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ALL CIE PHYSICS DOUBTS HERE !!!

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Deadly_king:

--- Quote from: ruby92 on October 10, 2010, 04:38:23 pm ---Physics A2 oct/nov 2008 paper 4 q 1aiii
why is it that the normal reaction force=force of gravity - centerpital force.
shouldnt it be plus?

Also m/j 2007 paper 4 q1 bi

--- End quote ---

Nov 08 p4 No 1

a)(iii) The normal reaction exerted by a planet on a mass is equal to the weight of the mass.

Normally for an object undergoing circular motion,
Resultant towards centre = Centripetal force

Resultant force towards centre of the planet will be : Gravitational force - Weight

Hence
Gravitational force - Weight = Centripetal force ---> Weight = GMm/R2 - mR2

Since Weight = Normal reaction = GMm/R2 - mR2

Jun 07 p4 No 1

b)(i) Gravitational potential energy is given by the formula = -GMm/R

In the question you are asked to find the change in gravitational potential.
Let ^Ep = Change in gravitational potential energy
Uf = Final gravitational potential energy = -GMm/3R
Ui = Initial gravitational potential energy = -GMm/2R

Hence ^Ep = Uf - Ui
         ^Ep = (-GMm/3R) - (-GMm/2R)

Simplify this equation and you'll get your required solution which is ^Ep = GMm/6R

ashish:

--- Quote from: Deadly_king on October 11, 2010, 05:15:34 am ---Hmm......Ashish......your spring constants are ok. But am afraid the extensions are not  :-[

Anyway dude, you should know that for spring constant,
When springs are in parallel, their net spring constants are added together. ( Same as for a capacitor and opposite to that of resistors)

When they are in series they are calculated as shown KR = [1/k + 1/k]-1 or 1/KR = 1/k + 1/k

For extension,
When in series, net extension is the sum of the extension of each spring.
When in parallel, it is calculated as shown eR = [1/e + 1/e]-1 or 1/eR = 1/e + 1/e

Let's back to the question now :
1. Total extension will be the sum of extension by each springs when they are connected in series. Each spring extends by e
Hence total extension will be 2e
Spring constant will be [1/k + 1/k]-1 = k/2

2. Since the two springs are connected in parallel, total extension will be e/2 and net spring constant will be 2k
eR = [1/e + 1/e]-1 = e/2
KR = k + k = 2k

3. Total extension : 3e/2
Net spring constant : 2k/3
Use the values you obtained in part(ii)
Total extension = e/2 + e = 3e/2
For the two springs in parallel, net spring constant will be 2k. Hence spring constant of the whole system will be [1/2k + 1/k]-1 = 2k/3

NOTE : kR and eR are the resultant spring constants and resultant extensions respectively.

--- End quote ---

the answer i gave were in terms of k an L :(

Deadly_king:

--- Quote from: ashish on October 11, 2010, 11:26:10 am ---the answer i gave were in terms of k an L :(

--- End quote ---

Yeah.......the question asked in terms of e ;)

ashish:
lol i i gave him these as i did a similar question in class in which it was asked in terms of k and l ::)

WARRIOR:
a simple test if anyone has the time to do it , good for basics practise

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