Physics: Post your doubts here!

[leadingguy]

hey can anyone please explain how to calculate change in momentum for these values: Mass=0.045 kg, Initial velocity=4.2 m/s Final velocity=-3.6 .The negative sign is creating a confusion. the formula is given : mass*(initial+final) in the mark scheme.it's Nov/2002 P2 Qs3.thank you.

momentum means the sum of mass intio velocity p = mv
change in momentum means the difference in mv before and after colision or after the particular action.

so finding the change in momentum frst consider the starting momentum/initial momentum

which is mv = (0.045)(4.2) = 0.189kgms-1.....taking velocity here positive considering the direction.
now when particle collides or rebounds back the direction will be changed, as velocity is vector quantity.
therefore it is represented wid a negative sign.

take out the momentum after collision mv = (0.045)(-3.6) = -o.162kgms-1
now U are having two momentums jxt take out the differnce

(mv) - (mv).... (0.189) - (-0.162)
delta p = change in momentum = o.351 ANS

 

[rz123]

http://www.xtremepapers.com/CIE/International A And AS Level/9702 - Physics/9702_w09_qp_11.pdf
please explain these questions ---> 10, 15, 22. Thank you very much.

as the previous questions are answered by Usman a.k.a Mani (thanks to him) so u r left with 22 which is Mr Young Modulus = Stress over Straaain! , u need to know the strain on your shoulders, lets get over with the stress first. which will be Force divided by Pi d^2 / 4. i.e 20N / (pi 5* 10^-4)^2 / 4 which will give u 10.2 into 10^-7 Pa. now strain is equal to Stress over Y.M divide it nd u will get 5.1 into 10^ -4% which is A. But the freaking marking scheme says its B. :|

 

[rz123]

one more question paper, please.
http://www.xtremepapers.com/CIE/International A And AS Level/9702 - Physics/9702_w08_qp_1.pdf
questions 27, 34, 36.
Thank you very much

ok so 27th: speed of g x u v i m r rays: 3 into 10^8 !!! so we need to know lambda i.e wavelength produced which will be twice the distance between the adjacent maximas ( i dnt knw why i have to make it twice but it works or maybe twice the distance between maximas gives u wavelength) anyway so 3 into 10^8 divided by .030 meters is = 1 into 10^10Hz i.e C!

34: its simple contrary to the first impression to this mcq: lets calculate the total current which the battery can supply for 2 second intervals: I = Q/t so 100,000 C / 2 s = 50,000 A! Now simply divide it with 200 A to get the number of times it can give this much current, provided the battery isn't gonna get recharged.

36: ok so we want to know the internal resistance of the battery in order to calculate the current in the circuit. well we are given with the voltage of the battery 9 volts and the terminal voltage 7.5 volts! so the volts used by the internal resistance is 1.5 volts!!!

lets make two equations as we know the current is same: so I = 1.5 / R1 ( eq 1)
(R1 being the internal resistance)

I = 9 / 15 + R1 (eq 2) now solve these two equations simaltaneously u will get R1 to be 3 Ohms!!! hurray!!!

so now just simply calculate the current. I = V / R => 9 / 18 = .50 Amperes which is B!

 

[Anoushay]

momentum means the sum of mass intio velocity p = mv
change in momentum means the difference in mv before and after colision or after the particular action.

so finding the change in momentum frst consider the starting momentum/initial momentum

which is mv = (0.045)(4.2) = 0.189kgms-1.....taking velocity here positive considering the direction.
now when particle collides or rebounds back the direction will be changed, as velocity is vector quantity.
therefore it is represented wid a negative sign.

take out the momentum after collision mv = (0.045)(-3.6) = -o.162kgms-1
now U are having two momentums jxt take out the differnce

(mv) - (mv).... (0.189) - (-0.162)
delta p = change in momentum = o.351 ANS

yes i understand that but don't we take final momentum-initial momentum?

 

[hassam]

how the image is formed in MRI.....I HAVE READ IT MULTIPLE TIMES BT I NEVER SOME TO UNDERSTAND HOW THESE protons cud every make an image.....i cnt get the basic idea.....

 

[Anonymous']

yes i understand that but don't we take final momentum-initial momentum?

hmm.. actually, you can use : final momentum - initial momentum, but the answer will be negative, so you just ignore the minus sign.. that's what my teacher told me.. heheh..

 

[Anonymous']

yes i understand that but don't we take final momentum-initial momentum?

or, you just remember this, if the object is hitting a vertical wall, and then it rebounds, the velocity is changing its direction, therefore, change in momentum will be initial + final momentum, but, if the object just goes straight without hitting anything, and it changes its speed (not with negative sign), you must use final momentum - initial momentum.. heheh..

 

[Anonymous']

yes i understand that but don't we take final momentum-initial momentum?

oh! one more thing, if you use final - initial momentum, the answer will be a negative value, it is true because the the mass is moving in the opposite direction (negative velocity), so as the momentum, as momentum is a vector quantity, therefore it has direction.. so, you just ignore the minus sign.. heheh..

 

[leadingguy]

yes i understand that but don't we take final momentum-initial momentum?

AoA

hmm.. actually, you can use : final momentum - initial momentum, but the answer will be negative, so you just ignore the minus sign.. that's what my teacher told me.. heheh..

as was quoted wisely by anonymus! U can ignore sign! as it jxt shows the direction. We are asked fr change in momentum. MArks will be given for that.

 

[Oliveme]

10) f=ma
60=30a a= deceleration
a= -2ms-2 (negative sign indicates that itx decelaration)


now apply formula fr acc. >>> a=(Vf- Vi)/t

-2 = (Vf - 3)/0.50

Vf = 2ms-1 ans.

15) according to conservation of momentum same will be the momentum at both sides
so at left side is MV= 2 * 2 =4

so will the momentum at right side MV= 1 * 4 = 4
now we know the velocities of both the objects moving!
take out the kinetic energy of both trolleys movingthese velocities
by the formula 1/2mv^2
the K.E of left will be 8j
of right will be 4j
the total stored in the spring will be 8+4 = 12j

Thank you very much. Allah bless you.

 

[Oliveme]

34) q is the charge! it can be found out by Q= IT where I is the current in AMperes and t is the time in sec.

now a car needs 200A till 2 seconds to start means it needs Q=IT 200*2 400C of charge.


now consider car being fully charged means it have 100KC
of charge!
cnvrt kilo coulomb into coulomb
means 100810^3 = 100000C

now divide it by the charge needed per start for the car!


100000/400 = 250 times is the ans!

Thank you.

 

[Anoushay]

AoA




as was quoted wisely by anonymus! U can ignore sign! as it jxt shows the direction. We are asked fr change in momentum. MArks will be given for that.

ok so if we solve it like that and get negative sign answer but ignore it ,our marks won't get deducted.and thank you to both of you.

 

[TheDealer]

http://www.xtremepapers.com/CIE/International A And AS Level/9702 - Physics/9702_w09_qp_11.pdf
Help on Q5 please.

 

[rz123]

http://www.xtremepapers.com/CIE/International A And AS Level/9702 - Physics/9702_w09_qp_11.pdf
Help on Q5 please.

just simply calculate the initial velocity at yellow signal. see what quantities u got here. s = 1000m , decc= .2ms^-2, v will be 0 as the train stops at red singnal so we can apply this formula:-
v^2- u^2 = 2as.
u^2= 2 into .2 into 1000 it comes 400 take under root, u will get 20 m/s . i.e the safe speed to drive ur train

 

[TheDealer]

:OOOOOO Damn that should've been easy. -_-
Thanks man!

 

[TheDealer]

What about Q13 of the same paper?

 

[rz123]

What about Q13 of the same paper?

to calculate the tension: use the torque given for wheel q => T = F into s so 3 = F into radius of Q wheel .05. tension in the BELT is 60 N,
same force will be transmitted to the next pulley : so 60 into the radius of p pulley will give u the torque (radius is the perpendicular distance from the line of action of the force). divide this radius by 1000 to get in meters. it comes 4.5 Nm. option D.

 

[leadingguy]

Thank you.

U welcome

 

[leadingguy]

ok so if we solve it like that and get negative sign answer but ignore it ,our marks won't get deducted.and thank you to both of you.

welcome

 

[TheDealer]

http://www.xtremepapers.com/CIE/International A And AS Level/9702 - Physics/9702_w05_qp_1.pdf
Q16 pleasseee!!!!