Wednesday, May 13, 2015

Physics 9702 Doubts | Help Page 140

  • Physics 9702 Doubts | Help Page 140

Question 691: [Waves > Superposition]
(a) Explain the principle of superposition.

(b) Sound waves travel from source S to a point X along two paths SX and SPX, as shown in Fig.1.

(i) State phase difference between these waves at X for this to be the position of
1. a minimum,
2. a maximum.

(ii) Frequency of the sound from S is 400 Hz and speed of sound is 320 m s–1. Calculate wavelength of the sound waves.

(iii) Distance SP is 3.0 m and distance PX is 4.0 m. Angle SPX is 90°. Suggest whether a maximum or a minimum is detected at point X. Explain your answer.

Reference: Past Exam Paper – June 2013 Paper 23 Q5

Solution 691:
(a) When waves overlap / meet, the resultant displacement is the sum of the individual displacements of the waves.

1. Phase difference for minimum = 180 º / (n + ½) 360 º

2. Phase difference for maximum = 0 / 360 º / (n360 º)

Speed v = f λ
Wavelength λ = 320 / 400 = 0.80 m

{Path difference = 7 – 5 = 2m. Wavelength λ = 0.80m. In terms of λ, a path difference of 2m is equal to 2 × (λ / 0.80) = 2.5 λ.}
Path difference = 7 – 5 = 2 (m) = 2.5 λ
Hence a minimum is detected at point X.
OR maximum if phase change at P is suggested
{If a phase change occurred at point P, then a maximum could be obtained at point X. The condition for destructive interference: 180 º / (n + ½) 360 º is for coherent sources (waves from these sources are in phase)}

Question 692: [Kinematics + Dynamics]
A trolley moves down a slope, as shown in Fig.1.

Slope makes an angle of 25° with the horizontal. A constant resistive force FR acts up the slope on trolley.
At time t = 0, trolley has velocity v = 0.50 m s−1 down the slope.
At time t = 4.0 s, v = 12 m s−1 down the slope.
(i) Show that acceleration of the trolley down the slope is approximately 3 m s−2.
(ii) Calculate distance x moved by the trolley down the slope from time t = 0 to t = 4.0 s.
(iii) On Fig.2, sketch variation with time t of distance x moved by the trolley.

(b) Mass of the trolley is 2.0 kg.
(i) Show that component of the weight of the trolley down the slope is 8.3 N.
(ii) Calculate resistive force FR.

Reference: Past Exam Paper – November 2014 Paper 21 Q4

Solution 692:
Acceleration a = (v – u) / t or (12 – 0.5) / 4
Acceleration a = (12 – 0.5) / 4 = 2.9 (2.875) (= approximately 3 m s–2)

(ii) {Distance travelled = Average speed × time}
Distance x = (u + v) t / 2 = [(12 + 0.5) × 4] / 2 = 25 m

(iii) A line with increasing gradient is drawn with a non-zero gradient at origin.
{The gradient of a distance-time graph gives the velocity.
There is a constant acceleration on the trolley, so its speed increases with time. That is, the distance travelled in a unit time becomes greater with time. This is represented by an increase in gradient of the graph.
Now, initially, the speed of the trolley is 0.5ms-1. So, the gradient cannot be zero at the origin.}

(graph can be drawn better)

(i) Weight down slope = 2 × 9.81 × sin 25° = 8.29 / 8.3 N

(Resultant force F = ma)
8.3 – FR = 2 × 2.9
Resistive force FR = 2.5 N (2.3N if 3 used for a)

Question 693: [Gravitation]
(a) Define gravitational potential at a point.

(b) Gravitational potential φ at distance r from point mass M is given by expression
φ = – GM / r
where G is the gravitational constant.
Explain significance of the negative sign in this expression.

(c) A spherical planet may be assumed to be an isolated point mass with its mass concentrated at its centre. Small mass m is moving near to, and normal to, surface of the planet. Mass moves away from the planet through a short distance h.
State and explain why change in gravitational potential energy ΔEP of the mass is given by the expression
ΔEP = mgh
where g is the acceleration of free fall.

(d) Planet in (c) has mass M and diameter 6.8 × 103 km. The product GM for this planet is 4.3 × 1013 N m2 kg–1.
A rock, initially at rest a long distance from planet, accelerates towards the planet. Assuming that the planet has negligible atmosphere, calculate speed of the rock as it hits the surface of the planet.

Reference: Past Exam Paper – June 2012 Paper 41 & 43 Q1

Solution 693:
(a) The gravitational potential at a point is the work done in bringing unit mass from infinity (to the point).

(b) The gravitational force is (always) attractive.
EITHER as the distance r decreases, the object/mass/body does work
OR work is done by masses as they come together

Force on the mass = mg
(where g is the acceleration of free fall /gravitational field strength)
Gravitational field strength, g = GM / r2
if r is at h {that is, the distance of the mass m is at a distance h from the point mass M – h is a distance close to the surface.}, g is constant
{This is the same as the acceleration of free fall on Earth. Close to the surface of Earth, g is about 9.81ms-2. However, as we go further away from the Earth into the space, the value changes. For g to be constant, we should be close to the surface of the Earth.
As stated in the question above, h is a short distance from the surface of the planet. So, the distance r should also be a short distance from the surface of the planet for g to be constant. If r is at h, then it is close to the surface of the planet.}
ΔEP = force × distance moved = mgh

{The following formula relates the gravitational potential energy to the gravitational potential. This is obvious from the definition of the gravitational potential at a point.}
Change in gravitational potential energy, ΔEP = mΔφ
{Consider the distance of the small mass changing from r1 to r2.
1/r1 – 1/r2 = (r2 – r1) / r1r2}
ΔEP = mΔφ = GMm(1/r1 – 1/r2) = GMm(r2 – r1) / r1r2
if r2 ≈ r1 (r2 is approximately equal to r1 – that h is small), then (r2 – r1) = h and r1r2 = r2 (since r1 is assumed to be approximately equal to r2, the product is either r12 or r22. Let call it r2.)
{ΔEP = GMm h / r2}
let g = GM / r2
ΔEP = mgh

{From the conservation of energy, the potential energy is converted to kinetic energy.}
½ mv2 = mΔφ
{Distance r is the radius, not the diameter. So, the diameter should be halved.}
v2 = 2 × GM/r = (2 × 4.3 × 1013) / (3.4 × 106)
Speed v = 5.0 × 103 m s–1

Question 694: [Dynamics > Equilibrium]
Climber is supported by a rope on a vertical wall, as shown in Fig.1.

Weight W of the climber is 520 N. The rope, of negligible weight, is attached to climber and to a fixed point P where it makes an angle of 18° to the vertical. Reaction force R acts at right-angles to the wall.
The climber is in equilibrium.
(a) State conditions necessary for the climber to be in equilibrium.

(b) Complete Fig.2 by drawing a labelled vector triangle to represent the forces acting on the climber.

(c) Resolve forces or use vector triangle to calculate
(i) tension T in the rope,
(ii) reaction force R.

(d) Climber moves up the wall and the angle the rope makes with the vertical increases. Explain why magnitude of the tension in the rope increases.

Reference: Past Exam Paper – June 2011 Paper 23 Q2

Solution 694:
Resultant moment = zero / sum of clockwise moments = sum of anticlockwise moments
Resultant force = 0

shape and orientation correct and forces labelled and arrows correct
angles correct / labelled
{Tension T in rope = Weight W + Reaction R}

(i) {The vertical forces should be balanced.}
T cos18° = W
Tension T = 520 / cos18° = 547 N      (Scale diagram: ± 20 N)

(ii) {The horizontal forces should be balanced.}
Reaction force R = T sin18° = 169 N             (Scale diagram: ± 20 N)

(d) The angle θ is larger, hence cos θ is smaller. Tension T = W / cos θ. Hence the tension T is larger.


  1. How to do question 8 c ii? I don't get why you subtract the 17.7 but you add the binding ebergy per nucleon of helium?

    1. To which paper are you referring to?

    2. november 2009 paper 41. sorry, forgot to include that

    3. See question 710 at

  2. Replies
    1. For Q1, see solution 697 at

      For Q24, see solution 440 at

  3. s13_13 question number 5(iii) how are you able to obtain 7-5?? how did the 7 get there for the phase difference that is! please help!

    1. Possible paths from S to X.
      1st path: S to X directly – from Pythagoras’ theorem, this is (√[3^2 + 4^2] =) 5m.
      2nd path: (S to P) + (P to X). This is 3 + 4 = 7m.

    2. thankyou so much!! for the reply for this website! really appreciated! ^_^

  4. Q 692 what does it mean by grad cant be 0 initially?

    1. the gradient gives the speed. In the question, it is stated that the initial speed is 0.5m/s, so it cannot be zero.


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