HSC Physics Question Thread

[Happy Physics Land]

Ahh, ok I see. Thanks for the explanation Happy Physics Land!

One last thing though: Why is the ring permanently repelled when the switch is closed. Wouldn't the ring only be momentary, similar to Faraday's experiment with the primary and secondary coil wrapped around the wooden block - because the changing magnetic flux is only changing when the power supply is turned on/off?

Hey Meckenza:

Good question actually. My teacher suggested that this case is perhaps different to Faraday's experiment. In this case, we've found out that the ring being suspended in air is repelled away from the solenoid. Originally I would have picked D, but if you think about it,  is there a  force there to bring the ring back to its original position? Clearly we dont have any evidence of a restoring force acting on the ring to bring it back to its original position. So as long as there is flux the ring would just stay there because the current would keep on circulating in the ring and the same poles between ring and solenoid would cause them to keep being repelled. Its quite a tough question!

Best Regards
Happy Physics Land

[jamonwindeyer]

hi,
we have a test on 'the standard model' coming up, and i was wandering if you could please help me with this question?
when a muon and an anti-muon collide they can annihiate each other and release their mass-energy as 2 photons. assuming that these two photons are identical,
a) what will each of their energies be
b) what wavelength will they have
c) why does there need to be 2 photons produced and not just one?
d) in what directions would they have to travel relative to each other and why?
e) in what part of the electromagnetic spectrum are they located?
i don't have any answers to these so i even the ones that i have attempted, i don't know if the answer is right or wrong

thankyou soooo much in advance

Hey mq123! I think we have another case of the WA course not overlapping with ours, so I can't give full solutions. I don't have all the info. However, the help I can give:

For Part A, you would need the total energy of the muon/anti-muon system. I wager this would be some combination of their masses, and perhaps their speed. Once you have this, the conservation of energy dictates that this energy must now be contained in the two photons. So,



For Part B, use the formula linking a photons energy to its frequency, and then convert this to a wavelength. In one step, it looks like this:



For Part C/D, I think your answer lies in the Conservation of Momentum. When two particles collide, they are moving (presumably) in opposite directions. There is momentum in one direction, and then an identical momentum in the opposite direction, and so the total momentum of the system is likely to be zero (same mass, velocities in opposite directions, and so the momentum of one cancels out the other when we add them as vectors). Photons, being a particle, must obey this principle, and so the total momentum of the photons must also be 0. Thus, there must be two photons, as they move in opposite directions to obey the Conservation of Momentum. We can't have just 1, because there would then be momentum in one direction, and the momentum in the opposite direction would have been "lost." Let me know if this is a little confusing and I will elaborate.

For Part E, take your wavelength and map it to the EM Spectrum, it will probably be in the visible spectrum.

I hope this helps! 

[jamonwindeyer]

Hey Meckenza:

Good question actually. My teacher suggested that this case is perhaps different to Faraday's experiment. In this case, we've found out that the ring being suspended in air is repelled away from the solenoid. Originally I would have picked D, but if you think about it,  is there a  force there to bring the ring back to its original position? Clearly we dont have any evidence of a restoring force acting on the ring to bring it back to its original position. So as long as there is flux the ring would just stay there because the current would keep on circulating in the ring and the same poles between ring and solenoid would cause them to keep being repelled. Its quite a tough question!

Best Regards
Happy Physics Land

Depends on if we naturally assume the presence of gravity or not. Gravity is our restoring force if it is there. I would also assume that the copper ring has resistance and thus, any induced currents will dissipate very quickly. Fair enough that it isn't specifically in the question, but I would naturally assume these things. It's a matter of theoretical versus practical Physics!

[Happy Physics Land]

Depends on if we naturally assume the presence of gravity or not. Gravity is our restoring force if it is there. Fair enough that it isn't specifically in the question, but I would naturally assume   

The question is either too ambiguous or too hard l reckon, my friend is find your notes very helpful by the way.

[jamonwindeyer]

The question is either too ambiguous or too hard l reckon, my friend is find your notes very helpful by the way.

It is difficult and definitely ambiguous. I don't think this was in a HSC Paper, I don't remember seeing it in the last few years anyway    awesome! I'm glad they are of help 

[monique.degiovanni]

anyone know bohr's postulates there seems to be multiple answers

[Happy Physics Land]

anyone know bohr's postulates there seems to be multiple answers

Hey Monique!

Nice question! And I would certainly expect there to be several answers because bohr's postulate indeed encompasses quite a few concepts. (I haven't done in depth studies in regards to this but I did some research and here are some responses to your question!)

First postulate: Electrons in an atom exist in STATIONARY STATES. Bohr stated that electrons orbit the nucleus WITHOUT EM emission and any permanent change in their motion must be accompanied by a complete transition from one stationary state to another. Problem with this postulate is that Bohr cannot explain this very effectively.

Second Postulate: Transmission between stationary states produces/absorbs EM Radiation. When an electron moves between stationary states, it is accompanied by the emission or absorption of a photon whose equation can be given mathematically as ΔE=hf. Thus, EM radiation is produced by the movement of electrons between energy states which account for Planck's "atomic oscillators."

Third Postulate: The ANGULAR MOMENTUM of a stationary electron is quantised. If you recall from Rutherford's model of the atom the electrons orbited in circular orbits at ANY RADII. If this coincided with Bohr's 2nd postulate, it would mean that every element can emit a FULL SPECTRUM as any transition would be possible. As this is not the case, it would imply that electrons orbited at FIXED RADII.

Hope these informations can help you Monique! If anyone has done/is doing from quanta to quarks, you may feel welcome to add any extra information!

Best Regards
Happy Physics Land

[Neutron]

Yo fam,

Okay this is actually sorta dumb but I'm having the biggest brain fart.. Where's the armature and what does it do again? Like I keep getting confused when I look at diagrams as to which part's the coil, the armature and the axle D: Thanks guys

Neutron

[RuiAce]

Yo fam,

Okay this is actually sorta dumb but I'm having the biggest brain fart.. Where's the armature and what does it do again? Like I keep getting confused when I look at diagrams as to which part's the coil, the armature and the axle D: Thanks guys

Neutron

The armature is literally the coil. I remember getting that confused for so long and realising oh wait they're the same thing.

An axle is not necessary for a motor. On a generator, you probably want an axle so that you have something to spin the actual generator. (This is of course, for an ordinary generator you built or used as a play toy; not for large scale electricity production)

[Castform99]

Hi, I need help with some questions:

1. A projectile has a time of flight of 8.0s and a range of 1120m.
a) What maximum height does it reach?
b) At what velocity is it projected?

2. The model rocket has a pre-launch mass of 94.2g, of which 6.24g is solid propellant. It is able to deliver a thrust of 4.15N for a period of 1.2s. Assuming that the rocket is fired directly up, determine:
a) The initial rate of acceleration and g force
b) The final rate of acceleration and g force just prior to exhaustion of the fuel

3. Bill is selected for the space trip to Alpha Centauri a distance of 4.3 light years from Earth. It is planned for them to travel at 0.9c. If his body can withstand a force of 3g, how long will it take to accelerate them to this speed(ignoring relativistic effect)?

[Stylo]

Yo

Having a bit of trouble understanding how a synchronus motor works. Anyone be able to just briefly explain it?

Cheers

[Happy Physics Land]

Hi, I need help with some questions:

1. A projectile has a time of flight of 8.0s and a range of 1120m.
a) What maximum height does it reach?
b) At what velocity is it projected?

2. The model rocket has a pre-launch mass of 94.2g, of which 6.24g is solid propellant. It is able to deliver a thrust of 4.15N for a period of 1.2s. Assuming that the rocket is fired directly up, determine:
a) The initial rate of acceleration and g force
b) The final rate of acceleration and g force just prior to exhaustion of the fuel

3. Bill is selected for the space trip to Alpha Centauri a distance of 4.3 light years from Earth. It is planned for them to travel at 0.9c. If his body can withstand a force of 3g, how long will it take to accelerate them to this speed(ignoring relativistic effect)?

Hey Castform!

The solution to question one is posted before, if there are any obscurities in my solutions please dont hesitate to ask!




Best Regards
Happy Physics Land

[jamonwindeyer]

Yo

Having a bit of trouble understanding how a synchronus motor works. Anyone be able to just briefly explain it?

Cheers

Hey Stylo! Sure thing, a quick explanation coming up 

So, as shown in the diagram below, a synchronous motor (very similar to an AC Induction Motor has a rotor surrounded by three pairs of electromagnets. For a synchronous motor, the rotor is a permanent magnet.

Now, the electromagnets which make up the stator are fed 3 Phase AC Current. Without going into technicalities, what this means is that the three pairs of electromagnets turn on in sequence: Pair 1, Pair 2, Pair 3, Pair 1, etc, one pair at a time. This happens at a speed proportional to the frequency of the AC current fed into the motor.

Now, as the electromagnets turn on, the magnet rotates to align itself with the resultant magnetic field. Since the electromagnet pairs turn on and off in sequence, the magnet rotates to follow. This is what causes the rotation.

An AC Induction Motor operates on very similar principles, except the rotor is not magnetised. Instead, eddy currents formed in the rotor generate the magnetic field required for the rotation of the rotor. This can be modelled by taking a conductive disk, suspending it from string, and moving a magnet around it in a circle. The disc will chase the magnet. AC Induction motors work off that same principle 

[Happy Physics Land]

Hi, I need help with some questions:

1. A projectile has a time of flight of 8.0s and a range of 1120m.
a) What maximum height does it reach?
b) At what velocity is it projected?

2. The model rocket has a pre-launch mass of 94.2g, of which 6.24g is solid propellant. It is able to deliver a thrust of 4.15N for a period of 1.2s. Assuming that the rocket is fired directly up, determine:
a) The initial rate of acceleration and g force
b) The final rate of acceleration and g force just prior to exhaustion of the fuel

3. Bill is selected for the space trip to Alpha Centauri a distance of 4.3 light years from Earth. It is planned for them to travel at 0.9c. If his body can withstand a force of 3g, how long will it take to accelerate them to this speed(ignoring relativistic effect)?

Hey Castform:

Solution to question 2 is posted below. The main formulae I used were newton's 2nd law and the g-force formula, and also the weight formula. The quantity 1.2s can fundamentally be ignored here, it only indicates the amount of time the fuel is gonna last for but this is kind of irrelevant to solving the question. So essentially its a red herring.




Best Regards
Happy Physics Land

[Happy Physics Land]

Hi, I need help with some questions:

1. A projectile has a time of flight of 8.0s and a range of 1120m.
a) What maximum height does it reach?
b) At what velocity is it projected?

2. The model rocket has a pre-launch mass of 94.2g, of which 6.24g is solid propellant. It is able to deliver a thrust of 4.15N for a period of 1.2s. Assuming that the rocket is fired directly up, determine:
a) The initial rate of acceleration and g force
b) The final rate of acceleration and g force just prior to exhaustion of the fuel

3. Bill is selected for the space trip to Alpha Centauri a distance of 4.3 light years from Earth. It is planned for them to travel at 0.9c. If his body can withstand a force of 3g, how long will it take to accelerate them to this speed(ignoring relativistic effect)?

Hey Castform:

Here is your solution to the third question. The trick with this part is remembering to use v = u +at. This formula is not limited only to projectile motions, it can be applied versatilely across all physics calculations. In this case, because initially the rocket doesnt have any velocity, hence u=0 m/s. IF this question wanted us to consider the relativistic effects as well, it would be harder because relativistic effects of length contraction and time dilation would be different at each stage of our voyage, since velocity is gradually increasing and this has an impact upon the lorentz factor.



Best Regards
Happy Physics Land

UPDATE: thanks Lazydreamer for spotting my mistake. The value of "a" that I used in v = u +at was 9.8 when it was meant to be 19.6 (i.e. the acceleration that I calculated using g-force formula). This would change the result by twofold (i.e. just multiply the final result by 2 to get the correct answer)