HSC Physics Question Thread

[bsdfjnlkasn]

Hey there, I'm a tad confused with what to include my response to the following:

Define the term ‘g force’, and discuss the relevance of g force during the early stage of rocket launching

I find the definition for 'g force' in my textbook really unsatisfactory so if someone could please explain the concept of these forces to me- that would be greatly appreciated!!

[Kle123]

Hey there, I'm a tad confused with what to include my response to the following:

Define the term ‘g force’, and discuss the relevance of g force during the early stage of rocket launching

I find the definition for 'g force' in my textbook really unsatisfactory so if someone could please explain the concept of these forces to me- that would be greatly appreciated!!

The CONCEPT-
G force is a person’s apparent weight expressed a multiple of their true weight on earth. G-force is a scale used to measure different forces that are experienced at different rates of acceleration (when in contact with an accelerating body).
As a rocket accelerates people will feel different forces in comparison to another person. This variation is due to the different mass of different people which consequently correlates to different weight force(W=mg) and the force a body experiences due to Newton's second law.

If this raw scale of force due to acceleration was used to describe the effects on every person, it would be highly complicated. Thus g-force scale is used instead to simplify and communicate the same relative forces on different masses, as it only takes into account acceleration in which everyone experiences the same.
_______________________________________________________

During early stage of rocket launching, where rockets acceleration is greatest, g-force is significant due to the effects such as unconsciousness due to blood draining away(Inertia) from certain areas of the body and death if extreme g-forces are sustained. Consequently techniques have to be utilised such that astronauts aren't seriously harmed

[jamonwindeyer]

OMG that makes so much sense thanks!
One thing though... how would we decrease our orbital radius? Do we provide thrust in front of direction of motion to slow down? But wouldn't that cause us to follow an elliptical orbit/or slowly fall into the Earth's atmosphere / or does slowing down automatically drop us into a lower orbit?

So to reduce orbital radius do we slow down through providing forward thrust then accelerate to the required speed a lower orbit (which is what i'm thinking)?

Hmm, yep I think from memory the best way to answer it is indeed your way! Provide an opposing thrust forwards, this reduces the orbital radius. Maintain this orbit to catch up to the space station, then get back into the initial orbit to dock

[f_tan]

Hi, not sure how to answer these past HSC questions:

1. Two significant problems that will affect a manned spaceflight to mars are:
    - the changes in gravitation energy
    - protecting the space vehicle from high-speed electrically charged particles from the sun.
    Use your understanding of physics to analyse these problems (8 marks)

2. In 1970 NASA launched Apollo 13, their third mission planned to land humans on the Moon. Half-way to the Moon a huge explosion crippled the    spacecraft. The only way home for the astronauts was to fly around the back of the Moon and then fire the rocket engine to take the craft out of lunar orbit and put it into an Earth-bound trajectory

At the completion of the rocket engine burn, mission leader Jim Lovell was heard to say, 'We just put Isaac Newton in the driver's seat.'

Given that the spacecraft returned safely to Earth, justify Jim Lovell's statement (4 marks)


Any help is appreciated, thank you!

[jamonwindeyer]

Hi, not sure how to answer these past HSC questions:

1. Two significant problems that will affect a manned spaceflight to mars are:
    - the changes in gravitation energy
    - protecting the space vehicle from high-speed electrically charged particles from the sun.
    Use your understanding of physics to analyse these problems (8 marks)

2. In 1970 NASA launched Apollo 13, their third mission planned to land humans on the Moon. Half-way to the Moon a huge explosion crippled the    spacecraft. The only way home for the astronauts was to fly around the back of the Moon and then fire the rocket engine to take the craft out of lunar orbit and put it into an Earth-bound trajectory

At the completion of the rocket engine burn, mission leader Jim Lovell was heard to say, 'We just put Isaac Newton in the driver's seat.'

Given that the spacecraft returned safely to Earth, justify Jim Lovell's statement (4 marks)


Any help is appreciated, thank you!

Hey f_tan!

That first question you should answer in two parts. The first bit, you are talking about changes in gravitational potential energy. You could (and should) interpret this two ways - The challenge in obtaining that energy, and the challenge in then getting rid of it safely to land on Mars. Your answer to this part of the question should discuss GPE (perhaps include the formula), and discuss the methods by which we can obtain and shed this energy as required. It will likely require a discussion of orbits, as well as the rotational and orbital motion of the earth (think - How do we use the earths rotation to make it easier to gain GPE?)

The second part involving high speed electrically charged particles, you are discussing insulate shielding, much the same as you would discuss in a re-entry into the atmosphere question

The second question is, basically, asking you to explain the Physics principles involved in the described manoeuvre. Essentially what happened is that the spacecraft was crippled while in orbit around the moon. The spacecraft maintained that orbit until a suitable time, where the engines were used to escape that orbit and enter an orbit around the earth (not so much an orbit, but a trajectory, as it had them gradually descend to the surface). Your job in answering the question is to explain how Newton's Laws play a role in that scenario. Newton's law of Universal Gravitation will need mentioning, since that is what governs the nature of orbits, and the 2nd Law (\(F=ma\)) is related to the acceleration of the rocket during the engine burn

I hope that helps

[f_tan]

Hey f_tan!

That first question you should answer in two parts. The first bit, you are talking about changes in gravitational potential energy. You could (and should) interpret this two ways - The challenge in obtaining that energy, and the challenge in then getting rid of it safely to land on Mars. Your answer to this part of the question should discuss GPE (perhaps include the formula), and discuss the methods by which we can obtain and shed this energy as required. It will likely require a discussion of orbits, as well as the rotational and orbital motion of the earth (think - How do we use the earths rotation to make it easier to gain GPE?)

The second part involving high speed electrically charged particles, you are discussing insulate shielding, much the same as you would discuss in a re-entry into the atmosphere question

The second question is, basically, asking you to explain the Physics principles involved in the described manoeuvre. Essentially what happened is that the spacecraft was crippled while in orbit around the moon. The spacecraft maintained that orbit until a suitable time, where the engines were used to escape that orbit and enter an orbit around the earth (not so much an orbit, but a trajectory, as it had them gradually descend to the surface). Your job in answering the question is to explain how Newton's Laws play a role in that scenario. Newton's law of Universal Gravitation will need mentioning, since that is what governs the nature of orbits, and the 2nd Law (\(F=ma\)) is related to the acceleration of the rocket during the engine burn

I hope that helps

Thank you so much!!

[kiwiberry]

Hey, when back emf cancels out supply emf in a motor, why does it move at a constant speed if there's no net force?

[RuiAce]

Hey, when back emf cancels out supply emf in a motor, why does it move at a constant speed if there's no net force?

Think about what no net force means.

By Newton's second law of motion, we know that \(\Sigma F=ma\). Since all of our objects never have 0 mass, if net force = 0 then acceleration = 0.

If something is not accelerating, then its velocity does not change. It either remains at rest, or remains at the same velocity it was always at..


Worth mentioning: In fact, Newton's first law of motion (law of inertia) also states that an object tends to stay in its current state of motion (whatever velocity it was in) unless acted upon by an external force.

[jamonwindeyer]

Hey, when back emf cancels out supply emf in a motor, why does it move at a constant speed if there's no net force?

Just to add to Rui's answer - This is for ideal motors - Meaning no friction! If there was friction, it would slow down and gradually stop due to frictional forces

[RuiAce]

Just to add to Rui's answer - This is for ideal motors - Meaning no friction! If there was friction, it would slow down and gradually stop due to frictional forces

Hey Jamon, just gonna digress for a bit. Imagine if someone successfully made the frictionless surface

[jamonwindeyer]

Hey Jamon, just gonna digress for a bit. Imagine if someone successfully made the frictionless surface

No one could stop them

[kiwiberry]

Think about what no net force means.

By Newton's second law of motion, we know that \(\Sigma F=ma\). Since all of our objects never have 0 mass, if net force = 0 then acceleration = 0.

If something is not accelerating, then its velocity does not change. It either remains at rest, or remains at the same velocity it was always at..


Worth mentioning: In fact, Newton's first law of motion (law of inertia) also states that an object tends to stay in its current state of motion (whatever velocity it was in) unless acted upon by an external force.

Just to add to Rui's answer - This is for ideal motors - Meaning no friction! If there was friction, it would slow down and gradually stop due to frictional forces

Ohhh I see! Thanks guys

[strawberriesarekewl]

I know this sounds like a very silly question but at school our teachers for the sciences are making us do past HSC questions in class and I know for a fact most HSC students leave HSC papers till the HSC and do past trial papers in order to prep for trials. Is it a good idea to do a couple of HSC questions during non-HSC periods? I have this feeling that the teachers will take some past trial questions for our school exams...

[jamonwindeyer]

I know this sounds like a very silly question but at school our teachers for the sciences are making us do past HSC questions in class and I know for a fact most HSC students leave HSC papers till the HSC and do past trial papers in order to prep for trials. Is it a good idea to do a couple of HSC questions during non-HSC periods? I have this feeling that the teachers will take some past trial questions for our school exams...

Hey strawberries! Welcome to the forums

So I see where you are coming from, but doing HSC questions now can only be a good thing. Practice is always a good thing. There are heaps of HSC exams to practice with now, so those combined with Trials means you won't have a shortage come September. It doesn't really matter what exams you do in the lead up to the HSC - Obviously HSC exams are a little better but everything is going to be almost equally useful

Plus, doing HSC questions now, you won't remember the questions in September - You can always redo them

[strawberriesarekewl]

Hey strawberries! Welcome to the forums

So I see where you are coming from, but doing HSC questions now can only be a good thing. Practice is always a good thing. There are heaps of HSC exams to practice with now, so those combined with Trials means you won't have a shortage come September. It doesn't really matter what exams you do in the lead up to the HSC - Obviously HSC exams are a little better but everything is going to be almost equally useful

Plus, doing HSC questions now, you won't remember the questions in September - You can always redo them

Thanks for the solid advice

A quite "cliche" question but why is GPE negative? I know it has to do with the reference point being infinity or something and that when an object is being brought closer to the earth it has "negative work done" but it still kind of confuses me how GPE is negative