Physics 9702 Doubts | Help Page 13
Question 70:
[Matter > Young modulus]
Steel wire and brass wire are joined
end to end and are hung vertically with steel wire attached to point on the
ceiling. Steel wire is twice as long as brass wire and has half the diameter.
Large mass is hung from end of brass
wire so that both wires are stretched elastically.
Young modulus for steel is 2.0 × 1011Pa
and for brass is 1.0 × 1011Pa.
What is the ratio of extension of
steel to extension of brass?
A 2 B
4 C 8 D 16
Reference: Past Exam Paper – November 2012 Paper 11 Q25
Solution 70:
Answer: B.
Young modulus, E =
stress / strain
Stress = Force, F /
Area, A
Strain = extension, e
/ original length, L
E = (F / A) / (e / L) = FL / Ae
Extension, e = FL / AE
Let length of brass wire
be L and let diameter of brass wire = d
Length of steel wire = 2L
and diameter of steel wire = d/2
The weight, F is the same
for both. Area is proportional to d2.
For steel, extension, es ∝
(2L) / (d2/4) Es = 8L / (2.0x1011 d2)
For brass, extension, eb ∝ L /
d2 Eb = L / (1.0x011 d2)
Ratio = es / eb
= [8 / 2.0x1011] / [1 / 1.0x1011] = 4
Question 71: [Forces
> Moments]
Spindle is attached at one end to
centre of lever of length 1.20 m and at its other end to centre of disc of
radius 0.20 m. String is wrapped round disc, passes over a pulley and is
attached to 900 N weight.
What is the minimum force F, applied
to each end of lever, that could lift the weight?
A 75 N B 150 N C
300 N D 950 N
Reference: Past Exam Paper – June 2009 Paper 1 Q13 & June 2014 Paper 11 Q13
Solution 71:
Answer: B.
Distance of Force F from the centre
of the spindle = 1.20 / 2 = 0.60m.
The forces F cause a
clockwise moment.
The 900N weight acts at a
distance 0.20m (equal to the radius) from the spindle, causing an anticlockwise
moment.
So for minimum force, the clockwise
moment should be equal to the anticlockwise moment.
F (0.60) + F (0.60) = 900 (0.20).
1.20F = 180N
Minimum Force F
= 180 / 1.20 = 150N
Question 72: [Forces
> Resultant force]
Tractor of mass 1000 kg is connected
by tow-bar to trailer of mass 1000 kg. Total resistance to motion has constant
value of 4000 N. One quarter of this resistance acts on trailer.
When tractor and trailer are moving
along horizontal ground at constant speed of 6 m s–1, what is the
force exerted on tractor by the tow-bar?
A 0 N B 1000 N C
3000 N D 4000 N
Reference: Past Exam Paper – June 2014 Paper 12 Q10
Solution 72:
Answer: B.
The tractor {where
the driver is} and trailer {found at the back}
are moving. {The question involves equilibrium of an
object moving with constant velocity}
Resultant force on the trailer is
zero during motion {since the speed is constant,
resultant acceleration, and hence resultant force is zero}, so the (forward) force exerted by tractor on trailer is
(= 4000/4 =) 1000 N. (since one quarter of the resistance acts on the trailer)
This force is equal and opposite
to the force exerted on tractor by the tow-bar.
Question 73: [Energy
> Centre of mass > Potential energy]
Uniform solid cuboid of concrete of
dimensions 0.50 m × 1.20 m × 0.40 m and weight 4000 N rests on flat surface
with the 1.20 m edge vertical as shown in diagram 1.
What is the minimum energy required
to roll cuboid through 90° to position shown in diagram 2 with the 0.50 m edge
vertical?
A 200 J B 400 J C
1400 J D 2600 J
Reference: Past Exam Paper – June 2014 Paper 13 Q14
Solution 73:
Go to
43/m/j/13 no. 4(c) pls
ReplyDeleteExplained as solution 237 at
Deletehttp://physics-ref.blogspot.com/2014/12/physics-9702-doubts-help-page-38.html
salam! in q73, in the diagram on the right-hand side, the cuboid should not technically fall as the point of contact is in line(blue-dotted line) with the centre of mass, so why does it fall?
ReplyDeleteWslm. Well, it may be in some kind of equilibrium, but that equilibrium is not stable. In reality, you would expect it to fall, right?
Delete+ the question asks for the minimum energy required to the cube to roll as shown. This minimum energy is the energy required to raise the centre of gravity as shown. Afterwards, it may fall to the right (that is, roll) or regain its original position.
there are 3 types of equilibrium: stable, unstable and neutral. the cuboid should be in stable equilibrium in position 2 as the centre of gravity is in line with point of contact of cuboid and surface. so in that position(position2), the cuboid wont fall
ReplyDeleteconsider it to be as in diagram 1 first - its vertical. A force is required to move it to the position we are discussing.
DeleteBy applying the force, the cuboid is moving. From its motion, it reaches the position being discussed.
But if you want to consider more laws of physics, the cuboid has been made to be in a state of motion. From Newton's 1st law, the cuboid would tend to continue to be in a state of motion unless a force is applied to stop it. But no force is being applied. So, after reaching the position I drawn, the cuboid would tend to continue to move and it will fall. That's its inertia.
Well, it's actually a simple case. Try to do it on your own in reality and you'll observe the cuboid would fall
yeah u r ryt! this is definitely an everyday example and i really wanted to know the physics behind it. JazakAllah!
Deleteis there a website like this for chemistry???
ReplyDeleteSorry, I don't know
DeleteThe tension in a spring of natural length lo is first increased from zero to T1, causing the length to increase to l1. The tension is then reduced to T2, causing the length to decrease to l2.
ReplyDeleteWhich area of the graph represents the work done by the spring during this reduction in length?
May/June 2002, Question#24.
check q488 at
Deletehttp://physics-ref.blogspot.com/2015/03/physics-9702-doubts-help-page-94.html
can you please do question 10 b i and ii from w10qp43 ?
ReplyDeleteSee solution 603 at
Deletehttp://physics-ref.blogspot.com/2015/04/physics-9702-doubts-help-page-119.html
Thanks
ReplyDeleteAdmin
9702/13/m/j/11 Question 14
ReplyDeleteGo to
Deletehttp://physics-ref.blogspot.com/2015/02/9702-june-2013-paper-11-worked.html
Hi, for question 71, why isn't the two forces F considered as a couple? Why are they not treated with the torque formula of force * distance?
ReplyDeleteThanks
It can be done this way. The magnitude of only one force is considered but the distance is taken as 1.20m.
DeleteThe answer would be the same.
How Does 900N Weight Produces ACW moment ?
DeleteSorry Got it .
DeleteHey sorry I don't get why 900N weight produces ACW moment. Could someone pls explain to me? Tqvm
DeleteThe 900 N force appears as the tension in the string.
DeleteConsider the spindle as the pivot. The 900 N in the string causes an anticlockwise moment on the disc since the pivot is the spindle. the disc moves in an anticlockwise direction
Salam,Can you explain me how did you get 4 in Q72
ReplyDeleteWslm.
DeleteThe question says "One quarter of this resistance acts on trailer."
june07. q7
ReplyDeletewhich paper?
Deletecould you please direct me to the answers for june 2015 paper 11,12,and 13 if they're available
ReplyDeletethanks!
you need to search for them. some of them are already solved, but not all.
DeleteTo do this, go to Google search and type '9702 physics reference' followed by the fist lines of the question.
9702_s14_qp_11 question no. 11. Thanks in advance
ReplyDeleteGo to
Deletehttp://physics-ref.blogspot.com/2014/11/9702-june-2014-paper-11-worked.html
can you explain banking part in circular motion
ReplyDeletecan u reply this?
DeleteI don't get what you mean by 'banking'.
Deletecan u explain this:
ReplyDeleteA bucket of water can be swung in a vertical circle so that the water remains in the bucket even though it is upside down at the top of the circle?
can u please answer my two qs that I send before
ReplyDeleteGo to
Deletehttp://physics-ref.blogspot.com/2015/12/circular-motion-in-vertical-plane.html
thanks
Deleteplease help me with q5b 23/mj/2013
ReplyDeleteSee solution 691 at
Deletehttp://physics-ref.blogspot.com/2015/05/physics-9702-doubts-help-page-140.html
for solution 73 what if the question was for maximum energy ? hen we will find the difference in the potential energy of the two positions given the question itself?
ReplyDeleteit would be irrelevant though, as any value above this minimum would cause it to roll
DeleteCan u plz q13 from oct/nov 2009
ReplyDeletewhich variant? try to see if it's available at
Deletehttp://physics-ref.blogspot.com/2014/05/physics-9702-notes-worked-solutions-for.html
Q13 speciman paper 2008 ownwards
ReplyDeletei do not have specimen 2008 for a level
DeleteI get almost all of your solutions.I just don't understand one thing why do you do all of this for us and how do you get all of the tough questions while we need to come here to try and understand.
ReplyDeleteIkr...
DeleteFor Q17, where is the pivot, is it the whole spindle is the pivot?
ReplyDeleteThe (vertical section of the) spindle may taken as the pivot.
Delete