HSC Physics Question Thread

[jamonwindeyer]

Hey there how would I find the height of the platform of this question?
I got the gradient = 0.73 which should give me the time I need to sub into one of the projectile equations. I'm just not sure which one to use and how direction plays into this because i'm putting -9.8 for g but then that gives me a negative distance (delta y) so i'm really confused???

Any help would be great!

You did the right thing! You would have gotten about -2.5 metres? That represents the fact that, after the time of flight, you are 2.5 metres BELOW your launch point. It's because the projectile is lower than when you started

Height of the platform is about 2.5 metres

Hey me again,

Sorry if this is really stupid but how would you count the dots in a question like this? Would you count the first one on the right or not because it's when the initial velocity is 0 i.e. when t=0 - or am I completely overthinking this - please help 

EDIT: I've attached the relevant questions and my answers - if someone could please help me out with c and check the rest I'd really appreciate it!!(Image removed from quote.)

You are over thinking a bit I think counting an extra dot will only make a slight difference to your estimates and thus your calculations, so I don't think it would matter!

For C - You need to identify that the horizontal velocity is constant. You can tell because the horizontal distance between each of your dots is relatively constant

The others look good on a quick glance - Your first one it is 0.67m below the HIGHEST point, not the initial point (you've redefined the origin to be the peak point by using the formula in that way). So just 0.67 metres should be the answer. For D, you might need to include the horizontal component of velocity as well using pythagoras?

[jamonwindeyer]

Hey is there anything I could add to my answers for projectile motion here (Q5)
I'm also stuck with how to interpret part d)

(This is a practice test by the way im not making you guys do an exam for me - there just weren't any answers provided online)

(Image removed from quote.)

For A, give a brief explanation why (horizontal motion has no effect on vertical)

I think you've misinterpreted B, C and D - The vehicle is accelerating, the frame of reference is not inertial. So the person outside the vehicle would see a parabolic motion - But the person inside the vehicle would see the mass accelerating horizontally (because the car is accelerating away from the falling mass).

If the person in the vehicle couldn't tell they were moving, they'd need to say, "Well, some magical force must have accelerated that mass." This reflects the fact if we are in a non-inertial reference frame, we need to introduce 'fictional' forces to account for observations

[bsdfjnlkasn]

For D, you might need to include the horizontal component of velocity as well using pythagoras?

Hey Jamon!

Thanks so much for the prompt replies I was just wondering if you could offer a bit more guidance for part d) as I just don't see where we can get enough information to do a vector sum (and then apply pythagoras). We're not given any angles so can't resolve the horizontal/vertical components and so I'm genuinely lost with how to approach this question.

Thanks again for everything!

[jamonwindeyer]

Hey Jamon!

Thanks so much for the prompt replies I was just wondering if you could offer a bit more guidance for part d) as I just don't see where we can get enough information to do a vector sum (and then apply pythagoras). We're not given any angles so can't resolve the horizontal/vertical components and so I'm genuinely lost with how to approach this question.

Thanks again for everything!

All good! Hmm, you are right - I don't see how we can find the horizontal component either. Unless you somehow use the distance in B as a measure, but that would mean the diagram has to be to scale and they don't specify that...

It must just mean vertical velocity as you did initially? Unless anyone else has any ideas

[bsdfjnlkasn]

All good! Hmm, you are right - I don't see how we can find the horizontal component either. Unless you somehow use the distance in B as a measure, but that would mean the diagram has to be to scale and they don't specify that...

It must just mean vertical velocity as you did initially? Unless anyone else has any ideas

Ok phew

Here's a similar question - could you again check that I've approached everything correctly? Does the last question also then mean I should find the vertical velocity?

[TooLazy]

Hey me again,

Sorry if this is really stupid but how would you count the dots in a question like this? Would you count the first one on the right or not because it's when the initial velocity is 0 i.e. when t=0 - or am I completely overthinking this - please help 

EDIT: I've attached the relevant questions and my answers - if someone could please help me out with c and check the rest I'd really appreciate it!!(Image removed from quote.)

Horizontal velocity is constant, acceleration = 0.
Only force acting on the projectile is gravity (neglecting air resistance)
If you know the horizontal distance and time, you can use the formula x=ut to get the horizontal velocity

[TooLazy]

Ok phew

Here's a similar question - could you again check that I've approached everything correctly? Does the last question also then mean I should find the vertical velocity?(Image removed from quote.)

For the overall velocity in part d,
you need to first calculate both components of the final velocity and add them together using a vector diagram.
This  will enable you to find the final velocity

[khitnay]

Hi! Anyone who could help me with this physics assignment would be deeply appreciated
My assignment requires me to build either a motor, a generator or a transformer with complex parts such as commutators and a complex design.
I have chosen to build a three phase (to make the design complex) AC generator with stationary coils and a rotor with magnets on it unlike the statonary magnetic field and rotating coil as learnt in the curriculum. However, I'm not sure whether such a model would require slip ring commutators or even how to start building it.
All the models of alternators I've seen on youtube are self excited with a dc generator connected. Is it possible to build an alternator that is not self excited? If so, how should I go about building it?

[RuiAce]

Hi! Anyone who could help me with this physics assignment would be deeply appreciated
My assignment requires me to build either a motor, a generator or a transformer with complex parts such as commutators and a complex design.
I have chosen to build a three phase (to make the design complex) AC generator with stationary coils and a rotor with magnets on it unlike the statonary magnetic field and rotating coil as learnt in the curriculum. However, I'm not sure whether such a model would require slip ring commutators or even how to start building it.
All the models of alternators I've seen on youtube are self excited with a dc generator connected. Is it possible to build an alternator that is not self excited? If so, how should I go about building it?

Since the three-phase motor is an example of an induction motor, if you had a commutator it wouldn't be for the motor itself; it would be for the magnet rotor. Which may make the design a bit unnecessarily complicated.

This chose is pretty advanced. (Back when I had this assignment I just built an ordinary DC motor.) You have to somehow ensure that the wires are going to be running in sync. I haven't examined the validity of these links but here is a video and an article you can read about it. (Taken from a quick Google search.)
https://www.youtube.com/watch?v=DAYz3v8HwEE
https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwiAzoGVzebSAhWGkJQKHddvDRYQFggsMAM&url=https%3A%2F%2Fwww.electrical4u.com%2Fconstruction-of-three-phase-induction-motor%2F&usg=AFQjCNH4ZsiQclUPg6gWP-Ald3gXaBCstg&sig2=VEuh92PB7wn5HcQxExYEkg

Horizontal velocity is constant, acceleration = 0.
Only force acting on the projectile is gravity (neglecting air resistance)
If you know the horizontal distance and time, you can use the formula x=ut to get the horizontal velocity

Thank you for the answer but I have a request. To help ease the students' understanding, please write down formulae as they are given on the data sheet

[arunasva]

https://drive.google.com/file/d/0B3wlCPQFQN1tNkxUYXRXNGZSYU0/view help with this if anyone can. Please T.T

[jamonwindeyer]

https://drive.google.com/file/d/0B3wlCPQFQN1tNkxUYXRXNGZSYU0/view help with this if anyone can. Please T.T

Welcome to the forums!! remember that G-force is closely related to weight force. The G-force experienced by the astronauts will be equal to:



This formula acknowledges the effects of gravity normally, being added to any additional acceleration \(a\) of the rocket. We notice that the only thing that will differ between the astronauts is their mass - Everything else is constant. So, the ratio of the G-forces is equal to the ratio of their masses! For this reason, I'm fairly certain the answer should be B

[arunasva]

I have an exam in 8 hrs (HSC OUTSIDE AUSTRALIA) I am so done for dont know how to use light years. Please help.

[Syndicate]

I have an exam in 8 hrs (HSC OUTSIDE AUSTRALIA) I am so done for dont know how to use light years. Please help.

Hi arunasva,

9 light year is just a unit of time. For the time dilation formula, it doesn't matter what units are used for the time as long as your proper time, and dilated time has the same units.

For question a, t0 = 9 light year, v = 0.75c, t =? (9 is the proper time, as it is the actual time).
t = t0Y
t = 9 x 1.512
t = 13.61 light years

b) Use the speed = distance/ time formula. Why? Because the time calculated by the astronaut is the proper time in this case.

distance = 9 lights years
Speed = 0.75 c

Time = 9/0.75
time = 12 light years

[arunasva]

Yup there was a light years question. I messed it up. Messed up physics pretty darn bad. Sigh im stuck only 5 units and physics is so fucking hard dont even get projectile motion look at the question and sit there not knowing what to do. I should've dropped Physics in Year 11 but I got 90 % and parents were like nah we can't let you drop that. Ill Fail again in the trials. Oh well.

[jamonwindeyer]

Yup there was a light years question. I messed it up. Messed up physics pretty darn bad. Sigh im stuck only 5 units and physics is so fucking hard dont even get projectile motion look at the question and sit there not knowing what to do. I should've dropped Physics in Year 11 but I got 90 % and parents were like nah we can't let you drop that. Ill Fail again in the trials. Oh well.

Don't give up! If you go through the year thinking you'll fail, you'll fail. If you go through the year willing to go hard and improve, you never know what can happen! Never too late to turn things around my friend! You're always welcome to pop any questions here and we'll help you any way we can