HSC Physics Question Thread

[Sukakadonkadonk]

Hey! So we use the centripetal force formula:




We have mass and we have radius (half the diameter, 0.25 metres) - We just need velocity! We know it completes 10 revolutions in 65.2 seconds, so we use that to calculate velocity:



Once you've got that, sub into your original formula for the answer!! Doing this I get:



Hmm. That's odd - Anyone see what I've done wrong here? Maybe we need to compensate for gravity with an upwards force as well to keep the circle horizontal but that seems a bit odd for how they've worded it

Surely it's not A, technically it is the closest but come on...

Thanks for helping but the answer they got was B, 8.5 newtons and yeah... I still don't get it haha.

[blasonduo]

Hi, how would this be done?

Thanks.

I think I have it, but I am about to leave home right now, i'll be back in 2 hours, and edit this if no one else has it Is iit B? because my working said B

Sorry for the delay,

[jamonwindeyer]

I think I have it, but I am about to leave home right now, i'll be back in 2 hours, and edit this if no one else has it Is iit B? because my working said B

Sorry for the delay,

It is indeed B and you are my saviour and hero

[pikachu975]

Hey! So we use the centripetal force formula:



We have mass and we have radius (half the diameter, 0.25 metres) - We just need velocity! We know it completes 10 revolutions in 65.2 seconds, so we use that to calculate velocity:



Once you've got that, sub into your original formula for the answer!! Doing this I get:



Hmm. That's odd - Anyone see what I've done wrong here? Maybe we need to compensate for gravity with an upwards force as well to keep the circle horizontal but that seems a bit odd for how they've worded it

Surely it's not A, technically it is the closest but come on...

I tried as well and got your answer. I tried using the 4U method of w = 2pi/T and then F = mrw^2 and also using your method and I got the same as you so I'm not sure what's wrong

[blasonduo]

Hi, how would this be done?

Thanks.

So, My way gave me 86.14 N (This was applying absolutely every force, and I think even here I could've done something VERY wrong)

HOWEVER

From just fiddling with potential errors with the answers I found that equating speed as 6.52 (YES this is 100% wrong) did give you the answer.




This gives the EXACT answer for B. From the answer of D having the exact decimals, I assume the answers are incorrect.

[Sukakadonkadonk]

So, My way gave me 86.14 N (This was applying absolutely every force, and I think even here I could've done something VERY wrong)

HOWEVER

From just fiddling with potential errors with the answers I found that equating speed as 6.52 (YES this is 100% wrong) did give you the answer.




This gives the EXACT answer for B. From the answer of D having the exact decimals, I assume the answers are incorrect.

Ohh, thank you very much.
Do you think the question is wrong then? Or is Jamon's method viable in solving this with the given info.

[blasonduo]

Ohh, thank you very much.
Do you think the question is wrong then? Or is Jamon's method viable in solving this with the given info.

Yes, it must be wrong. Follow Jamon's work

I will sleep on this question and will post if anything changes (doubt it will)

[Charlie_Sparkes]

Hey Everyone

Hoping I can just get an explanation of how to tackle projectile motion questions... I have Physics trials tomorrow and I'm having some difficulty with separating Horizontal and vertical components and which formulas to use etc.

Thanks again, Charlie.

[Shadowxo]

Hey Everyone

Hoping I can just get an explanation of how to tackle projectile motion questions... I have Physics trials tomorrow and I'm having some difficulty with separating Horizontal and vertical components and which formulas to use etc.

Thanks again, Charlie.

The motion questions should be on a formula sheet (pretty standard but I didn't do HSC so can't be sure). Note this is from a VCE perspective
You usually have to tackle the horizontal and vertical components separately.

If you're given the velocity at an angle, you have to figure out the magnitude of the velocity in each direction. I remember, if it's the adjacent side it's hypotenuse * cosx if it's opposite then hypotenuse *sine(x) (SOH CAH TOA, and sinx=O/H so O=Hsinx). Basically just resolve into each direction whichever way you remember

Vertically, gravity is the force acting on the object, the acceleration of which is g (9.8 ). Remember to take into account directions, if you're taking up as positive gravity will be negative and vice versa. This is typically the only force acting on it so g is the only acceleration you have to worry about.
So vertically, you know a, and you should know 2 other variables (typically initial velocity and distance - remember to take into account directions). From this you can find the vertical component of its velocity as it hits the ground, or the time it's in the air (common), or whatever other variable they want you to calculate. The vertical component is what determines the time in the air, as the object will continue travelling horizontally until it hits the ground.

Horizontally, there's no force acting on it (usually). This means its speed won't change! So you can calculate the horizontal distance it travelled (s=vt), and the velocity will stay constant. This is usually the only horizontal thing you'll have to do.

If they want you to find the velocity it hits the ground at, you can use the horizontal velocity and vertical velocity and use pythag to find the overall velocity (V²=V_horizontal²+V_vertical²)
Also, there's some symmetry in projectile motion. Keep in mind if you throw a ball up, it'll fall back in your hand with the negative of that velocity, so if an object is thrown at an angle at a certain velocity, it will land at the negative of that velocity (provided it doesn't travel any distance vertically)

Hope this helps and feel free to ask for anything more

[jamonwindeyer]

Hey Everyone

Hoping I can just get an explanation of how to tackle projectile motion questions... I have Physics trials tomorrow and I'm having some difficulty with separating Horizontal and vertical components and which formulas to use etc.

Thanks again, Charlie.

Awesome response from Shadow above, I also wrote this guide which includes a big section on projectile motion. It might be helpful!

Good luck for tomorrow!!

[Mymy409]

This question has totally stumped me.

What is currently used to define the standard metre?
(A) The speed of light
(B) The signals from GPS satellites
(C) The wavelength of light from a krypton lamp
(D) The distance between two lines on a platinum iridium bar

[Shadowxo]

This question has totally stumped me.

What is currently used to define the standard metre?
(A) The speed of light
(B) The signals from GPS satellites
(C) The wavelength of light from a krypton lamp
(D) The distance between two lines on a platinum iridium bar

I'm not sure whether you're expected to know this (we didn't for VCE Physics) but if not you can just look it up. According to Wikipedia
"The metre is defined as the length of the path travelled by light in a vacuum in
1/299 792 458 seconds."
So, a)
But using the process of elimination you should be able to see that b) and d) are unlikely, as they're not really well defined or constant. Wavelengths are often not precise (usually we get a range for different kinds of light. Even a laser has a narrow range for example) so c) is unlikely. a) we know that c is a constant and well defined and so that can be used to define a meter

[jamonwindeyer]

This question has totally stumped me.

What is currently used to define the standard metre?
(A) The speed of light
(B) The signals from GPS satellites
(C) The wavelength of light from a krypton lamp
(D) The distance between two lines on a platinum iridium bar

Ditto above, though you don't necessarily need to know the exact value - This is one of those 'rote learn' dotpoints

[Bubbly_bluey]

Hey guys! I got B using the right hand palm rule but the answer is A. Does anyone know why??
Thanks

[kiwiberry]

Hey guys! I got B using the right hand palm rule but the answer is A. Does anyone know why??
Thanks

By Lenz's law, the current generated will induce a magnetic field which opposes the external field, so you have to point your fingers in the opposite direction of the external field when using the right hand palm rule. On the top of the coil, palm will face left, fingers pointing up and current will flow out of the page - so it will flow from X to Y in the conductor