• 9702 June 2009 Paper 21 Worked Solutions | A-Level Physics

Paper 21

Question 1

{Detailed explanations for this question is available as Solution 1009 at Physics 9702 Doubts | Help Page 210 - http://physics-ref.blogspot.com/2015/10/physics-9702-doubts-help-page-210.html}

Question 2

Ball B of mass 1.2 kg travelling at constant velocity collides head-on with stationary ball S of mass 3.6 kg, as shown. Frictional forces are negligible. Variation with time t of velocity v of ball B before, during and after colliding with ball S shown.

(a)

Significance of positive and negative values for v:

The negative value indicates that the ball is moving in the opposite direction (after collision)

(b)

Use Fig to determine, for ball B during collision with ball S,

(i)

Change in momentum of ball B:

Change in momentum = 1.2(4.0 + 0.8) = 5.76Ns

(ii)

Magnitude of force acting on ball B:

Force = Δp / Δt (or mΔv / Δt) = 5.76 / 0.08 = 72N

(c)

Speed of ball S after collision:

5.76 = 3.6 x v

Speed, v = 1.6ms^-1

(d)

Using answer in (c) and information from Fig, deduce quantitatively whether collision is elastic or inelastic:

EITHER

Relative speed of approach = 4.0ms^-1 and relative speed of separation = (0.8 + 1.6 =) 2.4ms^-1. These are not equal, so the collision is inelastic.

OR

Kinetic energy before collision = 9.6J and Kinetic energy after collision = 4.99J. Kinetic energy is less after the collision / not conserved. So, the collision is inelastic.

Question 3

{Detailed explanations for this question is available as Solution 1091 at Physics 9702 Doubts | Help Page 232 - http://physics-ref.blogspot.com/2016/01/physics-9702-doubts-help-page-232.html}

Question 4

Spring having spring constant k hangs vertically from fixed point. Load of weight L, when hung from spring, causes extension e. Elastic limit of spring is not exceeded.

(a)

(i)

An elastic deformation is the change of shape / size / length / dimension when a (deforming) force is removed, returns to original shape / size

(ii)

Relation between k, L and e:

L = ke

(b)

Some identical springs, each with spring constant k, are arranged as shown. Load on each of the arrangements is L. For each arrangement, complete table by determining (i) total extension in terms of e, (ii) spring constant in terms of k.
{Force, L = (spring constant, k) x( extension, e). L is constant in all cases. e can be determined from the diagrams. E.g. 2 springs in parallel causes extension to be halved while 2 springs in series causes extension to be doubled. Spring constant, k is given by k = F / extension. In this question, F is constant – so, k is the reciprocal of the extension}

Arrangement 1:

Extension = 2e

Spring constant = ½ k

Arrangement 2:

Extension = ½ e

Spring constant = 2k

Arrangement 3:

Extension = (3/2) e

Spring constant = (2/3) k

Question 5

Two sources S1 and S2 of sound are situated 80 cm apart in air, as shown in Fig. 5.1. The frequency of vibration can be varied. The two sources always vibrate in phase but have different amplitudes of vibration.

Question 6

2 vertical parallel metal plates are situated 2.50 cm apart in vacuum. Potential difference between plates is 350 V, as shown. An electron is initially at rest close to negative plate and in uniform electric field between plates.

(a)

(i)

Magnitude of electric field between plates:

E = V / d = 350 / (2.5x10^-2) = 1.4x10⁴NC^-1

(ii)

Show that force on electron due to electric field is 2.24x10^-15N:

Force = Eq = (1.4x10⁴) (1.6x10^-19) = 2.24x10^-15N

(b)

Electron accelerates horizontally across space between plates.

(i)

Horizontal acceleration of electron:

Force, F = ma

a = (F/m =) (2.24x10^-15) / (9.1x10^-31) = 2.46x10¹⁵ms^-2

(ii)

Time to travel horizontal distance 2.50cm between plates:

s = ½at²

2.5x10^-2 = ½ (2.46x10¹⁵) t²  

Time, t = 4.5x10^-9s

(c)

Why gravitational effects on electron need not be taken into consideration in calculation in (b):

EITHER The gravitational force is normal to the electric force OR The electric force is horizontal while the gravitational force is vertical.

Question 7

{Detailed explanations for this question is available as Solution 595 at Physics 9702 Doubts | Help Page 117 - http://physics-ref.blogspot.com/2015/04/physics-9702-doubts-help-page-117.html}

Question 8

Spontaneous and random decay of radioactive substance involves emission of either α-radiation or β-radiation and/or γ-radiation.

(a)

For a spontaneous decay, the rate of decay / activity / decay (of nucleus) is not affected by external factors / environment / surroundings

(b)

Type of emission that:

(i)

Not affected by electric and magnetic fields:

Gamma / γ

(ii)

Produces greatest density of ionization in medium:

Alpha / α

(iii)

Does not directly result in change in proton number of nucleus:

Gamma / γ

(iv)

Has range of energies, rather than discrete values:

Beta / β