HSC Physics Question Thread

[jamonwindeyer]

hi!! how would you make an assessment of validity (like what would say/how would you structure) of secondary sources you've used

Hey!! In this context, validity refers to how 'correct' the information you have obtained is, and there are a few ways to assess this.

- Does the source have appropriate citations (citations indicate a well-researched article, and thus, a valid source)
- Is the source credible (a peer-reviewed paper, a government website, etc. are all valid sources)
- Does there exist any bias/self interest on the part of the author? (less a big deal in Physics than it is in other subjects)

Happy for anyone to jump in with extra suggestions 

[Spencerr]

Hey!! In this context, validity refers to how 'correct' the information you have obtained is, and there are a few ways to assess this.

- Does the source have appropriate citations (citations indicate a well-researched article, and thus, a valid source)
- Is the source credible (a peer-reviewed paper, a government website, etc. are all valid sources)
- Does there exist any bias/self interest on the part of the author? (less a big deal in Physics than it is in other subjects)

Happy for anyone to jump in with extra suggestions 

 Our school had a two mark question similar. On top of what Jamon mentioned the information needs to be CROSSCHECKED to ensure CONSISTENCY across different sources.

The capital words were the necessary ones for each mark awarded

[wyzard]

hi!! how would you make an assessment of validity (like what would say/how would you structure) of secondary sources you've used

An unspoken rule of the Information Age: Never believe what's on the internet immediately This is why assessing validity is important, to make sure the information squares with reality and real science.

To assess the validity; check to see if the source is credible like where is it being published, is it being peer-reviewed, running a quick background check on the author for his credentials.

Usually scientific articles from Google Scholar, usually published in scientific journals, and textbook citations are fine.

Science articles on news websites or popular science websites generally, on the other hand, are not as they're written by journalist for the general public, so the information in there are usually heavily trimmed down, or made more dramatic to attract attention. They are nonetheless useful as they provide the general ideas, and will usually include links to the actual scientific research papers so use those

On a final thought, sources from sketchy science websites, such as those promoting "New Free Unlimited Energy Generating Device" or "New Quantum Consciousness" is a big no no. For example, a quick google search for "free energy devices" leads me to this site: http://www.free-energy-info.com/, an example of a page where you don't want to cite your sources. I can't even verify the author(s) credentials, and reading the content gives me so much cringe on the misunderstanding of physics behind it.

[jamonwindeyer]

For example, a quick google search for "free energy devices" leads me to this site: http://www.free-energy-info.com/, an example of a page where you don't want to cite your sources. I can't even verify the author(s) credentials, and reading the content gives me so much cringe on the misunderstanding of physics behind it.

Oh my goodness, this site actually says:

Oliver Heaviside's equation E = mC2...

If that whole site isn't actually a giant troll, then I actually don't know what to do right now... 

[conic curve]

I do not understand this question and how the answer is 1/18

Qualitatively describe the effect on intensity if the distance is tripled and the strength of the source is halved?

[Spencerr]

I do not understand this question and how the answer is 1/18

Qualitatively describe the effect on intensity if the distance is tripled and the strength of the source is halved?

Intensity equals strength over distance squared.  It's and inverse law.  Someone else might be able to explain it more I'm depth but that's the formula used

[wyzard]

Oh my goodness, this site actually says:

Oliver Heaviside's equation E = mC2...

If that whole site isn't actually a giant troll, then I actually don't know what to do right now... 

hahahaha join me in cringing Yeah judging by the 3000 ebook they have available, I highly doubt they're trolling. I think they truly believe in building a free energy device, and that the government and big oil companies are suppressing such technology

[jakesilove]

Intensity equals strength over distance squared.  It's and inverse law.  Someone else might be able to explain it more I'm depth but that's the formula used

Yep, that's all you need! Sub in your initial values (ie. Strength = A, distance = d) and then try it with some new values (ie. Strength = (1/2)*A, distance = 3*d) and you should get 1/18th of the initial value!

[Loki98]

Could someone please explain this question to me =]

[conic curve]

Why is it hard to see Newton's first law in real life?

[jamonwindeyer]

Why is it hard to see Newton's first law in real life?

Newton's 1st Law is (roughly stated) that an object at rest stays at rest and an object in motion stays in motion, unless acted upon by an external force.

So, why doesn't a football we kick (for example) just fly upwards forever, why doesn't a car roll forever without accelerating? The answer is friction, in real life there is constantly frictional forces impeding the motion of objects, thus making it very difficult to ever find an actual situation where Newton's 1st Law hold valid 

[conic curve]

Newton's 1st Law is (roughly stated) that an object at rest stays at rest and an object in motion stays in motion, unless acted upon by an external force.

So, why doesn't a football we kick (for example) just fly upwards forever, why doesn't a car roll forever without accelerating? The answer is friction, in real life there is constantly frictional forces impeding the motion of objects, thus making it very difficult to ever find an actual situation where Newton's 1st Law hold valid 

Oh thank you thank you thank you

Another question but what is the action and reaction force between a rocket flying into space and it lifting off from the ground. I heard that there is a misconception where the smoke that the rocket exerts pushes the rocket up into space (apparently this isn't true)

[jamonwindeyer]

Could someone please explain this question to me =]

This question shows DC motors where the permanent magnets have been replaced by electromagnets. Further, the same current is used to drive these magnets as is used in the rotor/coil itself.

First, we must determine the polarity of the electromagnets. Remember that current flows from positive to negative. We use the right hand grip rule. Take your right hand and give a thumbs up. If your fingers are wrapping in the direction of the current, then your thumb points in the direction of the North Pole. Obviously, the opposite pole will be the south pole. This should allow you to determine the polarity of the magnets on either side of the coil.

For each option, with the left/right polarity are:

A – South/North (as in, on the left, the south pole is pointed towards the coil, and on the right, the North pole is pointed towards the coil)
B – South/North
C – South/South
D – South/South

In fairness, those diagrams are terrible to see this properly. We can now disregard options C and D, because the two poles are the same. We need one of each for a magnetic field.

Now we see which has the right direction by using the right hand slap rule (or your version). The magnetic field in both options goes from North to South, or, from right to left. Point your fingers in your right hand in this direction. Then, we point our thumb in the direction of current. Pick, for example, the left hand side of coil B. Current is going away from us, magnetic field is going to the left. We then slap in the direction of the force (meaning, which way that side of the coil will move). Using the rule, this is upwards.

If the left hand side of the coil is moving up, the coil is spinning clockwise. So, B is incorrect. We can use the same idea to prove that A is the answer. Look the left hand side of the coil in A. The magnetic field is going to the left, but now the current is going towards us. Use the slap rule here to prove to yourself that the resultant force will thus push upwards, and thus, that the coil will spin clockwise.

Therefore, the answer is A.

[jakesilove]

Oh thank you thank you thank you

Another question but what is the action and reaction force between a rocket flying into space and it lifting off from the ground. I heard that there is a misconception where the smoke that the rocket exerts pushes the rocket up into space (apparently this isn't true)

In very basic terms, the action is the gas being pushed DOWN from the rocket. This doesn't exert a force on the ground, thus exerting a force on the rocket, or anything like that; just imagine that the gas expands, and since it can only go down, it does.

However, we know that all momentum must be conserved. Therefore, if there is a spontaneous DOWN force, there has to be an equivalent UP force. This reaction projects the rocket upward!

[jamonwindeyer]

Oh thank you thank you thank you

Another question but what is the action and reaction force between a rocket flying into space and it lifting off from the ground. I heard that there is a misconception where the smoke that the rocket exerts pushes the rocket up into space (apparently this isn't true)

No that's pretty much right! The idea is that the rocket exerts a downwards force on the fuel particles, causing them to be accelerated downwards as exhaust. It is the reaction force to this that pushes the rocket upwards, and this is called the thrust force 

In very basic terms, the action is the gas being pushed DOWN from the rocket. This doesn't exert a force on the ground, thus exerting a force on the rocket, or anything like that; just imagine that the gas expands, and since it can only go down, it does.

However, we know that all momentum must be conserved. Therefore, if there is a spontaneous DOWN force, there has to be an equivalent UP force. This reaction projects the rocket upward!

Ditto