HSC Physics Question Thread

[arunasva]

Ooh 😀 I've heard that it was an unpopular option and was wondering why but it sounds good to me

Also could someone explain how the slingshot effect works? How do the satellites/rockets use the planet to gain velocity? I don't get it!!

Imagine you are flying close to a tree towards well Dwayne Johnson who  is running around the tree, he holds your hand, spins you around and tosses you away. You're gonna fly faster relative to the tree cos you have his speed around the tree added to your speed. Similarly a spacecraft (you) comes and latches onto the gravitational field of a planet(Dwayne) by making an elastic collision. The planet takes the spaceship around and its orbital velocity relative to the sun (the tree) is gained by the spacecraft and the planet slows down a lil'. However the speed relative to the planet is still the same, as you gained the planet's velocity relative to the sun and your velocity relative to the planet didn't slow down.

[jamonwindeyer]

Also could someone explain how the slingshot effect works? How do the satellites/rockets use the planet to gain velocity? I don't get it!!

^^ Love the explanation above!! I think of it like riding a bike and grabbing onto a moving car - The car drags you along and increases your speed without their own speed being affected in any significant way. Except for the slingshot effect, it is a planet pulling a probe with a gravitational field - The principle is the same, but of course there is the orbital aspect which is explained better above

I wrote some nifty summaries of all the course content if it helps as you are working through, they are about halfway down in this list

[justwannawish]

Imagine you are flying close to a tree towards well Dwayne Johnson who  is running around the tree, he holds your hand, spins you around and tosses you away. You're gonna fly faster relative to the tree cos you have his speed around the tree added to your speed. Similarly a spacecraft (you) comes and latches onto the gravitational field of a planet(Dwayne) by making an elastic collision. The planet takes the spaceship around and its orbital velocity relative to the sun (the tree) is gained by the spacecraft and the planet slows down a lil'. However the speed relative to the planet is still the same, as you gained the planet's velocity relative to the sun and your velocity relative to the planet didn't slow down.

I'm going to remember that analogy now haha! Thank you so much.

^^ Love the explanation above!! I think of it like riding a bike and grabbing onto a moving car - The car drags you along and increases your speed without their own speed being affected in any significant way. Except for the slingshot effect, it is a planet pulling a probe with a gravitational field - The principle is the same, but of course there is the orbital aspect which is explained better above

I wrote some nifty summaries of all the course content if it helps as you are working through, they are about halfway down in this list

Just checked it out (the maths post was so amazing and rockets are finally starting to make more sense now

[Mymy409]

When an electron is passing through a magnetic field, a centripetal force acts on it, right? What other forces? What needs to be equated?

[mary123987]

When an electron is passing through a magnetic field, a centripetal force acts on it, right? What other forces? What needs to be equated?

hey to explain this simply If A partlicle with charge q is moving with velocity v perpendicularly to a magnetic field of strength B, the particle will experience a ,magnetic force given by F=qvBsinθ .Now from my understanding you are not required to explain where this formula is from or even how to derive it(its currently beyond our understanding) . To answer your question with respect to the force acting on it , it really depends on the circumstance as it will not always be experiencing centripetal force however if it is orbiting something like a nucleus (which to be honest i dont think ive ever seen a past hsc paper ask) then yeah you can say the force keeping it in orbit is centripetal force .However in general terms the force experienced is from the interaction with the magnetic field . Also centripetal force generally applies in the circumstance above and if this force is perpendicular to the direction of motion (I.e. it is a centripetal force) and will cause the particle to undergo uniform circular motion (if no other forces are in play).
if not perpendicular and the  particle enters at an oblique angle, it will follow a helical path. hope that makes sense let me know if your'e confused

[Dragomistress]

I would like to ask, why is the change in potential energy 52J since the work done on the object is translated to the potential energy gained during this time. So if I was to move it parallel to the ground, it will also gain potential energy?

[hinakamishiro]

Hey guys! Can I please have some please with these two questions? Any explained working out would be really helpful. Thanks! 

[sidzeman]

Hey guys! Can I please have some please with these two questions? Any explained working out would be really helpful. Thanks! 

For question 19 - we know that length contracts for objects travelling at near light speeds. The key thing to recognise here is "when observed from the trains frame of reference" - therefore we know that the length of the train will be contracting. If it was from the trains frame of reference, the platform would appear to be contracting. Therefore we know lo (200) and lv (160 which it contracts to be the same length as the platform), and the rest is just simple subbing into the formula.

[blasonduo]

Hey guys! Can I please have some please with these two questions? Any explained working out would be really helpful. Thanks! 

Sidzeman is right! a simple substitution! If you need further help, I'll be happy to!

20) This will be a bit tricky to explain, but it uses triangles!!

It is launched horizontally, so the initial VERTICAL acceleration is 0. We also know the Horizontal velocity is constant.

Knowing that after 2 seconds, the angle is 45 degrees, we know that this triangle in ISOSCELES, which means that the horizontal AND vertical components have to be the same!

As we know gravitational acceleration is a constant 9.8 PER SECOND, after TWO seconds, the vertical velocity of the ball is:

9.8 X 2 = 19.6

And as we know the horizontal and vertical velocities HAVE to be equal, it means the horizontal velocity is ALSO 19.6.

The find the resultant velocity, we must find the hypotenuse so

(19.6)^2 + (19.6)^2 = v^2

v = 27.72

Which is D!!

[Shadowxo]

I would like to ask, why is the change in potential energy 52J since the work done on the object is translated to the potential energy gained during this time. So if I was to move it parallel to the ground, it will also gain potential energy?

It's because it's on a slope, and the surface is frictionless. The work done on it results in it gaining g.p.e. and k.e. with the k.e. being converted into g.p.e. as it continues travelling up the slope. So the final g.p.e. = work done. If it was travelling parallel to the ground, the g.p.e. would never change and all the work done would be converted into k.e. instead.

[justwannawish]

How do I tackle question 19 from the 2014 paper?
The rest length of a train is 200 m and the rest length of a railway platform is 160 m. The train rushes past the platform so fast that, when observed in the platform’s frame of reference, the train and the platform are the same length.

[pikachu975]

How do I tackle question 19 from the 2014 paper?
The rest length of a train is 200 m and the rest length of a railway platform is 160 m. The train rushes past the platform so fast that, when observed in the platform’s frame of reference, the train and the platform are the same length.

so Lv = 160m as the person on the platform sees the train's length contract from 200 to 160. Hence sub in Lv = 160 and Lo = 200

[Bubbly_bluey]

Hey guys!
Just a quick question about "other consequences of Special Relativity": Does this mean time,length and mass dilations? What does it mean by some other consequences?
Thank you!:D

[felix12345]

Hey, I was wondering if anyone could provide an explanation to this question - answer is b.
Thanks

[pikachu975]

Hey, I was wondering if anyone could provide an explanation to this question - answer is b.
Thanks

p = mv so since velocity is constantly slowed down by friction F, then since p is proportional to m then p will decrease linearly therefore B