VCE Physics:

Common Exam ErrorsRevision Advice

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Common Exam Errors

Common Errors: Connected Bodies

Examples: Horse pulling a cart, Mass over edge pulls another along table.

Error: Apply net force on system to each mass.

Misuse internal forces.

Consider: Questions 3 & 4 from 2010 Exam.

Average: 0.7 out of 2 (35%), Only 10% got 2 marks.

Average: 1.1 out of 2 (55%), 51% got 2 marks. Consequential on Q’n 3.Common errors: assume a = g or loss of GPE of mass 2 = gain in KE of mass 1

Common Errors: Connected Bodies

Method:1. Label all forces acting2. The whole system and each mass of the

system accelerates at the same rate3. Apply Newton’s 2nd Law: Net Force = Mass

x Accel’n applies to each mass in the system and to the whole system as well.

4. Forces between masses in the system are examples of Newton’s 3rd Law.

Accel’n = a

Accel’n= a

m2g

T

T

m1g

N

Net force on m1 equals T, but T is unknown, so use Net force on the system of two masses:

Net force on (m1 and m2) = m2g, but Net F = Ma, so m2g = (m1 + m2) x a.

Substituting, 0.10 x 10 = (0.1 + 0.4) x a, solve for a.

Conseq Q’n: use value of accel’n. Two methods: i) Use equations of motion to find v, the calculate KE, or ii) Use Work Done by force on m1 = Gain in KE

Common Errors: Circular Motion

Examples: Banked curves, crests in road

Error: Include centripetal force as another force, misunderstand reaction force.

Consider: Questions 5 & 6 from 2010 Exam.

Average: 0.9 out of 2 (45%), 51% got zero)

Average: 1.5 out of 3, 48% got 3, 48% got zero

Common Errors: Circular Motion

Examples: Banked curves, crests in road

Error: Include centripetal force as another force, misunderstand reaction force.

Method: Better to only refer to centripetal acceleration and then apply Newton’s 2nd Law.

Consider: Questions 5 & 6 from 2010 Exam.

Vertically: mg = N Sin , Horizontally: N Cos = Net Force = ma = mv2/r,

so combining: Tan = gr/v2, now solve for angle.

mg

N

Common Errors: Reaction force

Examples: Platforms slowing down, landing on trampoline, going through a dip in the road

Error: Confusing directions, and net force with actual forces.

Consider: Question 3 from 2015 exam.

Average: 1 out of 2. 50% got zero marks.

Common errors: Assume centripetal force is a force acting on the mass, rather than an expression for net force. Also ignore reaction force.

Common Errors: Reaction force

Examples: Platforms slowing down, landing on trampoline, going through a dip.

Error: Confusing directions, and net force with actual forces.

Method:

• Define a direction as positive,

• Determine sign of acceleration,

• Draw in acting forces,

• Apply Newton’s 2nd Law

Accel’n

Force by earth on car, mg

Force by track on car, N

Net F = ma = mv2/r

N – mg = mv2/r

N = mg + mv2/r

N = 2.0 x 9.8 + (2.0 x 6.02 / 4.0)

N = 19.6 + 18 = 38 N

Common Errors: Collisions

Examples: Elastic, and inelastic collisions (both sticky and non-sticky)

Error: Assume momentum temporarily goes into ‘storage’; assume all rebound collisions are elastic.

Consider: Questions 15 - 17 from 2010 Exam.

36% got full marks

56% got full marks

35% got full marks

Common Errors: Collisions

Examples: Elastic, and inelastic collisions (both sticky and non-sticky)

Error: Assume momentum temporarily goes into ‘storage’; assume all rebound collisions are elastic.

Method: Include graphs on 2 page summary.

Common Errors: Gravitation

Examples: Satellite motion

Error: Confusion of net force with actual forces. Need for a tangential force.

Consider: Question 13 from 2009 Exam.

Average 0.8 out of 2, 44% got zero, 26% got full marks.

Common faults: Tangential force; wrong label; two arrows

Common Errors: Gravitation

Examples: Satellite motion

Error: Confusion of net force with actual forces. Need for a tangential force.

Method: Emphasise: a force inwards is all that is necessary for circular motion - “Keys on a string”.

Common Errors: Electromagnetic Induction

Examples: Induced EMF with changing Flux

Error: Don’t realise that only a changing magnetic flux induces an EMF.

Consider: Questions 8, 9 and 11 from 2010 Exam.

Average: 0.8 out of 2, 51% got zero, some drew sine waves, many missed the point of different gradients.

Average: 0.4 out of 1, both Faraday and Lenz were accepted.

Average: 1.0 out of 2, 33% get zero, Many said the induced flux opposed the original flux rather than the change in flux. Others had right reason, but wrong direction.

Common Errors: Electromagnetic Induction

Examples: Induced EMF with changing FluxError: Don’t realise that only a changing magnetic flux induces an EMF.

Method: Demonstration as an POE, with students drawing graph of flux (with direction) and observing induced EMF.

Consider: Questions 8, 9 and 11 from 2010 Exam.

Time Flux Induced EMF

0 – 1 Increasing steadily Constant andbut rapidly large

1 – 2 Constant Zero

2 – 4 Decreasing steadily Constant and but slowly smaller

Two possible answers

Magnetic Flux through loop

Before After Change

Change = Final – Initial or Initial + Change = Final

Induced Magnetic Field Direction of Current

Opposes change Use Hand Rule

Down the front: Q to P

Common Errors: Path Difference

Examples: Double Slit Interference of Light

Error: Assuming path difference is along the screen

Consider: Question 4 from 2010 Exam.

Average: 1 out of 2 = 50%. 48% got zero.

Two students are studying interference of light. They use a laser of wavelength 580 nm.

Common Errors: Path Difference

Examples: Double Slit Interference of Light

Error: Assuming path difference is along the screen

Method: Draw the light paths. Determine the path difference at specific points in multiples of /2. Avoid stock formulae.

Consider: Question 4 from 2010 Exam.

Y is the next node after the antinode at X, so the path difference has increased by/2.Path diff for Y = Path Diff for X + (580 / 2)

= 1160 + 290 nm = 1450 nm

Two students are studying interference of light. They use a laser of wavelength 580 nm.

Common Errors: Momentum and energy of photons and electrons

Examples: Comparing diffraction patterns between electrons and X-rays.

Error: Assume if momenta of both are the same, then their energies are also the same.

Consider: Question 10 from 2010 Exam.

Average: 0.6 out of 3, 20%

Common Errors: Momentum and energy of photons and electrons

Examples: Comparing diffraction patterns between electrons and X-rays.

Error: Assume that if momenta of both are the same, then their energies are also the same.

Method: Draw up concept map linking quantities with relationships.

Electron Photon

Momentum Wavelength Momentum

Velocity, v Kinetic Energy Energy Speed, c

p = h/p p = h/ p

p = mv E = p2/2m E = hc/ E = pc

KE = ½ mv2

For Electrons: KE has to be in Joules to determine its momentum and wavelength.

For Photons: Energy can be in either eV or Joules to determine its wavelength.

Steps: KE in eV to KE in Joules to electron momentum to electron wavelength.Wavelengths are the same, so then Wavelength to photon energy in eV

Revision Advice for Students

Vicphysics The Students page has:

• Several links to websites with advice of learning physics,

• Advice of revising and preparing for exams,

• Topic summaries.

Revision Advice for Students

• Prepare a one page summary as soon as you finish an Area of Study,

• Use it with extra problems until the weeks before the exam,

• Maintain a regular routine of doing exam type problems after finishing an Area of Study,

• Two weeks before the exam prepare the first draft of the two page summary.

Revision Advice for Students

In the two weeks before the exam:

• Use the draft with past exam and trail papers,

• Evaluate performance after each exam and revise draft of two page summary accordingly.

Exam Advice for Students

• Use the 15 minute reading productively,• Attitude: Remember if you are finding the exam

fairly hard, don’t panic, because the rest of the state is probably also finding it hard. The reverse also applies.

• Read the Question Carefully: The exam will have many instances where you have to read a graph or interpret data. In many cases the values will need to be converted to SI units, e.g. cm m, kN N, MPa Pa.

Exam Advice for Students

• Highlight data and important information as you read the question.

• Show working every time.

• Written response: Answer in point form.

• Don’t look for complexities in the question. Assume the simplest explanation.

Resources

• Past Exam Papers and Solutions

• Chief Assessor Reports

• Exam Statistics

• Practice Papers

Resources

Past Exam Papers and Solutions

Papers

• VCAA: 2002 – 2016

• Checkpoints (Cambridge) $29 per unit

Solutions

• Vicphysics: 1999 – 2016 (for students with marking scheme)

Resources

Chief Assessor Reports• VCAA General comments including:

* Areas of concern,* Advice.

For each question: * The percentage getting each score from 0 to

max, * Specific comment, e.g. common errors

Resources

Exam Statistics

• VCAA Score ranges for each letter grade and grade distributions for both exams and the internal assessment for males and females for 2011.

• Vicphysics Raw score cut offs for 1999 – 2016.

Resources

Practice Papers• STAV• NEAP• TSSM• Physics Trial Test Team• Itute• QATs• Kilbaha• Access Education• Lisachem