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ST STITHIANS BOYS’ COLLEGE

DEPARTMENT OF PHYSICAL SCIENCES

PHYSICS PRELIM EXAMINATION

Time: 2 hour Marks: 200

Examiner: Ms C Rossouw Moderator: Mr F Grey

Ms S Nagaroo

This test consists of the following Learning Outcome: LO1: Practical Scientific Inquiry and Problem solving Skills (SKILLS) LO 2: Constructing and Applying Scientific Knowledge (KNOWLEDGE) LO3: The Nature of Science and its Relationships to Technology, Society and the Environment (SCIENCE & SOCIETY) PLEASE READ THE FOLLOWING INSTRUCTIONS CAREFULLY

1. This paper consists of 16 pages and contains eight questions 2. All answers are to be written on the answer sheet and paper provided

3. Read the questions carefully

5. It is in your own interest to write legibly and to set your work out neatly

6. The data sheet is given.

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Question 1 Four possible options are provided as answers to the following questions. Each question has only ONE correct answer. Mark only the letter, corresponding to the correct answer. NB: ANSWER QUESTION ONE ON THE ANSWER SHEET PROVIDED 1.1 A conductor carrying conventional current as indicated (out of the page) is placed in

the magnetic field, as shown below.

The conductor will experience a force in the direction shown by which arrow?

1.2 Which one of the following velocity-time graphs is the correct one for the motion of

a ball being thrown up into the air and then caught by the thrower?

B

D C

A

A B C D

I

N S

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1.3 The diagram below shows a velocity vs time graph for a vehicle moving in a straight line.

Which one of the following is not a correct deduction from the information

presented on the graph alone?

A. The vehicle returned to its starting point at t = t1. B. The vehicle had its maximum velocity at R. C. The distance travelled by the vehicle is equal to the area under the graph. D. The vehicle started from rest at t = 0.

1.4 Consider a ball dropped from a height of 1m on earth and an identical ball dropped

from 1m on the moon. Assume both balls undergo free fall. The acceleration due to gravity on the moon is one sixth of that on earth. In what way do the following compare when the ball is dropped on earth and on the

moon?

Mass Final velocity Kinetic energy A The same Greater on earth Greater on earth B Less on earth The same The same C The same Less on earth Less on earth D Greater on earth The same The same

1.5 Which of the following statements is true for a body in free fall?

A. It experiences no acceleration. B. It has a constant acceleration of 10 m.s-2 down. C. There is no resultant force. D. The frictional force is equal to the gravitational force.

R  

t1  t  (s)  

v                  (m.s-‐1)  

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1.6 Cart X moves along a smooth track with momentum p. A resultant force F applied to the cart and it stops it in time t. Another cart Y has only half the mass of X, but has same momentum p.

How long does it take Cart Y to stop, if Cart Y experiences the same resultant force?

A. 0,5 t B. t C. 2 t D. 4 t

1.7 The diagram shows a circuit with three identical resistors. The battery has a negligible

internal resistance. Which of the light bulbs numbered 1 to 3 will light up?

A. 1 B. 2 C. 1 and 3 D. 2 and 3

     2  m  

Cart  X                  F          m  

Cart  Y                  F  

P   P  

3  1  

2  

5  

 

– +12 V

20 Ω 40 Ω

60 Ω

1.8 Consider the following circuit containing two diodes. The current through the battery is 0,16 A and the battery has no internal resistance

The the resistance of the forward bias diode is:

A. 7,2 � B. 4, 8 � C. 60 � D. 15 �

1.9 A car’s tail-light is indicated as 12V: 4A, what is the rate of energy consumed when it is in operation?

A. 3 W B. 48 J C. 48 W D. 16 W

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1.10 The diagram shows a circuit with two identical resistors. The battery has a negligible internal resistance. The resistance of the ammeter can be ignored.

What will the effect on the ammeter and voltmeter readings be if the switch S is opened?

Ammeter reading Voltmeter reading A Increase Stay the same B Increase Decrease C Stay the same Increase D Decrease Stay the same

[10 X 2 = 20]

S  R  

V  

R  

A  

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Question 2 Vertical motion in one dimension

2.1 Peter throws a stone vertically upwards from the edge of a roof that is 5 m above the

ground. The stone reaches a maximum height of 2 m above the roof and then turns

and falls to the ground below.

2.1.1 What is the stone’s velocity at it’s maximum height above the ground? (1)

2.1.2 Calculate the velocity of the stone just after it has left Peter’s hand. (3)

2.1.3 Calculate how long it took for the stone to reach its maximum height. (2)

2.1.4 What is the stones displacement when it hits the ground? (2)

2.1.5 Calculate the magnitude of the stones final velocity when it hits the ground. (3)

2.1.6 Calculate how long it took for the stones entire journey. (3)

2  m  

5  m  

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2.2 A hot air balloon is rising vertically at a constant velocity. A man

in the hot air balloon accidentally drops his iPhone over the edge of the

basket of the hot air balloon. When the iPhone eventually reaches the

ground, it bounces once then comes to rest. The velocity-time graph

below represents the motion of the iPhone from the instant it is released

from the man’s hand until it comes to rest on the ground. Use the graph

to answer the questions that follow.

2.2.1 Determine the acceleration of the iPhone once it is dropped out of the hot air balloon

(1)

2.2.2 Using the values from the graph calculate the height above the ground of the hot air

balloon when the iPhone was dropped. (4)

2.2.3 Consider point P on your graph

a) What happened at point P? (1)

b) Calculate the time value for point P. (3)

c) Calculate the maximum height the iPhone will reach after it has bounced. (3)

2.2.4 Calculate what distance apart the hot air balloon and iPhone are at time P. (6)

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v

(m.s-1) 0

6

-30

3

P  

t (s) 4, 3 0, 6 3, 6   3,7

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Question 3 Vertical motion in two dimensions

Maria Sharapova hits a tennis ball with a speed of 35 m.s-1

horizontally straight down the tennis court from the baseline

(back line). The ball is struck at a height of 1.5 m above the

ground as shown in the diagram below. The net is 0.9 m high.

The baseline is 11.9 m from the net.

Assume that the effects of air resistance are negligible.

3.1 Draw a sketch of the path of the tennis ball. (1)

3.2 Calculate the time it takes for the ball to reach the net. (3)

3.3 Prove by means of calculations that the tennis ball will clear the net. (4)

3.4 Calculate how long the ball will be in the air for from the time it leaves the racket up

to the first bounce. (3)

3.5 Calculate the horizontal distance the ball will travel from the time it leaves the racket

up to the first bounce. (2)

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Baseline (point where ball leaves the racket)

11.9 m

The ball leaves the racket travelling horizontally at 35 m.s-1

1.5 m

Net

0.9 m

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Question 4 Momentum and energy

4.1 A sandbag on a trolley has a combined mass of 4 kg. The sandbag and trolley moves

from A down a frictionless inclined plane from rest. When it reaches the bottom of the

incline at point B, it continues towards C at a constant speed. A bullet of mass 0,15 kg

is fired with an initial velocity of 200 m.s-1 and gets imbedded in the sandbag.

Immediately after the collision the trolley-sandbag-bullet combination travels

backwards at a constant velocity of 5,3 m.s-1.

4.1 State the law of conservation of linear momentum. (2)

4.2 Calculate the velocity of the sandbag and trolley before the bullet got imbedded in it.

(4)

4.3 Explain what is meant by the term “impulse”. (2)

4.4 Calculate the initial momentum of the bullet. (2)

4.5 Calculate the magnitude of the change in momentum of the bullet. (2)

4.6 Calculate the resultant force exerted on the bullet if the collision lasted for 0,05 s.

(4)

4.7 State the law of conservation of mechanical energy. (3)

4.8 Calculate the mechanical energy of the trolley and sandbag at point B. (3)

4.9 Calculate the height of the slope. (4)

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   200  m.s-‐1  

 

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Question 5 Power and energy

In 2008 Google Inc. initiated a project to research ideas on changing

the world. Google Inc. subsequently searched the globe and selected

Shweeb as the project with the most forward looking transportation

vision. By integrating the unique properties of monorail and

recumbent cycle technologies, Shweeb delivers a personal, efficient,

and cost-effective transport solution with applications for urban

commuting, recreational and fitness markets. The Shweeb is a

system of personal, pedal-powered monorail pods that hang from an elevated track, a track

that hopefully in the future will stretch to destinations through out a city.

One such occupied capsule has a mass of 310 kg and is 5 m above the ground.

5.1 Define power. (2)

5.2 Calculate the gravitational potential energy of the capsule relative to the ground when

it is suspended at a height of 5 m above the ground. (3)

5.3 A crane is used to lift the capsule off the ground to the required height. Calculate the

power required by the crane if the capsule is lifted at a constant velocity of

0,056 m.s-1. (3)

5.4 During the assembly of the capsule to the monorail one of the mechanics drops a

spanner (mass 600 g) from the capsule to the ground 4,8 m below.

5.4.1 Calculate the speed of the spanner just before it strikes the ground.

Ignore effects of friction. (4)

5.4.2 State the Work Energy Theorem. (2)

5  m  

12  

 

A  

   V1  

V2  

2Ω  

5Ω   3Ω  

     r  

   

 

5.4.3 Calculate the net (resultant) force acting on the spanner while it penetrates

3 cm into the soil before coming to rest. (Assume uniform acceleration while

the spanner comes to rest.) (4)

5.4.4 Draw a labeled free body diagram indicating all the forces acting on the

spanner during its contact with the soil. (2)

5.4.5 Calculate the force applied by the ground on the spanner while it is moving

through the soil? (3)

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Question 6 Electrical circuits

6.1 Consider the following electric circuit. When the switch is open the reading on

V1 = 10 V and when the switch is closed the reading on the V1 = 9,6 V.

6.1.1 Describe how the volts are “lost” in a cell when the switch is closed. (2)

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A  

       

   

S1  

S2  

2,0 µF  

A  

Calculate:

6.1.2 the effective external resistance of the circuit. (4)

6.1.3 the reading on the ammeter. (3)

6.1.4 the reading on V2. (4)

6.1.5 the internal resistance of the battery. (4)

6.2 Consider the following circuit. A 12V battery is connected to a 2,0 µF capacitor and a

light bulb as shown in the diagram below. Ignore the internal resistance of the battery.

6.2.1 Define capacitance. (2)

Switch S1 is now closed (S2 remains open)

6.2.2 How will the reading on the ammeter change with time? (Increase, decrease or remain

the same). Explain your answer. (3)

6.2.3 Calculate the amount of charge stored in the capacitor. (3)

6.2.4 After some time switch S1 is opened and switch S2 is closed.

a) What would you observe the instant the switch is closed? (1)

b) How does the brightness of the bulb change? Give a reason for your answer.

(2)

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A dielectric capacitor consists of two metal sheets placed on either side of a layer of dielectric

material. Examples of such dielectrics are plastic like Teflon and polyethylene.

6.2.5 Give one environmental disadvantage of using plastic as a dielectric material. (2)

6.2.6 Give one example of how the above problem can be minimised. (2)

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Question 7 Motors and generators

7.1 A simple DC motor is shown below. The armature spins clockwise as shown below.

(Physics 3, Y H Ng et al, Aristo Educational Press 2009)

7.1.1 Give the energy conversions that take place in a DC motor. (2)

7.1.2 Name and give the function of part C. (2)

7.1.3 Determine the polarities of the magnets P and Q. (2)

7.1.4 What rule was used to obtain the above answer? (1)

7.1.5 State three methods of increasing the power of the motor. (3)

7.1.6 A DC motor can be changed into an AC generator. Identify two changes that

need to be made to achieve this. (4)

7.1.7 Draw a neat graph of current versus time for an AC generator. (One full

rotation). (2)

C  

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7.2 AC generators are widely used in wind turbines.

An experiment was performed to investigate how the area of

the blade surface affects the current produced at different

wind speeds of a wind turbine.

The following results were recorded during the experiment:

7.2.1 State a suitable hypothesis for this experiment. (2)

7.2.2 Give the two independent variables for this experiment. (2)

7.2.3 Plot a suitable graph to show the relationship between the blade surface area

and the current generated for the rotation speed of 6 m.s-1. (8)

7.2.4 Name one other factor, not listed in the table, that would also affect the

amount of current generated. State why this factor is not mentioned in the

table. (3)

7.2.5 Use your graph to find the current generated when the blade surface area is

16 m2. (2)

7.2.6 Use your graph and find which blade surface will generate a current of 0,35 A.

(2)

7.2.7 Give one advantage of using wind power instead of using fossil fuels. (2)

7.2.8 Give one disadvantage of using wind power as an only energy source. (2)

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Blade surface

area (m2)

Current (A) generated at wind speed

2 m.s-1 6 m.s-1

0,8 0,001 0,020

3,1 0,004 0,100

7,1 0,009 0,260

12,6 0,020 0,460

19,6 0,030 0,700

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Question 8 Doppler effect and Sonic booms

The photo below indicates a supersonic jet flying at mach 2,3.

8.1 What does the phrase “supersonic speed” mean? (1)

8.2 If the speed of sound is 340 m.s-1, calculate the speed of the jet. (3)

8.3 Make use of principles in physics and diagrams to explain the phenomenon of sonic

booms. (3)

The jet now reduces its speed to a constant velocity of 270 m.s-1 as it approaches an observer

on the ground.

8.4 Calculate the frequency of the sound emitted by the jet engine if the observed

frequency is 17 852 Hz (4)

When Concorde first broke the sound barrier back in the 70’s, it was banned from supersonic

flight overland. Now, once again, aircraft manufacturers are looking at overland supersonic

flight.

8.5 Give two reasons why jets where banned from overland flight. (2)

8.6 Give one way of making overland supersonic flight environmentally friendly. (2)

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THE END