# Physics: Post your doubts here!

#### a_wiserME!!

Ans is D, Because:
^R/R = ^I/I + ^V/V----- ^= Delta

R=V/I
R= 15/2=7.5

SO,
^R/R=^I/I+^V/V
^R=(^I/I+^V/V)R
^R=(0.2/2 + 0.5/15)*7.5 = 2
It's D.

The uncertainty in R can find by first finding the fractional uncertainties of I and V, and then adding them and multiplying by the the value of resistance calculated.

( 0.2/2 + 0.5/15 ) * 7.5

= -+1 ohm

thank you!

#### Rizwan Javed

View attachment 58723
can someone tell me why the ans isnt B???! :'(
In this question we're told that the velocity is positive upwards. Also, we know that at highest & lowest points the mass will come momentarily to rest. So when mass is at lowest point, the velocity will be zero, and afterwards, when it'll again start moving upwards, it's velocity will be positive. this elimintes the chance of B being the answer as after coming to rest, the velocity is negative showing that the ball is moving downwards now.

Hope you understood.

#### HumptyR

In this question we're told that the velocity is positive upwards. Also, we know that at highest & lowest points the mass will come momentarily to rest. So when mass is at lowest point, the velocity will be zero, and afterwards, when it'll again start moving upwards, it's velocity will be positive. this elimintes the chance of B being the answer as after coming to rest, the velocity is negative showing that the ball is moving downwards now.

Hope you understood.
So is C the ans?

#### Rizwan Javed

So is C the ans?
No. As I explained above, it'll be mometarily at rest at lowest point, so it's velocity will be zero there. So it can't be C.

It's D.

#### a_wiserME!!

In this question we're told that the velocity is positive upwards. Also, we know that at highest & lowest points the mass will come momentarily to rest. So when mass is at lowest point, the velocity will be zero, and afterwards, when it'll again start moving upwards, it's velocity will be positive. this elimintes the chance of B being the answer as after coming to rest, the velocity is negative showing that the ball is moving downwards now.

Hope you understood.

yup! got it

#### a_wiserME!!

and why is this B not A?? when it changes direction isnt it supposed to be subtracted?

#### HumptyR

View attachment 58724

and why is this B not A?? when it changes direction isnt it supposed to be subtracted?
Let m√2gh1 = V1 and -m√2gh2= V2
So change in momentum will be,

V1-V2
which is equal to

m√2gh1-(-m√2gh2)
so,

m√2gh1+m√2gh2
is the ans.
Hope u got that

Last edited:

#### Rizwan Javed

View attachment 58724

and why is this B not A?? when it changes direction isnt it supposed to be subtracted?
Sorry for getting late.

Look, if you take the downward direction to be negative, and the upward direction to be positive, then the momentum before collision will be : -m√(2gh1)
and the collision after collision will be m√(2gh2)

so then the change in momentum can be found by:

final momentum - initial momentum

= m√(2gh2) - (-m√(2gh1))
= m√(2gh2) + m√(2gh1))

P.S. You can also take the upward direction to be -ive and downward direction to be +ive, but this will result in the change in momentum to be :

- m√(2gh2) - m√(2gh1)) <--- this will be also correct, but this not an option given here.

#### a_wiserME!!

Let m√2gh1 = V1 and -m√2gh2= V2
So change in momentum will be,

V1-V2
which is equal to

m√2gh1-(-m√2gh2)
so,

m√2gh1+m√2gh2
is the ans.
Hope u got that
Sorry for getting late.

Look, if you take the downward direction to be negative, and the upward direction to be positive, then the momentum before collision will be : -m√(2gh1)
and the collision after collision will be m√(2gh2)

so then the change in momentum can be found by:

final momentum - initial momentum

= m√(2gh2) - (-m√(2gh1))
= m√(2gh2) + m√(2gh1))

P.S. You can also take the upward direction to be -ive and downward direction to be +ive, but this will result in the change in momentum to be :

- m√(2gh2) - m√(2gh1)) <--- this will be also correct, but this not an option given here.

oh yeah...i got it!! thanx a lot

#### HumptyR

2 A climber is supported by a rope on a vertical wall, as shown in Fig. 2.1
.
The weight W of the climber is 520 N. The rope, of negligible weight, is attached to the climber and to a fi xed point P where it makes an angle of 18° to the vertical. The reaction force R acts at right angles to the wall. The climber is in equilibrium. (a) Complete Fig. 2.2 by drawing a labelled vector triangle to represent the forces acting on the climber.

Q4

Q3

Q4 (speed)

#### Wolfgangs

Can anybody explain this?

#### Attachments

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#### The Sarcastic Retard

Anyways, this question was solved by myself.

#### shahzaib9291

Please guide me some good A Levels teachers in Karachi

#### zeejay

AOA everyone,
Actually I'm in a really confused state, my problem is related to M1 mechanics and physics....Alright, so we all know that when a simple object like a block of wood moves in a forward direction there is kinetic friction between the block and the floor in the opposite direction.... But things get tricky when it comes to an individual wheel, my mind is actually boggled about *HOW A WHEEL MOVES FORWARD?*. Several pages on Google state that the axle produces two tangential forces on the top and bottom of the wheel and this produces a torque about the axle. Then they state that the friction is opposite to the rotation of the wheel and so it acts in the forward direction causing the wheel to roll forward. But how can friction (forward) ever be greater than the backward force on the wheel produced by the axle. Some articles even mention static friction and rolling friction acting opposite to the direction of motion and all these contradicting statements on the web are confusing me further. Please help me with my query with a detailed and sensible explanation in terms of whatever I stated above. It would be really benign of you. Thanks in advance!

#### Catherine_1

Hey,

In this graph of X-ray spectrum, how is it possible for the photon to have maximum energy and yet zero relative intensity?:/
Since Intensity=Power/Area==>Energy/Area*Time?

Thanks.

#### Salonee B

u = 1.66 x 10^-27
Mass of hellium is 4u = 4 x 1.66 x 10^-27
Like 100km = 100000m.
thank you so, so much!!