VCE Physics Question Thread!

[lzxnl]

From memory, what one must do when adding two quantities is to deal with fractional error. Consider c=a+b. Then the fractional error in c is given by:



Where a, b, c are the mean values of a, b, c; and the Delta values are the error margins.

This is also how we deal with errors for division

Actually, you've quoted the rule for products. All of these rules originate from calculus.

If z = xy, then ln z = ln x + ln y
So differentiating gives
dz/z = dx/x + dy/y
If you associate dz as change in z, you get the rule for multiplication. Division works the same.

One way of thinking about it is, imagine I'm multiplying two numbers and one of them is 1% off while the second one is 2% off. The product is then roughly 3% off.

If z = x + y, dz = dx + dy. So you add the absolute errors; there's nothing more to it.

[Gogo14]

how can photons have momentum if it doesnt have mass

[KingKunta]

hey guys,
just curious would you advise the use of edrolo in learning all Unit 3&4 physics content (simply based on the fact that my teacher cannot teachto save her own life, she just an overall useless person)

cheers.

ps: the guy who goes through it is guy named Andy Matthews.

[Alevine]

how can photons have momentum if it doesnt have mass

Hi Gogo 14!
Fantastic question. It can seem bizarre that photons could even have momentum given that we're (probably) very accustomed to considering momentum in the 'classical' sense. That is, we're used to momentum being defined as the product of a given object's mass and velocity:
p = mv
However, this is not the only definition we have of momentum. We can also define the momentum as p = h / lambda
Whilst light does not have an associated mass, it certainly does however have a wavelength - and (using this formula) we can use that wavelength to calculate the photon's momentum.
In terms of WHY photons can still have momentum (despite being mass-less), a great experiment to consider is Compton Scattering. With Compton Scattering, we see in some situations that when photons of light are involved in a "collision" of one form or another they have a tendency to behave similarly to particles. In the experiment, photons of light collide with electrons in a graphite block. The interaction that follows is very similar to one billiard ball simply rolling into another. Once the photon hits the electron, the electron 'rolls' away (we get recoil electrons) and the photon loses some of its energy from the collision, subsequently increasing it's wavelength (since E = h / lambda, a decrease in the photon's energy means an increase in the photon's wavelength).

I hope some of that helps you get a sense of where all this comes from? Even though the idea is strange, there is more than one way that we can think about momentum.

[Alevine]

hey guys,
just curious would you advise the use of edrolo in learning all Unit 3&4 physics content (simply based on the fact that my teacher cannot teachto save her own life, she just an overall useless person)

cheers.

ps: the guy who goes through it is guy named Andy Matthews.

Lmao hey KingKunta hahahah
Soooo Edrolo. Um, it's okay?
Personally I kinda....idk
I wasn't the biggest fan of Edrolo in year 12 but it's not a bad resource. That being said, (and again, this is just my personal opinion) I think Edrolo is much better for revision than it is for learning things for the first time.
Even though it's arduous, reading through a (good) year 12 physics textbook, as well as doing tOns of practice questions and regular revision is probably the best way to go in terms of learning and mastering things for yourself.
No self promo lol but maybe this article will help? https://atarnotes.com/vce-physics/
Hope everything works out!

[Gogo14]

Hi Gogo 14!
Fantastic question. It can seem bizarre that photons could even have momentum given that we're (probably) very accustomed to considering momentum in the 'classical' sense. That is, we're used to momentum being defined as the product of a given object's mass and velocity:
p = mv
However, this is not the only definition we have of momentum. We can also define the momentum as p = h / lambda
Whilst light does not have an associated mass, it certainly does however have a wavelength - and (using this formula) we can use that wavelength to calculate the photon's momentum.
In terms of WHY photons can still have momentum (despite being mass-less), a great experiment to consider is Compton Scattering. With Compton Scattering, we see in some situations that when photons of light are involved in a "collision" of one form or another they have a tendency to behave similarly to particles. In the experiment, photons of light collide with electrons in a graphite block. The interaction that follows is very similar to one billiard ball simply rolling into another. Once the photon hits the electron, the electron 'rolls' away (we get recoil electrons) and the photon loses some of its energy from the collision, subsequently increasing it's wavelength (since E = h / lambda, a decrease in the photon's energy means an increase in the photon's wavelength).

I hope some of that helps you get a sense of where all this comes from? Even though the idea is strange, there is more than one way that we can think about momentum.

Ok, now im a bit confused about what momentum is because I always thought that it was mass times velocity. So what really is momentum?

[Syndicate]

Ok, now im a bit confused about what momentum is because I always thought that it was mass times velocity. So what really is momentum?

I don't know how correct I am, but I think because photons have energy, it can have momentum due to the mass-energy equivalence theory. So if the photons have x amount of energy, than it will have Y amount of mass (which is basically X divided by c, the speed of light, squared). Since they are already moving at the constant speed (speed of light), they will have momentum.

[Gogo14]

Thanks also I have a few qs about waves:
1. I dont really undersstand the concept of natural frequency, i know that its the oscillation that an object will naturally take, but why? I just don't understand the concept it kinda seems wierd to me.
2. Question 3 attached, Why is u moving up?
3. Q19. Dont know how to do

[gamma032]

Hey everyone. In regards to transformers, my textbook and worked solutions for questions indicate that they only work with an AC input current.

I can understand why a constant DC supply would not induce an emf (no change in flux through the secondary coil) but wouldn't a variable DC supply, such as that generated by a generator with a slit ring, still be able to induce an emf?

[Bri MT]

Ok, now im a bit confused about what momentum is because I always thought that it was mass times velocity. So what really is momentum?

Not sure if this is a proper definition, but I think of momentum in relation to inertia so my definition is " A measure of a body's resistance to change in it's motion"

Thanks also I have a few qs about waves:
1. I dont really undersstand the concept of natural frequency, i know that its the oscillation that an object will naturally take, but why? I just don't understand the concept it kinda seems wierd to me.
2. Question 3 attached, Why is u moving up?
3. Q19. Dont know how to do

2.
Q ans.   The gradient is positive
understanding: The graph shows a snapshot of the wave at that current moment. From the graph we can see that the wave is transverse therefore the only possible answers are up, down, or not moving. As the particle is not at a peak, it hasn't reached the end of it range in either direction, so we know it isn't still. Based on the location of the particles surrounding it, we can deduce that it is moving up.
3. Given that the location of the antinode is the x-co-ordinate of the location of the peak, the distance between two consecutive antinodes is the wavelength. If you're still stuck let me know.

Hey everyone. In regards to transformers, my textbook and worked solutions for questions indicate that they only work with an AC input current.

I can understand why a constant DC supply would not induce an emf (no change in flux through the secondary coil) but wouldn't a variable DC supply, such as that generated by a generator with a slit ring, still be able to induce an emf?

Your textbook has generalised. You are right a DC power supply can induce emf through a transformer, when it is changing. This is something to be careful of in questions as when it is connected or disconnected there will be emf generated, and listing this can get you marks. In your situation with a slip ring, I'm not sure if that would still be referred to as DC? but yes there would be emf generated when the current changes direction every half turn.

[A TART]

I'm wondering why my textbook says the car is travelling in the same direction as the truck in this question:

A brand-new Rolls Royce rolls off the back of a truck as it is being delivered to its owner. The truck is travelling along a straight road at a constant speed of 60 km h−1. The Rolls Royce slows down at a constant rate, coming to a stop over a distance of 240 m. It is a full minute before the truck driver realises that the precious load is missing. The driver brakes immediately, leaving a 25 m long skid mark on the road. The driver’s reaction time (time interval between noticing the problem and depressing the brake) is 0.5 s.

My Working:


Solution given in the textbook:

It is important to remember that, at the instant that the Rolls Royce rolls off the truck, it is moving in the same direction as the truck.
After 1 minute, it has moved a distance of 1000 m (a constant speed of 60 km/h is equal to 1 km/min).
During the driver’s reaction time, the distance moved by the truck is 8.3 m (the distance moved in 0.5 s at a constant speed of 60/h 16.67 m/s 0.5 s).
The braking distance of the truck is 25 m.
The total distance moved by the truck is 1000 m+8.3 m+25 m+1033 m.
The distance moved by the Rolls Royce is 240 m (in the same direction as that of the truck). The Rolls Royce is, therefore, 793 m behind the stopped truck.

Wouldn't the car go in the opposite direction?

PS: What does everyone think of the Jacaranda textbook? Personally I hate it.

[Shadowxo]

Initially they're both travelling in the same direction, but the car rolls off the truck because it's moving slightly slower than the truck. From the truck's perspective, the car is moving away but really it's just travelling a little slower than the truck so the truck is travelling faster, so the car falls off
(Just a little explanation but that's the basic idea)

[-273.15]

Hello
Could someone please explain to me why the stopping voltage in the photoelectric effect is negative?

Thanks so much

[sweetiepi]

Hello
Could someone please explain to me why the stopping voltage in the photoelectric effect is negative?

Thanks so much

I'm a tad rusty on the PE effect, however I believe that the stopping voltage is negative is due to the fact that a positively charged plate will attract the emitted electrons, a neutral plate will attract any electrons headed towards it, and an increasingly negatively charged plate will 'repel' the electrons, hence the current (flow of electrons) becomes zero.

[-273.15]

Hello
another question sorry
For a photoelectron to be emitted (in Photoelectron effect), the frequency of the electromagnet radiation and thus the energy of the photon must be greater than the threshold frequency and thus the work function.
However, can these be equal or does it have to be greater in order for photocurrent to be observed?
If they were equal, would it just mean that no excess kinetic energy is released??

Thank you!