HSC Physics Question Thread

[Neutron]

All good, no dramas at all no reason to apologise! I'll throw to Jake and/or diiiii for that question 

Okay In the meantime, do you think you could help me with this Space calculation question? :O

A special vehicle bound for Mars has a weight of 2.45x10^3 N at rest on the ground on Earth. It is loaded aboard a space craft and launched. The initial acceleration of the craft as it lifts fro the ground is 28.4 ms^-2 . What is the force applied to the vehicle by its mountings at the time?

The answer is 9.55 x 10^3 N and I've tried all the methods I know and could not get it :O

Sorry I have a lot of questions, my trials in like a few days D: pls shoot me

Neutron

[RuiAce]

Okay In the meantime, do you think you could help me with this Space calculation question? :O

A special vehicle bound for Mars has a weight of 2.45x10^3 N at rest on the ground on Earth. It is loaded aboard a space craft and launched. The initial acceleration of the craft as it lifts fro the ground is 28.4 ms^-2 . What is the force applied to the vehicle by its mountings at the time?

The answer is 9.55 x 10^3 N and I've tried all the methods I know and could not get it :O

Sorry I have a lot of questions, my trials in like a few days D: pls shoot me

Neutron

[conic curve]

Hey conic! Yep, that's exactly it, Newton's 1st Law of Motion (Inertia)  in terms of structuring an answer around it, you shouldn't need an extended response, just explain what happens, why it happens (inertia), and how it is potentially dangerous (sharp objects, blunt trauma, etc)  physics responses are fairly free-form! Don't think about it too much, just put down what you know 

Thanks Jamon

Another question I find difficulty in answering is this: A man wishes to drill some holes in the paneling of his car, but for the sake of safety, the holes must be well away from any electrical wiring. Describe the procedure he could use to detect the whereabouts of electrical wiring in the cavity, using only a small compass

Like is this even a physics question? lol

[jamonwindeyer]

Thanks Jamon

Another question I find difficulty in answering is this: A man wishes to drill some holes in the paneling of his car, but for the sake of safety, the holes must be well away from any electrical wiring. Describe the procedure he could use to detect the whereabouts of electrical wiring in the cavity, using only a small compass

Like is this even a physics question? lol

It definitely is! I'll leave answering the question to you, but the principle lies within the fact that currents have magnetic fields around them. This is caused by the moving electric charges. To explain why it happens is some really complex Physics arising out of special relativity and electrodynamics, so not necessary to go into any more detail than "because it does."

The magnetic field lines form circles around a current, the direction being given by the right hand rule. Give yourself a thumbs up with your right hand. If your thumb is the direction of the current, the magnetic field has a direction indicated by your fingers, the field lines wrap around the current. This isn't strictly necessary for the question, though it might come in handy.

So the electrical wires produce magnetic fields! The compass will always align itself with any magnetic field! Therefore, I'll leave you to come up with a method that uses the compass to find out where the wires are 

[conic curve]

It definitely is! I'll leave answering the question to you, but the principle lies within the fact that currents have magnetic fields around them. This is caused by the moving electric charges. To explain why it happens is some really complex Physics arising out of special relativity and electrodynamics, so not necessary to go into any more detail than "because it does."

The magnetic field lines form circles around a current, the direction being given by the right hand rule. Give yourself a thumbs up with your right hand. If your thumb is the direction of the current, the magnetic field has a direction indicated by your fingers, the field lines wrap around the current. This isn't strictly necessary for the question, though it might come in handy.

So the electrical wires produce magnetic fields! The compass will always align itself with any magnetic field! Therefore, I'll leave you to come up with a method that uses the compass to find out where the wires are 

By the way this is from the module electrical energy in the home

Do we have to talk about safety though and ways to reduce the chances of getting electrocuted?

Also do we have to talk about (in this question) what happens when we get electrocuted?

[jamonwindeyer]

By the way this is from the module electrical energy in the home

Do we have to talk about safety though and ways to reduce the chances of getting electrocuted?

Also do we have to talk about (in this question) what happens when we get electrocuted?

Nope, the question only wants a description of the procedure itself, nothing else is necessary here 

[conic curve]

I don't understand this question, mainly because I'm not given much information:

A stone is dropped from a 220m cliff. Calculate the:

a)time taken to reach the bottom (2 marks)
b) Velocity just before impact (2 marks)

I know that we have 220m as part of our information and we need to let one side be positive but what other information am I missing and how would I execute the question with all the information I have?

Thanks

[jamonwindeyer]

I don't understand this question, mainly because I'm not given much information:

A stone is dropped from a 220m cliff. Calculate the:

a)time taken to reach the bottom (2 marks)
b) Velocity just before impact (2 marks)

I know that we have 220m as part of our information and we need to let one side be positive but what other information am I missing and how would I execute the question with all the information I have?

Thanks

Howdy, so this is a pretty standard projectile question for Year 12, we'll use our kinematic formulae 

For the first one, we use the vertical displacement formula (this formula can be applied on any axis for which there is a constant acceleration). If we consider the initial height as zero, we want to find how long it takes the stone to reach -220 metres (remember acceleration is -9.8, and the initial vertical velocity is zero at the moment it is dropped)



For the second one, we have a time of flight until collision with the ground. To find the speed at this point, we have a simple formula to help:



That is, 67.13 metres per second towards the ground. Make sure you give a direction as is required for a velocity, rather than a speed 

[wyzard]

I don't understand this question, mainly because I'm not given much information:

A stone is dropped from a 220m cliff. Calculate the:

a)time taken to reach the bottom (2 marks)
b) Velocity just before impact (2 marks)

I know that we have 220m as part of our information and we need to let one side be positive but what other information am I missing and how would I execute the question with all the information I have?

Thanks

A word on the signs of acceleration and velocity. Their signs are determined by where you point the positive direction to be, if you decide upwards is the positive direction then acceleration due to gravity will be downwards. Likewise if you decide the positve direction to be downwards, then acceleration due to gravity will be downwards.

This assigning of positive direction is very often overlooked by students, hence the common confusion of when and where the acceleration due to gravity is positive or negative.

The more technical term for doing so is assigning a coordinate system, something they really drill into uni first year physics students doing mechanics

[conic curve]

A word on the signs of acceleration and velocity. Their signs are determined by where you point the positive direction to be, if you decide upwards is the positive direction then acceleration due to gravity will be downwards. Likewise if you decide the positve direction to be downwards, then acceleration due to gravity will be downwards.

This assigning of positive direction is very often overlooked by students, hence the common confusion of when and where the acceleration due to gravity is positive or negative.

The more technical term for doing so is assigning a coordinate system, something they really drill into uni first year physics students doing mechanics

Um...it said calculate it...

Need help with the following

1. A 200kg space probe fires its rocket motor while in deep space, exerting a force always perpendicular to its velocity. This causes it to accelerate at 2.6 ms^-2. The firing of the motor lasts 8.0 seconds

a)Determine the force exerted by the space probe's rocket motor (2 marks)

Is this asking for net force or reaction force or what?

b) Calculate the increase in the space probe's momentum (2 marks)

 (the formula isn't clicking into me right now)

c) Determine the acceleration of the vehicle during the first 8.0s of it's motion (1 mark)

Do we use F=ma for this?

24. A street outside a school has its speed limit reduced from 70 kmh^-1 to 40 kmh^-1. Describe, quantitatively and qualitatively, how the stopping distance of a vehicle would differ between these 2 speeds (3 marks)

This question requires us to describe the situation in terms of value and by visual observation but how am I supposed to do that?

[jamonwindeyer]

Um...it said calculate it...

wyzard is explaining conceptually the way the positives/negatives are assigned, it's a very important thing to understand 

1. A 200kg space probe fires its rocket motor while in deep space, exerting a force always perpendicular to its velocity. This causes it to accelerate at 2.6 ms^-2. The firing of the motor lasts 8.0 seconds
a)Determine the force exerted by the space probe's rocket motor (2 marks)
Is this asking for net force or reaction force or what?

This (if I interpret the question correctly) is simply asking for an application of Newton's 2nd Law, F=ma. You have the mass of the probe and its acceleration due to the motor, you can use that to find the force!

b) Calculate the increase in the space probe's momentum (2 marks)
 (the formula isn't clicking into me right now)

The formula you want here is that the change in momentum (also called the impulse) is linked to the force applied, and how long it is applied for, in the following way:



You have all the values you need here since you found force in the previous question.

c) Determine the acceleration of the vehicle during the first 8.0s of it's motion (1 mark)

Isn't that in the question?I'd love for someone else to have a look at this question, I think I've misinterpreted it somewhere

24. A street outside a school has its speed limit reduced from 70 kmh^-1 to 40 kmh^-1. Describe, quantitatively and qualitatively, how the stopping distance of a vehicle would differ between these 2 speeds (3 marks)

What you are looking for here is the idea of human reaction times. Say I take 1 second to respond to a hazard on the road. That means that my vehicle travels 1 second before I ever apply brake force. Then, when I do apply brake force, decelerating to a stop takes time as well, potentially another whole second. 2 seconds at 70km/h, for example, therefore equates to roughly another 20 metres of distance.

Qualitatively, you'd simply be describing how reducing speed reduces the distance taken to stop, since the distance travelled while the driver reacts is less, and the brake force will stop the vehicle faster. Quantitatively, you'd be doing rough calculations with the formula:



To find the distance travelled at some speed given some reaction time  for 3 marks, you'd want a couple of dot points of qualitative description plus the use of a formula (such as above) to estimate the difference in stopping time. It doesn't have to be exact 

[wyzard]

Um...it said calculate it...

Assigning the sign is part of the calculation, which you'll be using for the equations kinematic later. jamonwindeyer covered the calculation bit really well, was explaining why sometimes acceleration is positive, and why sometimes is negative

[Neutron]

Heyy another calculations question, for some reason I can never get these

A projectile of mass 1150 kg is fired vertically from the surface of an asteroid of mass 1.1x10^20 kg and radius 2.6x10^6m with a speed of 50 ms^-1. Determine the maximum distance the projectile reaches from the centre of the asteroid.

Thank you guys <3

[jakesilove]

Heyy another calculations question, for some reason I can never get these

A projectile of mass 1150 kg is fired vertically from the surface of an asteroid of mass 1.1x10^20 kg and radius 2.6x10^6m with a speed of 50 ms^-1. Determine the maximum distance the projectile reaches from the centre of the asteroid.

Thank you guys <3

Hey! So, first we have to calculate the value of acceleration due to gravity on this asteroid. The relevant formula is



So, subbing our information in, you get



This seems pretty low; maybe I did the calculation wrong on that? You'll have to check me on that.

Now, we can use the formula



Where the initial velocity is 50m/s^2, the final velocity is zero (as it has to stop), and the acceleration is as described above.




Therefore it will reach a total of 2.6*10^6+1151000m=3751012m

Again, I think I must have calculated acceleration wrong, definitely check me on that one

[Neutron]

:O (just a bunch of maths errors hahaha)

The solutions found the initial kinetic and gravitational potential energy of the projectile, and then they let that value be equal to -6.67x10^-11 x 1150 x1.1 x 10^20 all over r (think this is like the gravitational potential energy formula but I have no idea why)