HSC Physics Question Thread

[jamonwindeyer]

As a result of being hit from behind by a toy truck, a 500g toycar, initially at rest, rolls 12.0m across a floor that applies a constant retarding force of 1.2N to it. The car stops 2.0s after being hit. If the truck was in contact with the car for 0.12s calculate the impulse given to the car.

Hey! So this isn't the typical momentum question, because we have retarding forces involved. Therefore we can't apply the conservation of momentum (we don't have enough info anyway), so let's try something else:

We can calculate the work done on the toy car by the floor due to the retarding force:



This must have been the initial kinetic energy of the toy car, which was then taken away by friction. So, we can use that to find the initial velocity.



Meaning the initial momentum of the car was:



This is actually our answer! The impulse given to the car is equal to the change in momentum, and since the car started from rest, the change in momentum is its initial momentum!

There are definitely more ways to tackle this, but this is the way I'd choose

[brontem]

Can someone really simply sum up the components of an AC motor in as little words as possible (brief enough to fit on a flashcard ) Thank you 

[RuiAce]

Can someone really simply sum up the components of an AC motor in as little words as possible (brief enough to fit on a flashcard ) Thank you 

Components? It's the exact same as for a DC motor with one exception

No split-ring commutator - there's a slip ring commutator.


Or did you mean the AC induction motor?

[brontem]

Or did you mean the AC induction motor?

woops yes I meant induction motor

[RuiAce]

From all I remember:

The stator for the AC induction motor is a solenoid that produces a rotating magnetic field. The currents are indeed AC currents to ensure this happens.

The rotor looks like a series of bars that form what we call a "squirrel cage". In principle, due to the rotating magnetic field it is always subject to changing magnetic flux. The fact that it's rotational will cause, as a result of Lenz's law, the squirrel cage to rotate in the same direction the magnetic field is applied.



(Will let anyone else fill in more if they have something to say.)

[Spencerr]

From all I remember:

The stator for the AC induction motor is a solenoid that produces a rotating magnetic field. The currents are indeed AC currents to ensure this happens.

The rotor looks like a series of bars that form what we call a "squirrel cage". In principle, due to the rotating magnetic field it is always subject to changing magnetic flux. The fact that it's rotational will cause, as a result of Lenz's law, the squirrel cage to rotate in the same direction the magnetic field is applied.



(Will let anyone else fill in more if they have something to say.)

The stator consists of coil windings that are fed with an AC current to produce a rotating magnetic field. The rotor is a squirrel cage made from copper bars attached to a ring on each end. Faraday's law induces an emf in the copper bars and a current (as it is a conductor). The current flows in a direction to oppose the cause of motion i.e. the rotating B-field. In order to do so, the rotor rotates in the same direction, "chasing" the B-field. The speed of the B-field is always greater than the speed of the rotor and this difference is known as the "slip speed", if the speed were the same there would be no change in flux. The rotor is attached to a load to produce a torque and do work. There is no physical contact between the stator and the rotor so the induction motor is much more compact and reliant. However, it is low power.

[conic curve]

Can someone here please help me with this:

http://imgur.com/a/S4S24
http://imgur.com/a/pSih9

Thanks

[jakesilove]

Can someone here please help me with this:

http://imgur.com/a/S4S24
http://imgur.com/a/pSih9

Thanks

For the first question, you need to calculate the angle of incidence (ie. angle between the entrance-beam of light into the water and the central line). You can do this using Trig, knowing the two sides of that triangle. Then, you use Snell's law to calculate the angle of refraction (knowing the refractive index of air and water). Finally, having the angle in the bottom triangle, you can use trigonometry to figure out the bottom line of the Triangle. I won't do the maths here, because I'm sure you can do it; have a go and show me your working if you need further support!

[levendibigd]

A projectile was launched from the ground. It had a range of 70 metres and was in the air for 3.5 seconds.
At what angle to the horizontal was it launched?

[jamonwindeyer]

A projectile was launched from the ground. It had a range of 70 metres and was in the air for 3.5 seconds.
At what angle to the horizontal was it launched?

Hey there, welcome to the forums!! A bit of a mean question this one, they don't give much away 

Let's consider the horizontal component of velocity first. We know the range and time of flight, so it is actually a straightforward equation:



For vertical, we need to find the velocity required for the vertical displacement to be zero at t=3.5:



Now we form a right angled triangle with these values, and the angle we want is just the inverse tangent of the vertical and horizontal velocities:



This last bit might be a bit hard to picture, but remember that we can break a launch velocity into its vertical and horizontal components! This forms a right angled triangle, with the vertical side corresponding to vertical velocity, and the base to horizontal velocity let me know it this makes sense! 

[conic curve]

Someone please help me with these 2 Q's, greatly appreciated

Thanks

http://imgur.com/a/QVIou
http://imgur.com/a/uZb55

[RuiAce]

Hey Guys

Been stuck on this question for quite some time now. I've tried GPE and KE formulas and i seem to be either getting A or C, but the answer is B.

Cheers Guys.

P.S. I am absolutely convinced that the answer is D. May I please see the source of where you got this question from.

[jamonwindeyer]

P.S. I am absolutely convinced that the answer is D. May I please see the source of where you got this question from.

With 4 people (Jake, Spencerr, you and myself)  I think that is virtually a definite these sorts of errors are really annoying, imagine all the students who kick themselves over the question thinking they can't get it right? 

[RuiAce]

With 4 people (Jake, Spencerr, you and myself)  I think that is virtually a definite these sorts of errors are really annoying, imagine all the students who kick themselves over the question thinking they can't get it right? 

I just got word that it came from the 2013 Exam Choice M/C and the correct answer WAS in fact D.

[jamgoesbam]

Could someone please explain the answer to these multiple choice questions? Would be much appreciated! Thanks
(Answers in order of attachments: A, D, C)