speaking, the method employed by ystiang is called Complementary Cumulative Distribution Function. The solution is exactly the same as the one beautifully brought by ystiang, with a little extra information of how the principle of Cumulative Distribution Function works.

Alright thanks people!

Helium gas balloon will move forward, pendulum will move behind?
Weird guy.

You're right! As the box moves to the right, the air gets compressed to the left, making the left side of the box denser than the right side. The helium balloon actually gets pushed to the right!

I get it now .

But looking for explanations on the internet , there are some who use General Relativity to explain this . So is the explanation using air compression / buoyant force correct , or General Relativity , or both ?