Sec 3 Pure Physics MYE Post-mortem

[These slides will be uploaded on my blog after today]

What I’m doing today

• Eat Marshmellows!!• 4 Steps of Cognition for Examinations• Observations and feedback• Advice for future examinations• Answers to difficult questions• Checking of scripts• Collection of scripts• Final Words

4 Steps of Cognition for Examinations

• There are 4 cognitive steps to studying for examinations

• Step 1 – Comprehension• Step 2 – Consolidation / Internalization• Step 3 – Recall• Step 4 – Critical Application

Observations & Feedback

#1 – Many students don’t even understand basic concepts (e.g. don’t know how to draw FBD, Newton’s 3rd Law)• Step 1 (comprehension) not there• This is about understanding what is taught in

class. • Important to clarify what you don’t understand

as soon as possible after the Physics lesson ended.

Observations & Feedback

#2 – Students have understood concepts at one time, but are unable to recall (e.g. refraction, lens, kinematics equations & graphs)• Several possible causes

– Step 2 missing: did not consolidate learning– Step 3 missing: did not revise– lack of sleep: mind does not recall well when

poorly rested

Observations & Feedback

#3 – Higher order questions very poorly done• Very few students have accomplished steps 1-

3, hence step 4 also cannot• The only way to train Step 4 is to practice

many different higher-order questions. However, doing so if you haven’t even done Steps 1-3 is just going to demoralize yourself.

Observations & Feedback

#4 – Majority of students not meticulous. (e.g. marks deducted for s.f., units, careless errors)• No excuse for this – this is just about being

careful and meticulous• Physics paper will always have more than

enough time• Make sure you devote at least 10 minutes to

check units, s.f. and calculation steps.

Observations & Feedback

#5 – Students appear fatigued and unmotivated to study / check paper• This is about pacing your revision for exams

well. If you cram (and don’t sleep) for your first few papers, your later papers will suffer because your mind (and spirit) is fatigued

• Steps 1-2 should be done WAY before exams. Exam prep should only be Steps 3 and 4.

Advice for Future Exams1. Clarify lack of understanding ASAP after lessons2. Consolidate your own learning after a topic has

finished (e.g. make your own notes)3. Sleep is extremely important for Step 1 – so

getting enough sleep during regular term is important

4. Sleep is also extremely important for Step 3 – so making sure you have enough sleep throughout ENTIRE exam period is important

Advice for Future Exams

5. Plan and pace your revision well. Make a revision timetable (each day revise what topic)

6. I recommend: preparing for exams 3 weeks ahead of time, each day no more than 3 hours of revision.

7. Even during exam period, plan for 1 rest day per week to spend with family/friends.

8. Continue to eat well and exercise regularly during exam period – healthy body gives alert mind

Answers to Difficult Questions

MCQ Qn 12

• A Peregrine Falcon is able to reach the speed of 90 ms-1 as it dives from the sky to catch its prey. Assuming that it starts from rest and accelerates uniformly, what is the falcon’s acceleration if it is able to reach 90 ms-1 after diving a distance of 100 m?

• Strategy: need to apply BOTH a = (v-u)/t AS WELL AS draw own v-t graph to solve

MCQ Qn 17

• In the novel The Lord of the Flies, the character Piggy was killed by a falling boulder. Assuming the mass of the boulder is 80 kg and the velocity of the boulder decreased from 5.0 ms-1 to 0 ms-1 in 0.40 s upon impact with Piggy, what was the force exerted by the boulder on Piggy?

• Strategy: Draw FBD, determine acceleration, let resultant force = ma.

MCQ Qn 17 • Upon impact with Piggy, FBD of the boulder is

like this:• N > mg, hence• N – mg = ma• a = (0-50)/0.40 • = 125 ms-2 (ignore -ve sign)• N – (80)g = (80)(125)• N = 1800 N

Normal Contact Force, Boy on Boulder (N)

mg

Section B, 3(b)

• “An object having zero velocity but non-zero acceleration.”

• Possible. In fact, Qn 9(a) is an example of this.• Two other examples:

– At the start of free fall, when object dropped from rest: v = 0, a = g

– Throwing an object up, at the highest height the v= 0, but a = g

Section C, 8(a)(i)

• Explain how attaching a parachute allowed the mine to explode while still in the air.

• Some suggested, increased air resistance causes the bomb to explode. This is not possible. Parachute causes bomb to go down slower, and has less wind blowing against bomb (majority of air resistance is acting on the parachute, not on bomb)

8(b)(iii)

• While the ICBM is flying, it would consume fuel and its mass would gradually decrease, while the thrust force remains constant. In the given axes below, sketch the velocity-time graph of the ICBM, assuming that it is travelling in a straight line.

• T – mg = ma• a = T/m – g (divide throughout by m)• If T remains constant and m decreases, a will

increase over time.

9(a)(i)

v/ms-1

t/s 2 4 6

2

1

-2 P

Q

Fig 9.1

0

Describe the motion of P from 2 to 6 secondsAns: MUST show appreciation that at 4 s, P changed direction of travel (moving right to moving left)

9(a)(ii)

• At time t, dancer Q meets dancer P at the same position. Determine t, given that t is not 0 s.

• Strategy: need to first determine roughly where t would be. If you guess and check 4 s, P>Q, at 6 s, Q>P. Hence 4<t<6

• Next: need to determine what velocity of P would be

9(a)(ii)

• Area under Q = Area under P

t

x

x

9(b)(iii)

• Hence, comment if this provides evidence whether Jill was indeed pushed or not pushed down the hill.

• Initial velocity = 0• Only two possibilities – Jill was either pushed by

Jack, or she tripped and fall (and rolled down the slope)

• If Jill was pushed by Jack, her initial velocity would NOT be zero. Hence, she must have tripped and fall.

10(a)(ii)• Each cable can only withstand 200N of tension

before it snaps. Is it safer to hang the gong with a large or small angle θ?

• Recall: length of arrow represents magnitude of force.

• Note also that mg remains constant. Only T varies with angle.

θ θθ

10(a)(iii)

• if mg = 200N and tension = 200N each, then vector diagram forms equilateral triangle

θ

60°

60°

60°

Return and Checking of Scripts• I WILL NOT be releasing any statistics about the this

MYE paper. (% passes, % distinctions, top student, etc.)• I don’t think these statistics are helpful to you in any

way. I want you to focus on learning Physics, not on your marks.

• I also will NOT be asking you to counter-sign against your marks.

• If you prefer, I can send you a personal email to seek your verification before I key in your marks. Please email me at my preshigh email if you would like to do this.

Collection of Scripts

Final Words• We will be giving you your holiday homework

soon. It will need some effort, but will be quite fun if done well.

• For 3D, 3E: I may or may not be teaching you from Term 3 onwards.

• If I’m not teaching you, I hope you can remember what I’ve taught you about how to learn and what’s important.