2009 Physics Unit 3 Exam Suggested Solutions

[anonuser0511]

haha i was sitting there in an exam making elevator noises to get that one

i know i lost one on the LED, and perhaps one on the explanation of increase - since itute doesn't have it how did you guys prove it?

[TrueTears]

Using voltage divider formula: (Say the temperature was , I think this corresponded to 5000 ohms of resistance for thermistor, can't remember exactly)

So R must increase if you still want to get 8V.

[kurrymuncher]

arrrrrgh crap I only put two OFFs for the LED question.

[anonuser0511]

yea i put down voltage formula and subbed in random values

[danieltennis]

There were, but if people put down 2 points on one (rather than 4), then that's automatically a 88/90.. and so on.

2 points for what...?

For weightlessness question, 4 points for 4 marks.

From memory:
- apparent weightless is experienced by an object when in free fall and moving with an acceleration equal to the gravitional field strength at their location
- there is no force between the object and its surroundings
- Hence, N=0
- Gravitational weight force will remain the same since W = mg. Mass and g stays the same

I wrote another thing, but i can't really remember.

[anonuser0511]

arrrrrgh crap I only put two OFFs for the LED question.

yea same. if only we lived in some sort of string theory universe where LEDs in forward bias can light up without a complete circuit...

[anonuser0511]

There were, but if people put down 2 points on one (rather than 4), then that's automatically a 88/90.. and so on.

2 points for what...?

For weightlessness question, 4 points for 4 marks.

From memory:
- apparent weightless is experienced by an object when in free fall and moving with an acceleration equal to the gravitional field strength at their location
- there is no force between the object and its surroundings
- Hence, N=0
- Gravitational weight force will remain the same since W = mg. Mass and g stays the same

I wrote another thing, but i can't really remember.

i went with
-not experiencing true weightlessness as
w=mg
g>0
W>0

then
Fnet = mg - N
ma = mg - N
N = mg - ma
in freefall (a=g)
N = m(0)
   = 0
then i had some random example of here pulling something out of her pocket it'd appear to be floating

[IntoTheNewWorld]

Reading answers is so depressing =x

[dejan91]

arrrrrgh crap I only put two OFFs for the LED question.

Yeah screwed me up too. * Facepalms* But after you think about it, it MUST turn off as there isn't a complete circuit in that part of the parallel circuit. Pretty much the equivalent of cutting the wire at the reverse-biased LED.

[simplicity123]

how many marks do u think i'll lose if i put 720N for that apparent weight question. Im pretty sure my method was all right except that i misinterpreted the 2ms^-2 decelerating and used -2 instead, thus resulting in 720.

And with the explanation of the apparent weightlessness, i also gave an example of a body falling downwards, saying that the only forces acting on it are gravitational force and air resistance with 0 contact force. Isit ok if i said air resistance? Or does apparent weightlessness only mean gravity acting on the body only?

[dejan91]

Apparent weightlessness is when the acceleration of a body is that of gravity. i.e. in free fall. Air resistance is a resistive force, so having it will mean you're NOT falling at g ms^-2.

[TrueTears]

how many marks do u think i'll lose if i put 720N for that apparent weight question. Im pretty sure my method was all right except that i misinterpreted the 2ms^-2 decelerating and used -2 instead, thus resulting in 720.

And with the explanation of the apparent weightlessness, i also gave an example of a body falling downwards, saying that the only forces acting on it are gravitational force and air resistance with 0 contact force. Isit ok if i said air resistance? Or does apparent weightlessness only mean gravity acting on the body only?

It is meant to be -2?

[simplicity123]

i assigned negative as upward force and weight as downward. You think they'll just take a mark off for that?

[TrueTears]

question was worth 3 marks.

2 for working out.

1 for answer in the box.

[Mao]

Apologies for the delay [had to go to VicRoads... =S ]. First post has been updated. Paper + solutions attached, Special Relativity solutions will be coming later tonight