Tuesday, March 31, 2015

Physics 9702 Doubts | Help Page 100

  • Physics 9702 Doubts | Help Page 100



Question 513: [Forces > Momentum]
Which row correctly states whether momentum and kinetic energy are conserved in inelastic collision in which there are no external forces?
momentum                  kinetic energy
A         conserved                    conserved
B         conserved                    not conserved
C         not conserved              conserved
D         not conserved              not conserved

Reference: Past Exam Paper – June 2012 Paper 12 Q11



Solution 513:
Answer: B.
For any system in which there are no external forces, the momentum is always conserved. [C and D are incorrect]

If the system is elastic, kinetic energy is conserved.
If the system is inelastic, kinetic energy is not conserved. The energy may be lost as heat, sound and other forms of energy. [A is incorrect]









Question 514: [Kinematics]
Graph shows how velocity v varies with time t for a bungee jumper.

At which point is bungee jumper momentarily at rest and at which point does she have zero acceleration?
jumper at rest              jumper with zero acceleration
A                     Q                                             P
B                     Q                                             R
C                     R                                             Q
D                     R                                             R

Reference: Past Exam Paper – November 2013 Paper 11 & 12 Q7



Solution 514:
Answer: C.
The graph shown is a velocity-time graph.
When the jumper is at rest, her velocity is zero. Zero velocity is indicated when the curve touches the time axis. This is at point R. [A and B are incorrect]

The acceleration of the jumper is given by the gradient of the velocity-time graph. The gradient at a point in the curve is given by the gradient of the tangent at that point. For the gradient to be zero, the tangent should be a horizontal line. This occurs at point Q. [D is incorrect]










Question 515: [Current of Electricity]
Battery is marked 9.0 V.
What does this mean?
A Each coulomb of charge from the battery supplies 9.0 J of electrical energy to the whole circuit.
B The battery supplies 9.0 J to an external circuit for each coulomb of charge.
C The potential difference across any component connected to the battery will be 9.0 V.
D There will always be 9.0 V across the battery terminals.

Reference: Past Exam Paper – June 2011 Paper 11 Q32 & November 2014 Paper 11 & 12 Q31



Solution 515:
Answer: A.
The e.m.f. of a battery does not tell anything about its internal resistance. If the battery has some internal resistance, the p.d. across the battery terminals will not be exactly 9.0V, but less. [D is incorrect] This would also means that the amount of energy that the battery supplies to an external circuit for each coulomb of charge is NOT 9.0J. [B is incorrect]

Choice C is obviously incorrect. It should have been as follows. IF there is no internal resistance in the battery (and the wires have negligible resistance), the SUM of p.d. across the componentS connected to the battery will be 9.0V. [C is incorrect]

A battery being marked 9.0V means that each coulomb of charge from the battery supplies 9.0 J of electrical energy to the whole circuit (this includes the internal resistance of the battery).










Question 516: [Measurements > Uncertainties]
Micrometer is used to measure diameters of two cylinders.
diameter of first cylinder = 12.78 ± 0.02 mm
diameter of second cylinder = 16.24 ± 0.03 mm
Difference in the diameters is calculated.
What is uncertainty in this difference?
A ± 0.01 mm               B ± 0.02 mm               C ± 0.03 mm               D ± 0.05 mm

Reference: Past Exam Paper – November 2011 Paper 12 Q4



Solution 516:
Answer: D.
Let diameter of first cylinder = d1 = 12.78 ± 0.02 mm
Let diameter of second cylinder = d2 = 16.24 ± 0.03 mm

Difference in diameters = d2 – d1
Uncertainty in (difference in diameters) = Sum of uncertainties in d1 and d2
Uncertainty in (difference in diameters) = 0.02 + 0.03 = ±0.05mm









Question 517: [Kinematics + Momentum]
Small ball is thrown horizontally with speed of 4.0ms-1. It falls through vertical height of 1.96m before bouncing off a horizontal plate, as illustrated in Fig. 

Air resistance is negligible.
(a) For ball, as it hits the horizontal plate,
(i) state magnitude of horizontal component of its velocity
(ii) show that vertical component of velocity is 6.2ms-1

(b) Components of velocity in (a) are both vectors.
Complete Fig to draw a vector diagram, to scale, to determine velocity of ball as it hits horizontal plate

(c) After bouncing on plate, the ball rises to vertical height of 0.98m.
(i) Calculate vertical component of velocity of ball as it leaves the plate
(ii) Ball of mass 34g is in contact with plate for time 0.12s.
Use answer in (c)(i) and data in (a)(ii) to calculate, for the ball as it bounces on the plate,
1. change in momentum
2. magnitude of average force exerted by plate on ball due to this momentum change

Reference: Past Exam Paper – November 2009 Paper 22 Q3



Solution 517:
(a)
(i) Horizontal component of velocity = 4.0ms-1

(ii)
(v2 = u2 + 2as. Here u = 0, a = g = 9.8ms-2 and s = h = 1.96m)
v2 = 2gh = 2(9.8)(1.96)
Vertical component of velocity, v = 6.2ms-1

(b) Diagram must have a correct basic shape with the correct directions for the vectors.
Speed = (7.4 ± 0.2) ms-1 at (33 ± 2)o to the vertical




(c)
(i)
(v2 = u2 + 2as. At (max) vertical height, v = 0. So, 0 = u2 + 2(-9.8)(0.98))
EITHER v2 = 2(9.8)(0.98)      OR v = 6.2 / √2
Vertical component of velocity = 4.4ms-1

(ii)
1.
Momentum, p = mv
Change in momentum, Δp = 0.034 (6.2 – (-4.4)) = 0.36kgms-1

2. Force = Δp / Δt (however expressed) = 0.36 / 012 = 3.0N


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