Mrs Lindy Lee:

I am not sure if you have their contacts too? You may want to check with them if they go to AMK side. I managed to get my son's transport arrangement with Jonathan who comes to the east.

Thank you, Mrs Lee! Talked to BT & Tan, they'll be letting me know if there's any space. Seems like their bus passing through my area is full. Your help is much appreciated.

Does anyone have a contact for a private bus operator going from Ang Mo Kio/Lentor/North region to ACS(I)? I tried Jonathan Chan (mentioned earlier in this thread), but he said he doesn’t have enough boys to run that route.


I just accompanied my brother home from school via the Circle Line, and it’s a long way. Not good in the morning if we can get a schoolbus instead.

atutor2001:

From the above 2 sites, the definition of friction are :


A force on objects or substances in contact with each other that resists motion of the objects or substances relative to each other. Static friction arises between two objects that are not in motion with respect to each other.

Friction is the force between surfaces in contact that resists their relative tangential motion. Static friction occurs when the two surfaces in contact are not in relative motion; that is, when one surface is stationary relative to the other surface.


Unfortunately, in primary school, they only emphasizes that \"friction opposes motion\". I believe the definition of friction was not taught. Neither is the existence of static friction (as given by physics definition) being explained. This gives rise to questions as to whether there is friction when the object is stationary with no lateral force being applied. However, if the definition of friction is accepted and known to every student, then there will be no ambiguity. Everyone will accept that a stationary object has static friction, as defined by the definitions on friction.

Let's not be too quick to claim that those definitions settle the issue either way.

\"A force on objects or substances in contact with each other that resists motion of the objects or substances relative to each other\"
[*] This says that friction occurs on objects in contact with each other. It does not say that friction occurs for all surfaces in contact. Phenomenon A occurs in situation B, but situation B does not always imply phenomenon A.

\"Static friction occurs when the two surfaces in contact are not in relative motion\"
[*] Similarly, this definition doesn't use \"occurs whenever\" or \"occurs every time when\", and so does not cover all cases where surfaces are in contact without relative motion. What it's actually doing is defining a negative condition (\"not in relative motion\") in order to distinguish static friction from kinetic friction.

\"Friction is the force between surfaces in contact that resists their relative tangential motion\"
[*] This is a good definition. It's what we draw upon when we say: if there is no tangential motion to resist, then there is no frictional force.

Really, there's no point in debating what the proper answer is, because it's a matter of definition. What most of us can agree is on is that a P6 student can't be fairly expected to answer the question, if (1) the knowledge to answer it is beyond the syllabus, and (2) there are grounds for alternative answers given the definition of friction that is taught.

This is the sort of question that should be considered for nullification/striking off. I appreciate AWSP's arguments against questions that may accept too narrow a range of acceptable answers: if you ask an application question that isn't clear-cut, you should be willing to accept a broader scope of reasonable answers. This is the same thing we saw in the PSLE Chinese exam: multiple MCQ options that are acceptable (e.g. 偏僻，寂寞，孤单), but probably only one correct answer based on a narrow view of what counts as \"proper style\".

A stiff-minded approach to setting acceptable answers - or setting overly ambiguous questions - is simply a bad way to set exams.

AWSP:

But the question is trying to equate stationarity with absence of forces.

Was it? Why do you think so?


I think there are two main ways a P6 student might reason out the answer:

(Shown in diagrams: There is a box on a slope, and a box on a horizontal surface.)

(1) \"Actual friction\" reasoning: The box on the slope is obviously acted on by friction, since gravity is trying to pull it down the slope. However, the box on the surface isn't being pulled or pushed sideways by anything, so it's not being forced into motion. Since friction opposes motion, friction isn't acting on it.

(2) \"Potential friction\" reasoning: Friction occurs whenever surfaces are in contact. Both boxes are in contact with another surface, so friction is acting on them. The box on the horizontal surface can resist someone trying to move it, so that shows that friction is acting on it.

The trouble is that both lines of reasoning can be valid depending on what definition of \"friction is acting\" you go by. I think we all agree that the P6 syllabus doesn't require students to know how to determine if friction is acting on an object.
[quote]My main contention has always been it is extreme testing under the guise of \"application testing\": an unnecessary evil of PSLE.[/quote]I agree with you that the question is inappropriate for P6ers, because answering the question unambiguously relies on them having the next level of knowledge (secondary physics).

I tried checking which of my friends were working in the ST Education desk but it seems most journalists have left...though it would of course be better to approach MOE itself first.

@Sun_2010: Thanks! I'll hang around the forums, if there are interesting things to discuss

shaunshaun:

也有同感！其实我觉得小学中文程度应该是小朋友可以流畅去表达情感和思想，而不是吹毛求疵！

:salute:

paranoid:

我是一张白纸，我的 （1. 志愿 2. 目的 3. 作用） 是让同学们在我身上写出美美的字，然后交给老师批改。（or something like that)
Strictly speaking, 志愿 and 目的 should be accepted, given that the passage is personifying the piece of paper. On the other hand, one can reason that the paper is still an object and the personification is just a literary device hence we can say 作用。

That's interesting. If the phrasing is like that, then I would lean towards 作用, because the 然后交给老师批改 implies it's a specific procedure that the paper intends to goes through. I feel a 志愿/愿望 wouldn't be so specific. If it were 涂鸦 I would lean more towards 志愿/愿望.

Still, the options are kind of close and depend very much on the surrounding context, so it's still a bit tough on the students. Haiz.

Mum of 3:

I do not disagree with the teacher's answers. However, if we consider this question from another angle, why is the difference in habitat an advantage when the frog and tadpole feed on different things in the first place?

Even if the tadpoles eat different things from the frogs, they'll still affect the overall food supply. Here's one possibility:


- Let's say both tadpoles and animal A eat animal B
- Let's say animal A is eaten by frogs
- So if tadpoles eat animal B, the population of animal A will be reduced
- Thus, frogs will have less of animal A to eat

So it makes sense for a species to geographically diversify and spread out, even if it eats different things in two stages of its life cycle.

Sun_2010:

Case 3:
If a force of 0N is exerted on the block ( no force) it will not move.And the frictional force on it is 0

Yup, this is the same reasoning I presented earlier. It relies on an understanding that the total forces acting on the block must be 0 for it not to move.
[quote]If the table were to be wiped with a wet cloth, ( basically coating it with a film of water ) the cup would still not move.
Even a film of oil which is a better lubricant will not move the cup

The cup moves when water is poured due to surface tension of water.[/quote]Agreed. But primary schoolers can't be expected to know that. In this example, it would be better to say that the cup can be moved with a light push once it's placed on a lubricated surface.

In the case of the cup sitting on a dry table, there's no surface tension to push it, so it can't be argued that the friction is greater than 0.

Unfortunately, all this is irrelevant from a P6 point of view. We don't know what MOE has decided to be an acceptable answer. Has anyone ever managed to ask MOE for clarification on an exam question?

paranoid:

The concept of a perfectly lubricated surface might be subject to debate, though. If we push the block on the perfectly lubricated surface, would it be in perpetual motion for ever? In this non-perfect universe, there is no such thing as a zero.

Just because something may not practically exist doesn't stop it from being a useful illustration in a thought experiment. In school physics we often rely on perfect conditions, or limiting cases, to conduct our calculations. That allows us to separate the effects of different forces/actions from each other.


(More enrichment material!)

The reason the (net) frictional force is zero is really quite simple: a non-zero frictional force would need an equal and opposite force to cancel it out if the box were to remain stationary.

1) Let's call net frictional force Ffric.
2) Net vertical forces acting on box = weight + reaction force.
3) Net horizontal forces acting on box = Ffric.
4) But the box is stationary, so net force both horizontally and vertically must be 0.
5) Therefore Ffric must be 0.

This is why if you look at webpages, they'll say that static friction varies between 0 and limiting friction. Static friction is 0 when no force is being applied, and increases to exactly balance out any applied force until it hits the limiting (max) value. Once you're past that point, the object starts to slide.

ultimateschooler:

I am a Physicist. The answer can be argued both wàys in theory but in generàl students in p6 are taught that static friction is present even though Fs can be zero.

If school lessons aren't precise about when exactly \"friction is acting\", then it's not fair to ask a student that question in an ambiguous scenario. I didn't find any material in the PSLE Ultimate Science Guide that covered stationary objects, or a definition that settled the issue one way or the other.
[quote]Therefore be definition Fs > 0.[/quote]Fs is a limiting (maximum) value for static friction, not the actual frictional force (which varies from 0 to Fs).

In the end this comes down to a matter of phrasing and interpretation. Static friction can potentially occur for a stationary box on a stationary surface, but it doesn't actually occur until some sort of force is applied to the box (or the surface moves).

I wonder if SEAB takes feedback on this sort of matter? They ought to, since it could help to improve the quality of their question setting.

paranoid:

Back to the question. If the block is not moving, that means it could not overcome the frictional force between the horizontal surface and the block. Hence goes to show that there is frictional force in all setups.

The block only needs to overcome frictional force if there is an attempt to move it. Imagine a block on a perfectly lubricated surface. There is practically no friction between the block and the surface, yet the block may not be moving at all - it might just be sitting there, stationary. In this case, there's no friction to overcome, yet the block isn't moving. Therefore, friction is not necessarily acting on a block that isn't moving.


In my opinion, the issue here is whether

\"friction is acting on the block\"

should be interpreted as

(1) \"friction can act on the block\", or
(2) \"the frictional force acting on the block is greater than 0\"

I don't think P6 students can be expected to answer this directly without clarification in their textbooks. I couldn't find specific information on this topic in a PSLE science guidebook. I lean towards interpretation (2), but that's based on my knowledge of physics.

@atutor2001: Thanks! Physics puzzles are fun to think about.

verykiasumummy:

so does it mean that the heavier object will reach the ground first no matter whether is rolling down the ramp or free falling from a height??

That's what I think, but I haven't done calculations for the rolling case to confirm it. The basic idea from interpreting that last equation I gave is that air resistance depends only on speed, not mass. Therefore, comparing objects at the same speed, heavy objects can \"overcome\" air resistance more easily than light ones - they're slowed down less by air resistance. So they accelerate faster than light objects.

Still, it's important to keep sight of the original principle, which is that objects fall (accelerate) at the same rate, regardless of their masses.

atutor2001:

Thank you for the enlightenment. You are correct that rolling friction force does depends on mass and should cancel off. So if 2 objects of the same surface texture but of different mass roll down a ramp, the time to reach the bottom would be the same. The time taken should only depends on the coefficient of rolling friction. That is, if their surfaces are different, then the time taken will not be the same.

I see, this is interesting to know.
[quote]However, for free falling object, I tried googling and it appears that drag force is not dependent on the mass of the ball.[/quote]Intuitively, I'd guess that drag force wouldn't be dependent on mass, because drag occurs at the surface of the object, where the fluid particles are hitting the surface. The mass of the whole object therefore shouldn't matter. If the object were moving sideways (like an airplane), the drag force would depend on how fast it was hitting the air particles, not on how massive it was.

Sorry ah, I go ahead and work this out a bit. Once you combine drag force (Fdrag = -kv^2) with the pull of gravity, you get

net force = ma
Fgrav + Fdrag = ma
mg + Fdrag = ma
g + Fdrag/m = a
g - kv^2/m = a (where k is a constant)

Reading the above equation: For any particular velocity v, a greater m implies a greater acceleration. In other words, at any speed, a more massive object has a greater acceleration than a less massive one. A greater acceleration means the object speeds up faster. Thus, when air resistance is added into the equation, heavier objects actually fall faster (assuming all else, e.g. shape/area, is held constant).

I think this will extend to objects rolling down ramps as well. If air resistance is significant, then massive objects should roll faster. The effect will probably be pretty small unless you're rolling things down the Great Pyramid of Egypt.

This is really far from P6 Science, but I hope fellow posters will forgive us since it's after PSLE and no one will be doing much discussion for some time. :oops:
[quote]Well it is beyond me to understand such thing now. Thanks for giving a knock on my rusty brain.[/quote]No worries, interpreting formulae is often tricky and it took some thinking on my part too. It's quite fun sometimes.