Cooper's Physics Thread

[alchemy]

Hi
Physics Unit 2 is almost over for the year, and I've started revising for the Unit 2 exams. So far, so good; but I'm having doubts on how to do some of the checkpoints questions. Therefore I'll try to outline my method when asking a question so that people can point out where I went wrong. The topics in this thread will be predominantly Motion, Electricity, Light and Waves. Answer only as many questions as you want to, or have time for.

Thanks for your help.

The following information refers to Question 1-3.
A car battery is used in a remote house to power 2 lights and the radio. The circuit is connected in parallel. The lights both have a power rating of 30W, when running off a 12V battery. The battery can be assumed to have zero internal resistance.
1) What is the total power drain from the battery when the radio and both lights are switched on?
2) What is the current drain from the battery when the radio and both lights are switched on?
3) What the resistance of the whole circuit?

The following information refers to Question 4.
The two lights and the radio are connected in parallel. It would be possible to connect them in series.
4) What are at least two good reasons not to do this?

The following information refers to Question 5-7.
A battery is rated at 12V. When a voltmeter with a very high resistance is used to check it's voltage, however, the voltmeter reads 13.2V. Then the battery is used as a "back up" to run some emergency lights and an air compressor in the storage. Now the battery is supplying 20A, and the voltmeter reads 12V.
5) What is the EMF of the battery.
6) What is the internal resistance of the battery?
7) Explain why the readings of the voltmeter are different.

My answers:
1)

2)







3) =


4) When connected in series the current would be less across each component, limiting them from functioning to their maximum output.

5)





6) calculated above.

7) The voltmeter shows the reading across the battery terminals. The voltmeter has a very high resistance as mentioned, in particular a resistance higher than the battery's. Therefore it returns a higher reading of voltage.

[SocialRhubarb]

There seems to be no information given on the radio or the battery, but from what I gather from your working, the radio is functioning at a power of 30W, and the battery is providing a voltage of 12V, in which case all your working and answers are correct.

In terms of the series v parallel argument:
1. If all the devices are connected in series, turning off one device with a switch would turn off ALL devices in series, as you've broken the circuit with a switch.
2. If all the devices are connected in series, the highest power goes to the device with the lowest power rating, as and all devices receive the same current in series, so the devices with the highest resistances and hence lower power ratings will receive the greatest power, which would lead to, as you've said, most devices not functioning at their maximum output.

I think you've made a slight error in your calculation of the EMF of the battery.
You've stated that:

These statements are true, but the problem is that the 'R' in your equations actually apply to different resistances. In the first equation the 'R' in your equation refers to the internal resistance of the battery, however the R in your second equation actually refers to the external resistance, as the battery is outputting 12V at a current of 20 A, and this is not across the internal circuit of a battery.

The EMF of a battery is actually just 13.2V, as the voltmeter has a very high resistance, and would take all the available voltage provided by the battery, so the EMF of the battery must be 13.2V.

[alchemy]

John is completely invisible under water.

Questions:

1) What must John's refractive index be? Explain your answer.
2) Is it possible for John to see underwater? Explain your answer.

My answers:
1)




2) No, because he will be viewing the surface above him at a critical angle.

[alchemy]

John is completely invisible under water.

Questions:

1) What must John's refractive index be? Explain your answer.
2) Is it possible for John to see underwater? Explain your answer.

My answers:
1)




2) No, because he will be viewing the surface above him at a critical angle.

EDIT (I just answered my own question):
1) John's refractive index must be the same as the water (1.33) for anyone from the surface above (air) to see him. There is no need for any calculation here.
2) No, John will not be able to see anything underwater because light will not be able to pass to his eyes, since he has the same refractive index as water.

This question made me to think creatively, which is good in Physics, but it sure took a while.

[alchemy]

1. A car travelling with a constant speed of 80 km h–1
passes a stationary motorcycle policeman. The policeman
sets off in pursuit, accelerating uniformly to
80 km h–1 in 10.0 s and reaching a constant speed of
100 km h–1 after a further 5.0 s. At what time will the
policeman catch up with the car?


2. Vinh is investigating the bouncing ability of a golf
ball and a tomato. He drops both objects from a
height of 2.00 m and measures the rebound heights.
He found that the golf ball rebounded to 1.50 m and
the tomato just splattered without rebounding at all.
Calculate the speed of the golf ball as it rebounded.

[RKTR]

1. 80t = 0.5(80)(10)+0.5(80+100)(5)+100(t-15)
    t=32.5s


2. Mgh = 1/2 mv^2
     Gh=0.5v^2
      10(1.5)/0.5 =v^2
       v=root 30=5.48m/s

[alchemy]

1. 80t = 0.5(80)(10)+0.5(80+100)(5)+100(t-15)
    t = 32.5s

Can you please explain, in words, what you did here? Thanks.

[Stevensmay]

Can you please explain, in words, what you did here? Thanks.

So we want to find when the distances they have travelled will be equal. Seeing as speed*time = distance, we can set up an equality that allows us to solve for t.

Distance travelled by car at time t will be 80*t.

Distance travelled by the policeman is slightly more complex, as their speed is not constant.
We use our equations for constant acceleration in this scenario, specifically


Initial acceleration distance will be

Second stage of acceleration is

Finally we are at a constant speed, but 15 seconds has already elapsed, so we need to account for this when we go to calculate the distance travelled at this speed. Thus

So we add all these together to find the total distance the police car will have travelled at time t.


And seeing as we want the time when the distances of the two cars are equal, we set up the equality

[alchemy]

Haven't been doing Physics for ages but just thought I'd revise on things. The question is attached.

[clueless123]

The force that causes the masses to move is the weight force of the 8kg mass; which = 8g.
Total mass of the system = 8+5 = 13kg.
Acceleration for both masses is the same.
F = ma
8g = 13 * a
a = (8*9.8 )/13
~6m/s^-2

[Nato]

Haven't been doing Physics for ages but just thought I'd revise on things. The question is attached.

i would just look at the blocks individually, and labelling their respective forces.
so for block 1 there is the rightwards tension force (there is no leftwards friction, as states in the question).
so by performing newton's second law on block 1, we get T=5a.

now for the second block, the forces are acting on the vertical axis. so, there is the downwards force of gravity 'mg' and the upwards tension force.
so now let's perform the 2nd law on this object. Well, the net force would be mg -T=8a. In case you're wondering, the mg comes first, as it is moving downwards (mainly due to it having a larger mass that 5kg, duhhh LOL), meaning that 'mg' is greater than the tension force.
Now, with the forces resolved in the vertical axis of block 2: mg-T=8 , we can sub in some values making this: 8(9.-T=8a
which is 78.4-8a = T

now we basically have two equations with two unknowns, namely 'T' and 'a'. Now in this whole situation as they are both connected by the same rope, tension AND acceleration of the both block are the same, so we can just make the two equation equal to each other.

So now, we're left with 78.4-8a=5a

and i got  6ms^-2


i seriously hope this is correct ...LOL

[alchemy]

The force that causes the masses to move is the weight force of the 8kg mass; which = 8g.
Total mass of the system = 8+5 = 13kg.
Acceleration for both masses is the same.
F = ma
8g = 13 * a
a = (8*9.8 )/13
~6m/s^-2
i think..

I got that answer too, but I think it has to be exact and not approximate. It's kind of a trick question, I guess, because it's not asking to find the acceleration for the system which most questions generally ask for. This question was on the Physics Olympiad paper, btw.

[lzxnl]

The acceleration of block 2 IS the acceleration of the system; by treating the system with net force 8g and mass 13 kg, you are considering both blocks as one system.