HSC Physics Question Thread

[jakesilove]

Oh crap I totally forgot that I still had the job to derive this for you sire 123!! Im so sorry and thank you so much Jamon for remembering to derive the two formulae for sire 123!!! You are such a legend!

But yeah I dont feel like I have done my job properly so l will take the rocket propulsion proof a step further, way back to the first principles so that you can see how rocket propulsion is related to the Law of Conversation of Momentum, keeping in mind that when a rocket launches, the force with which gas acts on the rocket (F_{gr} = Force of gas on rocket) is equal to the force with which the rocket acts onto the gas (F_{rg} = Force of rocket on the gas).

(Image removed from quote.)

So yeah anyways sorry sire for such a late reply, hope this will help you to understand how the law of conservation of momentum is involved with rocket launch, great question!

Best Regards
Happy Physics Land

Absolute legend, also can't pass up a good derivation so I'm so sad I missed out on this/

Jake

[Happy Physics Land]

Absolute legend, also can't pass up a good derivation so I'm so sad I missed out on this/

Jake

Don't worry Jake, God has got bigger derivation plans for you ahead

[AladinX]

Hey Jake.
I'm currently a Year 12 student in Physics and am soon going to be given an assessment task requiring me to make a rocket launcher ( testing our knowledge of projectile motion ). Any ideas on where to start for such an assignment. What to use maybe to launch the rocket to the required distance?
Much appreciated, thanks.

[jakesilove]

Hey Jake.
I'm currently a Year 12 student in Physics and am soon going to be given an assessment task requiring me to make a rocket launcher ( testing our knowledge of projectile motion ). Any ideas on where to start for such an assignment. What to use maybe to launch the rocket to the required distance?
Much appreciated, thanks.

Hey AladinX!

Firstly, that's a seriously cool assignment! Tricky, but cool. I'm sure heaps of people on the forum will have great ideas, so hopefully everyone can contribute!

Secondly, I think we need a little more information about the project. Is the rocket launcher supposed to be accurate, or just launch a projectile further than a required distance? Is there a target? What is the required distance? How heavy is the projectile?

If you just need to launch a projectile greater than a required distance, and accuracy doesn't matter, I'd build yourself a good ol' fashion Potato Gun. Google the method, but it's cheap to make and works like a charm. You could vary the construction depending on how far you want it to go etc.

If you need accuracy, typically people build trebuchets and other similar launchers. Again, I can go more in depth when I have the answers to the above questions, but I'm thinking that building something like that would be really difficult. What I'm imagining is sort of "cheating" the question: if all that is required is launching a projectile a set distance, accurately, I would buy something like Hot Wheels tracks, set up a ramp and fling is to the point you need to get to. You can easily do the calculations, doesn't required difficult construction etc. etc.

Let me know more information! Hopefully other people on the forum have more ideas, because whilst I love theoretical Physics I am certainly not an Engineer.

Jake

[Happy Physics Land]

Hey Jake.
I'm currently a Year 12 student in Physics and am soon going to be given an assessment task requiring me to make a rocket launcher ( testing our knowledge of projectile motion ). Any ideas on where to start for such an assignment. What to use maybe to launch the rocket to the required distance?
Much appreciated, thanks.

Hey AladinX:

I havent had any experience with constructing a rocket model to test projectile motions, however I have found two very useful sites that outlines two different types of rocket launchers that you may be able to use to demonstrate your knowledge of projectile motion. You can use the IDEA from these two websites (not the same methods or results or safety of course) and twist them and changed them to avoid being accused of plagiarism. So yeah you can have a read of these two sample experiments to inspire some ideas for yourself!

http://www.scienceworld.ca/resources/activities/pop-bottle-rocket-part-ii-projectile-motion
http://www.arborsci.com/cool/projectile-motion-for-everyone

Best Regards
Happy Physics Land

[AladinX]

Hey AladinX!

Firstly, that's a seriously cool assignment! Tricky, but cool. I'm sure heaps of people on the forum will have great ideas, so hopefully everyone can contribute!

Secondly, I think we need a little more information about the project. Is the rocket launcher supposed to be accurate, or just launch a projectile further than a required distance? Is there a target? What is the required distance? How heavy is the projectile?

If you just need to launch a projectile greater than a required distance, and accuracy doesn't matter, I'd build yourself a good ol' fashion Potato Gun. Google the method, but it's cheap to make and works like a charm. You could vary the construction depending on how far you want it to go etc.

If you need accuracy, typically people build trebuchets and other similar launchers. Again, I can go more in depth when I have the answers to the above questions, but I'm thinking that building something like that would be really difficult. What I'm imagining is sort of "cheating" the question: if all that is required is launching a projectile a set distance, accurately, I would buy something like Hot Wheels tracks, set up a ramp and fling is to the point you need to get to. You can easily do the calculations, doesn't required difficult construction etc. etc.

Let me know more information! Hopefully other people on the forum have more ideas, because whilst I love theoretical Physics I am certainly not an Engineer.

Jake

Hopefully once I receive the formal assessment sheet and requirements I will be able to give you more information regarding it. Thanks for your help

[mijomo]

Hey guys, thanks so much for having this AMA, you guys are awesome!
My first assessment task in physics this year is an open ended investigation which requires me to analyse the relationship between the surface area of an object and the rate that it falls, and then write it up as a scientific report (aim, method etc.) Just got three quick questions for you:
1. Am i right in thinking that rate of fall or the rate that an object falls at is referring to velocity and not acceleration or just the time taken to fall?
2. What are the main things I should include in my discussion?
3. When we talk about validity, reliability and accuracy, what does accuracy mean in in a scientific report context?
Thanks again for doing this AMA, LEGENDS!

[jakesilove]

Hey guys, thanks so much for having this AMA, you guys are awesome!
My first assessment task in physics this year is an open ended investigation which requires me to analyse the relationship between the surface area of an object and the rate that it falls, and then write it up as a scientific report (aim, method etc.) Just got three quick questions for you:
1. Am i right in thinking that rate of fall or the rate that an object falls at is referring to velocity and not acceleration or just the time taken to fall?
2. What are the main things I should include in my discussion?
3. When we talk about validity, reliability and accuracy, what does accuracy mean in in a scientific report context?
Thanks again for doing this AMA, LEGENDS!

Hey Mijomo!

Firstly, I reckon this is an epic experiment that I would have spent WAY too much time on in my HSC year. That being said, love your teacher for giving it to you!

Let's go through your questions chronologically.

Is it asking about velocity, acceleration or time?

Well, in perfect Physics world the answer would be all of the above. That's because you SHOULD be able to derive any of the other components by gathering information about one: Calculating start and end velocity will give acceleration, acceleration will give time using projectile motion equations etc.

Therefore, if your question is "What should I even be measuring?" my answer would be "That depends on the accuracy of your equipment". We will discuss what "accuracy' means further down, but my overall answer re: what the dependent and independent variable should be is as followed.

Obviously, the thing that you should "change" is the surface area of the object. The thing that you should measure will either be
a) The time the object takes to fall a set distance.
b) The velocity of the object during the fall.
c) The acceleration of the object during the fall.

You should discuss these methods, deciding which will be MOST ACCURATE. I would guess that the TIME will be the most accurate: Use your phone to film the drop, then try and slow down the footage so you can see EXACTLY how long it took to fall. This will be very accurate, and you can do subsequent calculations from there.

What should I include in my discussion

You should include things that worked, things that didn't, reason for error, explanation of results etc. etc. Basically just explain what you did, why you did it, and how successful it was. There are heaps of guides online with what you should include in a discussion: Try find whatever your textbook says!

Accuracy

Accuracy refers to how close the measured value is to the real value. For instance, the more decimal places you measure a value to be, the more accurate that measurement is. So by filming the descent, and getting the point of landing to the millisecond, is quite accurate.

Accuracy refers to the success of the instruments, so just talk about limitations to accuracy depending on the equipment you use.


I hope that this helps! Sounds like an amazing task, so if you want to keep us updated with your progress/ideas/places you need help, I would be super interested. I would also recommend googling what the relationship between surface area and velocity SHOULD be, so you know what you're looking for.

Hope this helps! Fantastic question.

Jake

[Neutron]

Hey! I was just doing some homework and I was wondering how eddy currents cause flux leakage? So I'm doing the transmission process for electrical energy and I read somewhere that hysteresis losses can be reduced by laminating the iron core, which reduces the size of eddy currents formed but I was wondering how smaller eddy currents would help the flux leakage? Sorry if this seems dumb!

Neutron

[jakesilove]

Hey! I was just doing some homework and I was wondering how eddy currents cause flux leakage? So I'm doing the transmission process for electrical energy and I read somewhere that hysteresis losses can be reduced by laminating the iron core, which reduces the size of eddy currents formed but I was wondering how smaller eddy currents would help the flux leakage? Sorry if this seems dumb!

Neutron

Hey Neutron!

Not a dumb question at all! Basically, the solid magnetic core is NOT supposed to be used for eddy currents, its just to produce a Magentic field etc. So its use is as a magnet, rather than a cylinder.

However, being a conductive cylinder, and because of Lenz's law, the flowing current around the core will cause Eddy currents to be formed in the core. Remember, the purpose of the core is NOT to produce Eddy currents; that is just a result of the type of object it is.

Because of conservation of energy, any eddy currents produce must cause a reduction of energy elsewhere. Basically, this means energy is "lost" in the conversion process in the production of Eddy currents in the core, and those Eddy currents aren't useful in any way.

By laminating the core, it reduces the "size" and therefore the amount of energy removed by the Eddy currents. Essentially you put a sheet of insulator in between each sheet of metal, so the Eddy currents can't become really really big. By reducing the size of eddy currents, less energy is loss.

I hope this helps! Just remember that we don't WANT eddy currents to be produced in this process; that's just an unfortunate byproduct. By reducing the size of eddy currents, we reduce the energy lost!

Jake

[Neutron]

Thanks Jake but how do these Eddy currents cause flux leakage? Like I understand that they dissipate heat and all but the topic I'm studying right now is transformers (if that helps!)

Neutron

[Neutron]

And also, another question (sorry!), what are the benefits of the glass disc insulators' shape (still talking about the transmission tower oops) and without it, would the current just travel through the tower and be earthed? Like if the live wires were not separated from their supporting pylons, would all the electricity just be earthed? Thank you!

Neutron

[Happy Physics Land]

Thanks Jake but how do these Eddy currents cause flux leakage? Like I understand that they dissipate heat and all but the topic I'm studying right now is transformers (if that helps!)

Neutron

Hey Neutron:

I think Jake did a really great job explaining the lamination of soft iron cores to reduce hysteresis losses and I will just reinforce some of his statements and just tell you a bit about flux leakage. It's a good question and if l dont do a good job explaining this please tell me because it is a hard physics concept.

Two concepts you definitely shouldnt confuse are heat loss and flux leakage. Flux by definition is just the total number of magnetic field lines that passes through a perpendicular area. When a magnetic device keeps operating for a period of time, there will be residual magnetic field lines that remains for a short amount of time. An analogy to this is that when you turn on the stove for a while and when you suddenly turn it off its not gonna to immediately cooldown. So since there is a residual magnetic field, there will be magnetic field lines produced over an area (flux), and without your soft iron core, the flux wouldnt be able to have a path to pass through to the other coil. Hence a non-conductive plastic core or wooden core would lead to an absence of pathway for magnetic field lines to pass through and hence there will be a flux leakage (i.e. not all the flux will be transferred to the second coil in the transformer).

In regards to eddy currents, they are produced as a result of change in magnetic flux and EMF and Lenz's law etc (Im assuming you would be familiarised with this process).So it is reasonable to think that when there is a high value of eddy current flowing through the transformer, there will be a stronger residual magnetic field after the device is turned off, and so there is more flux leakage.

[Happy Physics Land]

And also, another question (sorry!), what are the benefits of the glass disc insulators' shape (still talking about the transmission tower oops) and without it, would the current just travel through the tower and be earthed? Like if the live wires were not separated from their supporting pylons, would all the electricity just be earthed? Thank you!

Neutron

Now this glass disc insulator questions (just a reminder, disc insulators can be all types of ceramics, glass is just a type of ceramic that is made in epoxy resin). Glass disc insulators take on a slope-shape to prevent the accumulation of moisture and dust to ensure a long conductive pathway for any spark discharge across the insulator. In addition, the metal links in glass insulators are isolated from each other, and the fibreglass is a non-conductor, so there is no continuity of conduction, which is exactly the purpose of an insulator.

If you have any further questions dont hesitate to ask!

Best Regards
Happy Physics Land

[mijomo]

Thanks so much for your reply Jake, greatly appreciated. Great minds must think alike because i did use my ipod to film the drops then counted how many frames the falling body was in the air for and worked out the time in seconds. I'm able to work out average velocity since i know what vertical height I dropped the metal sheets from and this gives quite a nice graph showing a trend that indicates that as the surface area was decreased the average velocity of the falling body increased! I suppose one limitation that I could talk about in my discussion is that fact that I can only calculate average velocity as opposed to instantaneous velocity which would better show when the object approached terminal velocity etc. Maybe using a data logger would have done the trick, or a background erected behind for the fall which had 10cm increments lined on it, which I could use in conjunction with a frame by frame analysis to see when uniform motion was reached. My teacher is always going on about trying to find a straight line relationship when you have a graph so that's what i'm now attempting to do. I was able to solve a differential equation, (Sum of forces acting) = m (dv/dt) = mg-F(drag), (with surface area proportional to the drag force), but it doesn't really give a straight line relation, more logarithmic. Reckon it's worth putting in, or is that over analysing the problem? Anyway, very interesting stuff, thanks again for your help!