Physics Marathon 2012 (3 Viewers)

[jenslekman]

heres a q from 2005 hsc

n 1970 NASA launched Apollo 13, their third mission planned to land humans on the Moon. Half-way to the Moon a hige 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.

 

[Parvee]

correctamundooooo

Pretty good questions, it just involves a lot of rearranging.

 

[barbernator]

heres a q from 2005 hsc

n 1970 NASA launched Apollo 13, their third mission planned to land humans on the Moon. Half-way to the Moon a hige 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.

As the spacecraft was crippled, the astronauts would attempt to return to earth with minimum fuel used. By using the slingshot effect around the moon, theorised by Isaac Newton, the rocket would be able to change its direction without any net loss of speed (insert diagram with rocket going around the moon with same speed but opposite direction at same point). This will results in minimal fuel being used to turn around 180 degrees back towards the earth. If the slingshot effect was not used, excess fuel would be used to slow the rocket and then accelerate in the opposite direction. To get the rocket out of the moons gravitational field, rockets will have to be fired when the rocket is returning to earth, yet the fuel required for this is much less than if the slingshot effect was not used.

 

[jenslekman]

As the spacecraft was crippled, the astronauts would attempt to return to earth with minimum fuel used. By using the slingshot effect around the moon, theorised by Isaac Newton, the rocket would be able to change its direction without any net loss of speed (insert diagram with rocket going around the moon with same speed but opposite direction at same point). This will results in minimal fuel being used to turn around 180 degrees back towards the earth. If the slingshot effect was not used, excess fuel would be used to slow the rocket and then accelerate in the opposite direction. To get the rocket out of the moons gravitational field, rockets will have to be fired when the rocket is returning to earth, yet the fuel required for this is much less than if the slingshot effect was not used.

im pretty sure moon's gravitational field wont be able to provide the slingshot effect that it needs because its not strong enough

 

[barbernator]

im pretty sure moon's gravitational field wont be able to provide the slingshot effect that it needs because its not strong enough

the slingshot effect occurs irrespective of mass. As space is a vacuum, momentum is conserved in a system and, by taking the moons tangential velocity as negligible (it could even provide a boost if the rocket went behind the moon) the overall momentum will remain in the system. Hence the speed will remain constant at the same distance from the moon, yet direction will be changed.

 

[Parvee]

New Question!
A loop of wire, carrying an electric current, is placed into a magnetic field. Describe the forces experienced by the loop and explain the result of these forces in terms of a DC motor. (5 marks)

 

[someth1ng]

New Question!
A loop of wire, carrying an electric current, is placed into a magnetic field. Describe the forces experienced by the loop and explain the result of these forces in terms of a DC motor. (5 marks)

A current carrying conductor in a magnetic field will experience a force perpendicular to the magnetic field. Only the current carrying wires that have a non-zero component that is perpendicular to the magnetic flux will experience a force.

(Insert Image showing the force experienced by a current carrying conductor in a magnetic field)

The force experienced by opposite sided wires with a non-zero component perpendicular to the magnetic flux in the coil will be constant assuming magnetic flux does not vary and current is constant however torque is constantly changing.

(Insert Image and label the direction of the force and use T=Fdsintheta to show that torque varies depending on the position of the coil)

The net force on the coil is always zero because on one either side on the coil, the direction of the force experienced is equal and opposite in direction. When these two forces are added together, they result in 0 but torque is only zero when the plane of the coil is perpendicular to the magnetic flux.

Sum of Forces=(nBIl)+(-nBIl)=0

The negative denotes opposite direction.

 

[someth1ng]

Here's a few questions from my school's 2011 trial:

Question 1: A magnet hovering above a superconductor, has become superconducting. Identify and justify one reason why the magnet may cease hovering.

Question 2: Explain the concept of a gravitational field.

Question 3: Justify factors that are understood to affect the strength of a gravitational field.

Question 4: For two appliances used in the home, recommend with justification the use of different types of electrical motors.

Question 5: A rocket of mass 7500 kg is launched from the Earth‟s surface into a uniform circular orbit of radius 7.5 × 106 m. Calculate the magnitude of the gravitational potential energy when the rocket is in this orbit.

 

[bleakarcher]

Here's a few questions from my school's 2011 trial:

Question 1: A magnet hovering above a superconductor, has become superconducting. Identify and justify one reason why the magnet may cease hovering.

Question 2: Explain the concept of a gravitational field.

Question 3: Justify factors that are understood to affect the strength of a gravitational field.

Question 4: For two appliances used in the home, recommend with justification the use of different types of electrical motors.

Question 5: A rocket of mass 7500 kg is launched from the Earth‟s surface into a uniform circular orbit of radius 7.5 × 106 m. Calculate the magnitude of the gravitational potential energy when the rocket is in this orbit.

Feel like taking part even though I'm not in year 12, also would like an analysis of my answers

Q2) All objects of mass (matter) in the universe possess a gravitational field would cause other objects of mass to be attracted towards it. By Newton's Universal Law of Gravitation, the force exerted upon two objects of uniformly distributed mass is given by the formula: F=Gm1m2/r^2 where r is the distance between the centres of mass, G=6.67*10^(-11) Nm^2/(kg)^2 (although this is simply a constant and has no physical significance) is the universal constant of gravitation and m1, m2 are the masses of the two bodies. It is clear from this equation that as r->infinity the magnitude of the gravitational force between the masses decreases meaning that the gravitational force is negligible for large values of r. It is also clear that objects of small mass have extremely little gravitational effect upon one another since G is very small. Whereas for large objects in space, the gravitational field of one body towards the other become much more obvious.

Q3) g=Gm/r^2 so the factors which influence the strength of an object gravitational field are it's mass alongi with density. (sorry don't really know much more)

Q5) lGPEl=Gm1m2/r=6.67*10^(-11)*6.97*10^(24)*7500/(7.5*106), not sure whether to add the radius of the Earth or not

 

[SpiralFlex]

Here's a few questions from my school's 2011 trial:

Question 1: A magnet hovering above a superconductor, has become superconducting. Identify and justify one reason why the magnet may cease hovering.

Question 2: Explain the concept of a gravitational field.

Question 3: Justify factors that are understood to affect the strength of a gravitational field.

Question 4: For two appliances used in the home, recommend with justification the use of different types of electrical motors.

Question 5: A rocket of mass 7500 kg is launched from the Earth‟s surface into a uniform circular orbit of radius 7.5 × 106 m. Calculate the magnitude of the gravitational potential energy when the rocket is in this orbit.

Can I has? I will give you any trial paper from any school you want. I have Ruse 2012 assess tasks.

 

[someth1ng]

Can I has? I will give you any trial paper from any school you want. I have Ruse 2012 assess tasks.

What 2012 papers do you have? :O

 

[SpiralFlex]

What 2012 papers do you have? :O

I'll be able to get nearly all school's ones after trials. But hopefully Tywebb will upload his collection. Anyway you guys get back to Physics. =)

Post some dodgy questions please!

 

[jenslekman]

the slingshot effect occurs irrespective of mass. As space is a vacuum, momentum is conserved in a system and, by taking the moons tangential velocity as negligible (it could even provide a boost if the rocket went behind the moon) the overall momentum will remain in the system. Hence the speed will remain constant at the same distance from the moon, yet direction will be changed.

oh sorry my bad - it even says go around the back of the moon in the question. but i was thinking that they would risk being slung out into open space and never return =P - but technically newton didnt theorise it :O - correct me if im wrong

 

[jenslekman]

okay new question

4 synchronised atomic clocks were put on two commercial planes (one going east one going west and each with 2 clocks) in 1970s to demonstrate time dilation. why do they put 2 clocks on the plane instead of one?

 

[barbernator]

okay new question

4 synchronised atomic clocks were put on two commercial planes (one going east one going west and each with 2 clocks) in 1970s to demonstrate time dilation. why do they put 2 clocks on the plane instead of one?

to increase reliability plain and simple

 

[jenslekman]

to increase reliability plain and simple

YUP.

 

[nightweaver066]

Bump

 

[someth1ng]

Bump

I sorta...can't be fucked? But this is what I'd say in the question.

Torque acting on the motor coil as it begins to rotate:

[Net EMF=Supplied EMF-Back EMF]

- Initially, there is no back emf as the coil is not rotating and hence, there is no change of flux for an emf to be induced.
--> Hence, the initial net emf is equal to the supplied emf through the coil which is large.
- As the coil speeds up, back emf increases resulting in a change of flux, inducing a back emf.
--> Therefore, the net emf through the coil is smaller than what it was initially.
- As the coil reaches maximum speed, it rotates at such a speed that back emf equals to supplied emf.
--> This means that net emf through the coil is zero and hence, torque is zero.

Also, as the coil rotates, the force supplied on the coils is also proportional to the cosine of the angle between the plane of the coil is the direction of magnetic flux.
.'. As the coil reaches maximum speed, torque on the coil is sinusoidal.

Contrast conductors and semiconductors in terms of their band structure and the electrical resistance. (2 marks)

 

[Kimyia]

Is this ok?
Conductors are materials were the valence electrons are delocalised and free to move into the conduction band. Hence, the valence band and conduction band are overlapped and so have little electrical resistance.
Semiconductors on the otherhand have most valence electrons used in bonding and thus few electrons are free to move. This means that unlike conductors, there is an energy gap between the valence and conduction bands that must be overcome for a semiconductor to be able to conduct electricity. This means that they have more electrical resistance than conductors.

Next one: the electrical supply network uses AC and a variety of transformers between the generating stations and the final consumer. Explain why transformers are used at various points in the network [4 marks]

 

[nirukk]

oh sorry my bad - it even says go around the back of the moon in the question. but i was thinking that they would risk being slung out into open space and never return =P - but technically newton didnt theorise it :O - correct me if im wrong

I was going to answer as to the slingshot effect, but the HSC marker's comments says: In weaker responses, candidates incorrectly applied the ëslingshot effectí to the motion of the rocket around the moon.