VCE Physics Question Thread!

[knightrider]

Another failing of VCE physics. Sigh.

Work is actually defined as a vector dot product of the force vector and displacement vectors (this will make more sense if you do spesh and I didn't check to see if you did). Essentially what it means is that the work done is dependent on the angle between the force and the displacement. For your question, if you push on an object for 500 m to the left with a constant force F, then apply the exact same constant force over 500 m to the right afterwards, you WILL have done no net work on the object. This means the object will be moving at the same speed as it was in the beginning.


Force * distance is quite dangerous to use.

Thanks so much lzxnl 

so we must use The work formula W=force*displacement ?

[Kel9901]

Thanks so much lzxnl 

so we must use The work formula W=force*displacement ?

force*change in displacement*cos (angle between force and direction of motion)

[Floatzel98]

Hey guys. I'm going over the checkpoints books now for motion because that was the AOS I did the worst in so far this year. And i just have a few questions I need help with.

In a laboratory class at school, Lee is given a spring with a stiffness of 20 N/m and an unstretched length of 0.40 m. He hangs it vertically, and attaches a mass of 0.40kg to it. The new length is 0.60 m. Lee pulls the mass down a further distance of 0.10 m. By how much has the potential energy stored in the spring changed?

I thought I would just go , but the answer says to go and I'm a bit confused why they do that. Why are they using the total length of the spring at those points for? Shouldn't they be using the stretched length?

Also with the attached I just can't seem to do question 103 properly. I don't even know if I'm doing it correctly. Are we supposed to use the graph or something?


 

[silverpixeli]

Hey guys. I'm going over the checkpoints books now for motion because that was the AOS I did the worst in so far this year. And i just have a few questions I need help with.

In a laboratory class at school, Lee is given a spring with a stiffness of 20 N/m and an unstretched length of 0.40 m. He hangs it vertically, and attaches a mass of 0.40kg to it. The new length is 0.60 m. Lee pulls the mass down a further distance of 0.10 m. By how much has the potential energy stored in the spring changed?

I thought I would just go , but the answer says to go and I'm a bit confused why they do that. Why are they using the total length of the spring at those points for? Shouldn't they be using the stretched length?

Also with the attached I just can't seem to do question 103 properly. I don't even know if I'm doing it correctly. Are we supposed to use the graph or something?

You're right of course they should be using Δx rather than total length, must be an error

I would do 103 by solving for x in: 6000 = 1/2 k x^2 with k the gradient of the graph, in the right units, from 102

Alternatively I you can find 'at what x the area under the F-x graph is 6000J' and you may realise that this is the exact same calculation as the first method

[Floatzel98]

If a question is asking us to find magnitude of an impulse and we give the units in kg m/s instead of Ns, would we be incorrect? I know on the VCAA exams they usually have a box for the answer with the units already there, but if we stated different units in our workings, would that be incorrect?

Another question as well. How does the impulse that the airbag exerts on the driver's head compare with the impulse that the drivers head exerts on the airbag? The Impulse is 56 Ns from the previous part of the question, but the answer for this question says they will be exactly the same. Should it not be equal and opposite? Impulse is a vector quantity isn't it?

[odeaa]

If a question is asking us to find magnitude of an impulse and we give the units in kg m/s instead of Ns, would we be incorrect? I know on the VCAA exams they usually have a box for the answer with the units already there, but if we stated different units in our workings, would that be incorrect?

You should get a mark, its not an incorrect unit for momentum at all. No way they can ping you if they didnt specify it, and on the exam it won't be an issue anyway

Another question as well. How does the impulse that the airbag exerts on the driver's head compare with the impulse that the drivers head exerts on the airbag? The Impulse is 56 Ns from the previous part of the question, but the answer for this question says they will be exactly the same. Should it not be equal and opposite? Impulse is a vector quantity isn't it?

I think that's just a badly worded question, it should say 'how does the magnitude compare' if they want that answer
You're correct, they are opposite and equal because and in the collision, the forces will be in opposite directions and so will the impulse

[Orson]

A beam of blue light, Hz, is incident normally on a perfect reflecting surface. The beam power is .

a) Calculate the number of photons that are incident on the reflecting surface each second.
b) What is the momentum of each photon in the beam?

I got the correct answer to part a) #ph = photons per second.

I am struggling with the second part. De Broglie?

[Floatzel98]

A beam of blue light, hz, is incident normally on a perfect reflecting surface. The beam power is .

a) Calculate the number of photons that are incident on the reflecting surface each second.
b) What is the momentum of each photon in the beam?

I got the correct answer to part a) #ph = photons per second.

I am struggling with the second part. De Broglie?

Well . Should be as easy as that. and is a constant.

[odeaa]

A comet is in orbit around the sun. The mass of the comet is 3.7x10^22 kg, and the radius of the comets orbit around the sun is 7.2x10^9m. Calculate the period of the comets orbit.

Is this even possible? All we have is the radius and mass of the planet, which is useless. Don't you need the mass of the sun (not given in data book afaik) to solve it? Note: its a 5 mark question, which is more than you would get for a substitution usually, so I'm a bit thrown off. Thanks

[lzxnl]

A comet is in orbit around the sun. The mass of the comet is 3.7x10^22 kg, and the radius of the comets orbit around the sun is 7.2x10^9m. Calculate the period of the comets orbit.

Is this even possible? All we have is the radius and mass of the planet, which is useless. Don't you need the mass of the sun (not given in data book afaik) to solve it? Note: its a 5 mark question, which is more than you would get for a substitution usually, so I'm a bit thrown off. Thanks

You can look up the sun's mass in your own data book for physics. For reference, it's about 1.99 x 10^30 kg (that number will change over time for reasons you don't need to worry about). Then, you have enough information.

Alternatively, you can compare the comet's orbit to Earth's orbit, which has a mean radius of 150 million km and a period of 365.26 days (I know, you're not expected to know the former).

[knightrider]

How would you do this question attached ?

[odeaa]

For a loop in a field, it has to be a complete loop for there to a flux doesnt it? Textbook explanation is a bit iffy

[lzxnl]

For a loop in a field, it has to be a complete loop for there to a flux doesnt it? Textbook explanation is a bit iffy

Erm...no. A magnetic flux is defined by essentially how much of a magnetic field goes through an area. This area does not have to physically exist as an object; it can be air, for all you know. You can have a voltage generated with a changing magnetic flux with an incomplete loop; you just won't get a current as the resistance of the air between the loop ends is huge.

knightrider, your question doesn't make a whole lot of sense as you can't really disturb a spring like that; you can't disturb part of a spring only

[odeaa]

Erm...no. A magnetic flux is defined by essentially how much of a magnetic field goes through an area. This area does not have to physically exist as an object; it can be air, for all you know. You can have a voltage generated with a changing magnetic flux with an incomplete loop; you just won't get a current as the resistance of the air between the loop ends is huge.

knightrider, your question doesn't make a whole lot of sense as you can't really disturb a spring like that; you can't disturb part of a spring only

Ah I see, I was a bit confused there. Thanks for clearing that up

[Orson]

Hey guys, I've been struggling with this style of question. I'm now good with these on horizontal planes, but I'm still struggling with on inclined planes.

Question [NEAP 2012 U3]:
A 4WD of mass 1800kg pulls a boat on a trailer at a constant speed up a boat ramp with an incline of 18 degrees. The combined mass of the boat and trailer is 700kg.

A frictional force of 300N acts between the boat trailer and the ramp, and a frictional force of 850N acts between the 4WD and the ramp.

a) Calculate the force exerted by the boat ramp on the 4WD (I'm good with this one). 1.7*10^4 N
b) Calculate the magnitude of the tension in the coupling joining the trailer and the 4WD. 2.5*10^3 N
c) Calculate the power developed by the 4WD if it moves the boat 15 meters up the ramp in 8 seconds. 1.7*10^4 W

Thanks heaps!