# Physics 9702 Doubts | Help Page 139

__Question 687: [__

__Kinematics]__**(a)**Drag force D on an object of cross-sectional area A, moving with speed v through a fluid of density ρ, is given by

D = ½ CρAv

^{2}
where C is a constant.

Show that C has no unit.

**(b)**Raindrop falls vertically from rest. Assume that air resistance is negligible.

(i) On Fig.1, sketch a graph to show
variation with time t of the velocity v of the raindrop for the first 1.0 s of
the motion.

(ii) Calculate velocity of the
raindrop after falling 1000m

**(c)**In practice, air resistance on raindrops is not negligible because there is a drag force. This drag force is given by expression in (a).

(i) State equation relating the
forces acting on the raindrop when it is falling at terminal velocity.

(ii) Raindrop has mass 1.4 × 10

^{–5}kg and cross-sectional area 7.1 × 10^{–6}m^{2}. Density of the air is 1.2 kg m^{–3}and initial velocity of the raindrop is zero. Value of C is 0.60.
1. Show that terminal velocity of the
raindrop is about 7ms

^{-1}
2. Raindrop reaches terminal
velocity after falling approximately 10m. On Fig.1, sketch variation with time
t of velocity v for the raindrop. The sketch should include the first 5s of the
motion.

**Reference:**

*Past Exam Paper – November 2012 Paper 22 Q1*

__Solution 687:__**(a)**

C = 2D / (ρAv

^{2})
Units of force D =

**kgms**^{-2}
Units of ρAv

^{2}= (**kgm**) (^{-3}**m**) (^{2}**ms**)^{-1}**=**^{2}**kgms**^{-2}
So, unit of C =

**kgms**/^{-2}**kgms**= no units (units cancel each other)^{-2}**(b)**

(i)

{The raindrop falls from
rest, so the speed v starts from zero.

Air resistance is negligible:
acceleration is constant (= 9.81ms

Straight line from (0,0) to (1,9.8) ± half a
square^{-2}), so gradient (which represents the acceleration) is constant.}
(ii)

{The change in E

_{K}is equal to the change in E_{P}. OR use the equation for uniformly acceleration motion}
½ mv

^{2 }= mgh or v^{2}= 2as (u^{2}= 0)
Speed v = (2 × 9.81 × 1000)

^{1/2}= 140ms^{-1}**(c)**

(i)

{At terminal velocity,
acceleration = 0. So, the weight (downward) is equal to the drag force on the
object (which is upward here).}

Weight = drag (D) (+ upthrust)

(ii)

1.

Weight, W = drag (D)

Weight W = mg = 1.4×10

^{-5}× 9.81
Drag D = ½ CρAv

^{2}= ½ × 0.60 × 1.2 × (7.1×10^{-6}) × v^{2}
Weight, W = drag (D)

1.4×10

^{-5}× 9.81 = ½ × 0.60 × 1.2 × 7.1×10^{-6}× v^{2}
Speed v = 7.33ms

^{-1}
2.

Line from (0,0) correct curvature to
a horizontal line at velocity of 7ms

^{-1}
Line reaches 7ms

^{-1}between 1.5s and 3.5s
{The correct method to
find the time at which terminal velocity is reached is to such that the area
under the velocity-time graph is 10m. The time t is not known, initial velocity
is zero and final velocity is 7m. However, such detailed values are not
required here since it is only a sketch.

If the object was falling
with constant velocity of 7ms

^{-1}since t = 0, to travel a distance of 10m, the time of fall would be = distance / speed [since speed = distance / time] = 10 / 7 = 1.4s. But the speed is initially zero and it increases until it becomes 7ms^{-1}, so the time of fall should definitely be greater than 1.4s. So, the lower range is 1.5s.
The upper limit for the
limit is given approximately. Students should be able to estimate some values.
We cannot choose a time of fall which is too big, nor one which is too small.

Finally, to remind you
again, the best method would be to use the area under the graph. But this is
more than required for just a sketch.}

__Question 688:__

__[Measurement]__**(a)**Young modulus of the metal of a wire is 1.8 × 10

^{11}Pa. Wire is extended and the strain produced is 8.2 × 10

^{–4}.

Calculate the stress in GPa.

**(b)**An electromagnetic wave has frequency 12 THz.

(i) Calculate wavelength in μm.

(ii) State name of the region of the
electromagnetic spectrum for this frequency.

**(c)**Object B is on a horizontal surface. Two forces act on B in this horizontal plane. A vector diagram for these forces is shown to scale in Fig.1.

A force of 7.5 N towards north and a
force of 2.5 N from 30° north of east act on B.

Mass of B is 750 g.

(i) On Fig.1, draw an arrow to show
approximate direction of the resultant of these two forces.

(ii)

1. Show that magnitude of the
resultant force on B is 6.6 N.

2. Calculate magnitude of the
acceleration of B produced by this resultant force.

(iii) Determine angle between the
direction of the acceleration and the direction of the 7.5 N force.

**Reference:**

*Past Exam Paper – November 2014 Paper 22 Q1*

__Solution 688:__**(a)**

Stress = Young modulus × strain = (1.8
× 10

^{11}) × (8.2 × 10^{–4}) OR = 1.476 × 10^{8}
Stress = 0.15 (0.148) GPa

**(b)**

(i) Wavelength = (3 × 10

^{8}) / (12 × 10^{12}) = 25 μm
(ii) Infra-red / IR

**(c)**

(i) The arrow should be drawn up to
the left of the 7.5 N force, approximately 5° to 40° to west of north

(ii)

1. A correct vector triangle or
working to show that magnitude of resultant force = 6.6 N

A triangle is formed with
the 2.5N vector at an angle of 60° to the vertical 7.5N vector. Since the resultant vector is
opposite to the 60° angle, the cosine rule can be used.

Let the resultant vector
be R.

R

^{2}= 7.5^{2}+ 2.5^{2}– 2(7.5)(2.5)cos 60°
Resultant vector R = 6.6N

(allow 6.5 to 6.7 N if scale diagram)

2.

{Resultant force F = ma}

Magnitude of acceleration a = 6.6 /
0.75 = 8.8 m s

^{–2}
[scale diagram: (6.5 to 6.7) / 0.75]
[scale diagram: 8.7 – 8.9 m s

^{–2}]
(iii)

{The acceleration is in
the same direction as the resultant force. Let the required angle be θ. Consider the triangle formed by the 3 vectors (same triangle as in
part (c)(ii)1.). Using sine rule,

sin θ / 2.5 = sin 60 / 6.6

Angle θ = sin

^{-1}(2.5sin 60 / 6.6) = 19°}
Angle = 19°

[

__use of scale diagram__allow 17° to 21° (a diagram must be seen)]

__Question 689: [Waves > Diffraction]__**(a)**Describe diffraction of monochromatic light as it passes through a diffraction grating.

**(b)**White light is incident on a diffraction grating, as shown in Fig.1.

The diffraction pattern formed on
screen has white light, called zero order, and coloured spectra in other orders.

(i) Describe how principle of
superposition is used to explain

1. white light at the zero order,

2. difference in position of red and
blue light in the first-order spectrum.

(ii) Light of wavelength 625 nm
produces a second-order maximum at an angle of 61.0° to incident direction.

Determine number of lines per metre
of the diffraction grating.

(iii) Calculate wavelength of
another part of the visible spectrum that gives a maximum for a different order
at the same angle as in (ii).

**Reference:**

*Past Exam Paper – November 2012 Paper 21 Q4*

__Solution 689:__**(a)**The waves pass through the elements / gaps / slits in the grating and spread into geometric shadow

**(b)**

(i)

1. The displacements add to give a
resultant displacement. Each wavelength travels the same path difference or are
in phase. Hence, this produces a maximum.

2. To obtain a maximum the path
difference must be λ or phase difference 360° / 2π rad. The wavelength λ of red
and blue are different, hence the maxima (of red and blue light) are formed at different
angles / positions.

(ii)

For diffraction grating: nλ = d sin θ

{Slit separation, d = nλ /
sin θ.

Number of lines, N = 1 / d
= sin θ / nλ.

For 2

^{nd}order diffraction, n = 2.}
Number of lines, N = sin 61° / (2 ×
625 × 10

^{–9}) = 7.0 × 10^{5}m^{–1}
(iii)

{The path difference nλ =
d sin θ is a constant for constructive interference and this constant can be given
by 2 × 625 since as given in part (ii), a maxima of order 2 is obtained when
the wavelength is 625nm. So, the path difference is 2 × 625.}

nλ = 2 × 625 is a constant (1250)

{Wavelength λ = 2 × 625 /
n. We need the wavelength for a different order, so n cannot be 2. Also, the
wavelength should be part of the visible spectrum.

We try for different
orders (values of n) and see if the wavelength lie in the visible spectrum.}

when n = 1 → wavelength λ = 1250
outside visible range

when n = 3 → wavelength λ = 417 in
visible range

when n = 4 → wavelength λ = 312.5
outside visible range

So, wavelength λ (= 417nm) = 420 nm

__Question 690: [Thermodynamics]__**(a)**The equation

pV = constant × T

relates pressure p and volume V of a
gas to its kelvin (thermodynamic) temperature T.

State two conditions for equation to
be valid.

**(b)**Gas cylinder contains 4.00 × 10

^{4 }cm

^{3}of hydrogen at a pressure of 2.50 × 10

^{7}Pa and a temperature of 290 K.

Cylinder is to be used to fill
balloons. Each balloon, when filled, contains 7.24 × 10

^{3}cm^{3}of hydrogen at a pressure of 1.85 × 10^{5}Pa and a temperature of 290 K.
Calculate, assuming that hydrogen
obeys the equation in (a),

(i) total amount of hydrogen in the
cylinder,

(ii) number of balloons that can be
filled from the cylinder.

**Reference:**

*Past Exam Paper – June 2006 Paper 4 Q2*

__Solution 690:__**(a)**Example:

fixed mass/ amount of gas

ideal gas

**(b)**

(i) Number of moles, n = pV / RT =
(2.5 × 10

^{7}× 4.00 × 10^{4}x 10^{-6}) / (8.31 × 290) = 415 mol
{1 cm

^{3}= 10^{-6}m^{3}}
(ii)

{From the equation, it can
be deduced that

p

_{1}V_{1}/ T_{1}= p_{2}V_{2}/ T_{2}= constant for different sets of p, V and T}
At 1.85 × 10

^{5}Pa,
{(1.85 × 10

^{5}) V_{1}/ 290 = (2.5 × 10^{7}) (4.00 × 10^{4}) / 290}
Volume of gas = (2.5 × 10

^{7}× 4.00 × 10^{4}) / (1.85 × 10^{5}) = 5.41 × 10^{6}cm^{3}
{1 balloon contains 7.24 ×
10

^{3 }cm^{3}of hydrogen. Let N be the number of balloons. As stated in part (a), one condition for the equation to be valid is that the amount of gas in the cylinder should be fixed. So, we should account for the amount of has that remains in the cylinder, which is 4.00 × 10^{4}cm^{3}.}
so, 5.41 × 10

^{6}= 4.00 × 10^{4}+ (7.24 × 10^{3}) N
Number of balloons, N = 741

21/O/N/14 Q.4

ReplyDeleteSee question 692 at

Deletehttp://physics-ref.blogspot.com/2015/05/physics-9702-doubts-help-page-140.html

What do you mean by visible spectrum?

ReplyDeleteVisible spectrum is part of the EM spectrum with frequencies / wavelengths that the human eye can see.

Delete