Bazza's 3/4 Physics question thread

[paulsterio]

It's because of direction, a spring will always exert a force in the direction opposite to which it is pulled.

Say I have a spring which looks like this (fixed to the wall on the left)
====

Now I pull it towards the right
========--->|| (where || represents my hand)

Displacement is to the right, but the spring will be pulling my hand to the left


(Aurelian got there before me!)

[WhoTookMyUsername]

For singificant figures shoudl we follow the "SF equal to least amount in question"?
Like if a question gives you 6.0 and 15 as values,
And 10.4 is the answer would you express it as 10?

Or is physics more a "appropriate number of decimal places" subject ?

[pi]

For singificant figures shoudl we follow the "SF equal to least amount in question"?
Like if a question gives you 6.0 and 15 as values,
And 10.4 is the answer would you express it as 10?

Or is physics more a "appropriate number of decimal places" subject ?

In physics, they don't really care about sig figs much. As long as it's reasonable, it's fine. It's not nazi like chem is.

[paulsterio]

Used to be Nazi, but not anymore, you have to give enough significant figures for your answer to be seen as correct though, so generally use your instincts and common sense and you should be fine.

[lokdog]

A drowsy driver in a small car (mass 800 kg) travelling east at 45 km/h drifts (sadly) into the path of an oncoming truck (12000 kg) travelling west at 90 km/h.  Assuming the small car remains intact after the collision, what speed will the combined car truck mass be travelling at?  Give a velocity and a direction (west or east).

Is the collision in the previous question elastic or inelastic?  You must show working to support your answer

[Mr. Study]

Convert 45 to m/s by dividing by 3.6.

Therefore 12.5 ms-1.

Convert 90 to m/s = 25

Now, Let east be positive and west be negative.

We can find the initial momentum by: (800 x 12.5) + (12000 x -25) = -290000
Now due to the conservation of momentum law, the final momentum will equal the initial momentum and since we know the car and truck have a combined mass, we can add the two masses to form:
Momentum Initial = Momentum Final = Mass x V
                                             -290000=12800V

Therefore v = -22.7 ms -1 or 22.7 m/s west.

(If you think about it, it does make sense as the truck has a higher momentum than the small car).

Now to find elastic or inelastic collisions we have to look at the kinetic energy.

We know the initial kinetic energy of the system is
                                                                                    0.5 x M1 x V1^2 + 0.5 x M2 + V2^2
                                                                                = (0.5 x 800 x 12.5^2) + (0.5 x 12000 x 25^2)
                                                                                = 3812500 J

Now the final kinetic energy is 0.5 x 12800 x 22.7^2 = 3297856 J

As we can see, the EK final does not equal EK initial. That would mean some energy was lost as heat/sound/what ever.

Someone check if my workings correct as I am more comfortable doing question by hand and paper instead of typing.

EDIT: Should mention lokdog that you're much better off making your own thread instead of posting in another users.

[WhoTookMyUsername]

Do we have to indicate to seperate N( normal points) for say e.g. A car on a road if it asks us to?

[pi]

Do we have to indicate to seperate N( normal points) for say e.g. A car on a road if it asks us to?

Do you mean for each wheel?

[Mr. Study]

I did an exam where the answer was two normal forces on a car. (The two tyres), But a VCAA exam just had the normal from the centre of mass.

[lokdog]

The “Pirate ship” is a carnival ride in which a large ship shaped carriage swings backwards and forwards in a vertical arc.

The radius of the arc is 10m and the ship can swing from 45˚ to the vertical one side to 45˚ from the vertical on the other side in 3 sec.

 If an occupant has a mass of 75kg what will their apparent weight be (at the bottom of the arc) when the ride is in operation?


Does anyone want my schools motion Sac for this year, I can upload it to mediafire. It was really challenging and would make some great revision for the exam.

[lokdog]

And thank you so much for the help Mr. study

[WhoTookMyUsername]

1) would you put 2 normal forces acting from 2 wheels of a car for example? Or just one
2) i there is a current leak across diode does that mean v across diode doesnt equal emf?
3) what is the difference between emf and v total?
4) is yeild point dependent on cross sec area ? (is it related directly to stress or strain, or both?)
5) does change in kinetic energy = - grav pot energy ( is negative sign appropriate?)
6) is strain energy toughness x new volume ( after extension  ) or old volume?
7 when a photodiode is reverse biased in a circuit, is the "reverse current" from positive to negative terminals of battery?
when something is earthed does any current flow through the earthing?
9 why do we use AC as opposed to DC
10) what is the beat definition of a semi conductor?
- conducts elec under certain cirumstances
Or
- has a conductivity between an insulator and conductor?
Thanks a lot

[paulsterio]

1) would you put 2 normal forces acting from 2 wheels of a car for example? Or just one
2) i there is a current leak across diode does that mean v across diode doesnt equal emf?
3) what is the difference between emf and v total?
4) is yeild point dependent on cross sec area ? (is it related directly to stress or strain, or both?)
5) does change in kinetic energy = - grav pot energy ( is negative sign appropriate?)
6) is strain energy toughness x new volume ( after extension  ) or old volume?
7 when a photodiode is reverse biased in a circuit, is the "reverse current" from positive to negative terminals of battery?
when something is earthed does any current flow through the earthing?
9 why do we use AC as opposed to DC
10) what is the beat definition of a semi conductor?
- conducts elec under certain cirumstances
Or
- has a conductivity between an insulator and conductor?
Thanks a lot

1) In reality, it's 2 normals on each wheel, but in physics I believe it's just one normal from the centre of mass.

2) In reverse bias, then yes, because it means that there is current flow and hence, the diode itself is not using up all the voltage

3) Same thing, as far as I know

4) Didn't do Structures

5) No, energy is scalar quantity, always positive

6) Didn't do Structures

7) No, reverse current is negative to positive

Are you talking about AC/DC, essentially it won't matter, earthing is a safety measure more than anything, it's so that if there is suddenly a large current, it won't pass through you to go to earth but will pass through the earth cable

9) AC can be stepped up with a transformer, high voltages means lower current, since Power Loss = I^2 R, lower current means lower power loss. With DC, no change in direction, hence, no change in magnetic flux, hence, transformers will not work.

10) Both - I think

[pi]

3) Not exactly the same thing, this explains it better than me: http://en.wikipedia.org/wiki/Electromotive_force#Electromotive_force_and_voltage_difference

8 ) Functional earth or protective earth?

9) More of a unit 4 question imo

10) They're both essentially two statements looking at the same thing from two different views

[WhoTookMyUsername]

thanks for the help

(what is the diff bewtween func earth and protective earth?) and how can we tell if current is flowing through it? I thought that since V=IR if R -> oo then I approaches zero?

with 5) i know energy is positive, but the CHANGE in energy can be negative (can it :S?). The negative there is to indicate when one increases the other decreases
with 7) i thought the current is the other way around ? http://mbed.org/media/uploads/harrisjunaid/photo_diode_amp_circuit.png
like the current is reverse as opposed to the diode, but is thus normal compared to the battery?

and pen or pencil in the exam? (is pencil allowed?)