Physics: Post your doubts here!

[Silent Hunter]

http://www.xtremepapers.com/papers/...and AS Level/Physics (9702)/9702_s07_qp_2.pdf

in question 2 (c)(i) why arent we using equations of motion?

 

[Brains]

WHEN WE ROUND OFF AN ANSWER TO AN APPROPRIATE NO OF SIGNIFICANT FIGURES, DO WE USE THE ROUNDED VALUE OR THE EXACT VALUE IN THE NEXT PART???

HELP.. PLZ REPLY

 

[xxxtoughxxx]

Plz anyone help me understand how Q. 2c of Oct/Nov 2011(21) is done..

(i) take moments about p,clockwise=anticlockwise,so 2.4x0.45=1.2xtension and 0.45 came from 0.75-0.30 cause da weight is at da center since rod is uniform (ii) sum of upward forces= sum of downwards so tension in A is 2.4-0.9

 

[xxxtoughxxx]

Show that a 100W lamp connected to a mains supply of 240V will have the same current as a 5W car lamp connected to a 12V? ans- current in both is 2.4A

can someone explain =D

actually da current in both will be 0.42A cause p=IV then I=P/V

 

[raamish]

power is the rate of doin work nd work is fxd then p=(fxd)/t and d/t is v then p=fv......and post da whole question 4 da cyclist

ty. and this is the whole question basically. A cyclist is travelling at constant speed so why will his chemical energy not convert to kinetic energy. I know that the energy change will be kinetic to heat and sound energy but why will the chemical energy not be converted to K.e thts the question.

 

[RGBM211]

actually da current in both will be 0.42A cause p=IV then I=P/V

i knw its 0.42 but in the book it says its 2.4

 

[Sandhya Mahat]

WHEN WE ROUND OFF AN ANSWER TO AN APPROPRIATE NO OF SIGNIFICANT FIGURES, DO WE USE THE ROUNDED VALUE OR THE EXACT VALUE IN THE NEXT PART???

HELP.. PLZ REPLY

exact value

 

[A.ELWY 7]

i knw its 0.42 but in the book it says its 2.4

then your book is wrong...100/240 is the same as 5/12 which will give an answer of 0.416

 

[A.ELWY 7]

ty. and this is the whole question basically. A cyclist is travelling at constant speed so why will his chemical energy not convert to kinetic energy. I know that the energy change will be kinetic to heat and sound energy but why will the chemical energy not be converted to K.e thts the question.

ok but do u have the answer or ur asking cause u NEED an answer?

 

[Silent Hunter]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Physics (9702)/9702_s07_qp_2.pdf

in question 2 (c)(i) why arent we using equations of motion?

anybody?

 

[Silent Hunter]

ok then
for the first row the power dissipated is obviously 0 because the circuit is incomplete so no electricty will flow
2nd row current will always go through the route with less resistance so power is 1.5kW as current "avoids" B
3rd current will only flow through A + C so total is 3.0kW
4th p= V^2/r since the resistors are in series voltage is shared equally since they have same electrical requirements so V is halfed while R remains constant so new voltage is (0.5v)^2 which is 0.25v^2 so P= 2*(0.25*240^2)/38.4 =750
5th 0.75kW + 1.5kW =2.25kW

hope it helped : D

can you please explain the highlighted part?

 

[Unicorn]

well an antinode is 1/4 of a wavelengh right? so given that l=45cm the antinode is 45 cm and wavelengh is 0.45x4=1.8 m aand u calculate the frequency from v=fxwavelengh

isn't the wavelength 60cm as calculated before? :S

 

[Unicorn]

How is P=Fv derived?
A question from pastpaper:Explain why a cyclist travelling at a constant speed is not making this trnasformation. Explain what transformations are taking place?

p= E/t e= work done W=f x s

p= (f x s)/t v=s/t

therefore p= fv

 

[Unicorn]

can you please explain the highlighted part?

ok from the diagram you can see that A and B are in series right? adn the switch S2 is closed and in row 4 they ask you for total power across them which is 0.75W

and C is in parallel with A and B so 0.75 + 1.5

I am treating A and B as 1 resisitor

 

[Unicorn]

Plz anyone help me understand how Q. 2c of Oct/Nov 2011(21) is done..

use one of the ropes as a turning point
in c(i) use A as the pivot and calculate momentum
in c(ii) use B as pivot

 

[Unicorn]

Show that a 100W lamp connected to a mains supply of 240V will have the same current as a 5W car lamp connected to a 12V? ans- current in both is 2.4A

can someone explain =D

P=IV
100/240 =5/12 A

5/12 A

 

[Unicorn]

anybody?

because there is no equation of motion which applies to what you have

you don't have v or t and all equations require either v or t or both

 

[lavanyamane]

http://www.xtremepapers.com/papers/...nd AS Level/Physics (9702)/9702_s10_qp_21.pdf
Q.5 (c)(i) , please?

 

[Silent Hunter]

because there is no equation of motion which applies to what you have

you don't have v or t and all equations require either v or t or both

okay thanks...
can these equations be applied in vacuum? places without air resistance and gravity?

 

[surajsingh143]

plz help me in question with explanation

may june p2 2D 2007 and may june p2 2008 6D