PSLE 2012 - Science
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AWSP:
Well, there is really no need for apologies. This is a forum where different and complementary views are shared.
I am sorry to see the answer as such. What is normal force? (does the full force acts on it?) what is force opposing force? A garbled reasoning is as good as a wrong answer. The danger is as what atutor2001 posted about \"bringing forward a wrong concept into later stages of learning\".MeKiasuLeh:
IMHO, a student could be expected to tackle this MCQ question in this manner:
Picture (c) - object moving down a ramp or slope
As stated, the object is in motion. This means that kinetic friction is at play because the frictional force is acting against the object zooming down the ramp/slope. Both the weight of the object and gravity contributed to the normal force. Therefore, there is force opposing force.
Picture (b) - stationary object on a ramp or slope
As stated, the object is stationary on a ramp or slope. So why did it not slide down like the one shown in picture(c) where the weight of object and gravity combined will generate the normal force? Here static friction is at play because the normal force is not able to overcome the frictional force. Thus the object stays where it is on the ramp/slope. Therefore, there is force opposing force.
Picture (a) - stationary object on level ground
Unless the object is a ball or an object without complementary flat surface to the ground it is resting on (i.e. can be affected by factors such as wind, etc), it is viewed as an object resting motionless on the level ground. The keyword here is 'level'. The full weight of the object is being pulled down by gravity alone. There is contact sure, but no force opposing force.
Therefore, (b) and (c) is the answer.
:
The issue which I am pushing for is not about marks. The ambiguity of the question is not removed in this question. That is the fault of the setter. And further to that the fully correct answer can only be derived by drawing on knowledge of vectors and static equilibrium. Not by the few rules given to the students in P6. Are the teachers able to articulate the concept to students when they teach without drawing upon higher concepts like force vectorisation? Is the setter taking this into consideration when he set the question?MeKiasuLeh:
The big question goes beyond this exams. Questions like the frog and tadpole are another cause for concern. What is the expectation in the marking scheme? Is the examiner going to mark all else wrong unless it fits into a certain concept that is deemed as the only correct concept to apply? Where a long list of answer abounds?Unfortunately, students and parents of their DCs whose answer is other than the above will likely to disagree. Conversely, those whose answer is similar to the above will agree. I suppose this will always be the case even when we obtained the official answer. This is life, and perhaps life of a kiasu parent.
Could we allow this trend to continue for next year and so on? I heard from my child that some of his classmates cried. So what if everyone gets the same marks for the flawed questions, can we compensate for those who are so affected by these question that they did not perform to their standard?
@ultimateschool proposes that \"force opposing force\" is Newtonian law. Absolutely correct. Perhaps \"force opposing force\" can also be a crude definition of friction where one surface, in this case the surface of the object (which I presumed also has a flat surface) is going against the surface of the level ground/slope/ramp. The force as exerted to move the object in one direction is being opposed by the frictional force (kinetic friction) as depicted in picture(c) to prevent it from moving (keyword here is \"is moving\") too quickly to the end of the slope, and the frictional force (static friction) as depicted in picture(b) to prevent it from sliding down (keyword here is \"stationary\") the slope.
Like I say, not everyone and like-minded people who hold a different view from mine will agree with the stuff I put forward no matter how hard I try. Likewise, those who concur with me would somewhat or wholly agree. But I tried, and also try very hard to avoid using jargons and formulas. Force opposing force? Well I hope it is just about sharing different views.
I really do not find this question flawed or ambiguous. Perhaps the problem is with the word \"stationary\" where many jumped to conclude static friction is present. Whatever the case may be, there will always be PSLE Science questions where many will deem as flawed and the like because of differing views. And this perception, even if official answers are made available (though unlikely before PSLE marking), will ever change.
I know it hurts to see children cried. Many aspiring Acers who have put in tremendous effort and given their all would probably do so. Sadly, this happen in every PSLE and in almost any circumstances, not just \"flawed\" questions. If this situation or this question really bothered you so much, then it will serve you better to consult the relevant organisation, which in this case is MOE I think.
But there is no cause for alarm for those who hold a different view because I gathered from this forum that a physicist and some teachers believe/profess the answer should also include picture(a). I am neither a physicist nor a teacher and, more importantly, not from MOE. So there you go...
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Lets settle the friction question with help from google. Below are 2 sites that provided the definition of fricition :
http://science.yourdictionary.com/friction
http://physics.info/friction/summary.shtml
In the study of physics, definition is very important and must be strictly adhered to so that everyone is engaged in a discussion with common understanding of each term used.
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. -
atutor2001:
Let's not be too quick to claim that those definitions settle the issue either way.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.
\"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. åå»ļ¼åÆåÆļ¼å¤å), 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. -
MeKiasuLeh:
***************
Yes, I have no doubt there will still be tons of views even after students verified their teachers. We just have to hope for the HOD for Science of schools pursuing this question on behalf of students will be proactive to seek clarification from MOE for the official version.verykiasumummy:
however the problem is, which parent / p6 student will go clarify with their teacher now?? they are all enjoying and relieving themselves...
and again, even when a parent or student volunteers to ask their teacher, will the sci teacher give us a wrong answer since it seems to have so many different perceptions?
come to think of it, after reading the views of the friction question from so many \"experts\" here. will u agree to any of the answers that the teacher gives? that is, if the teacher says, \"yes, friction exists in all surfaces of contact so all of the set ups have friction\" --- will all here agree??
and if the teacher says, \"no, friction only occurs when there is motion force, since all objects are stationary, there is no motion force to oppose in the stationary set ups A and B so there is no friction between them and only in C.\" ---- will all agree this instead?
i have guts feel that the question posted should be whether gravitational potential energy exists in all 3 set ups, not frictional force.
re-quote the question posted by mum of 3 :
I believe the MCQ question that has been hotly debated is whether frictional force acts on the object shown in 3 pictures which are : A) stationary object on level ground B) stationary object on a ramp or slope C) object moving down a ramp or slope. The choice of answers includes (i) A, B & C (ii) B & C (iii) C only - not in order.
IMHO, a student could be expected to tackle this MCQ question in this manner:
Picture (c) - object moving down a ramp or slope
As stated, the object is in motion. This means that kinetic friction is at play because the frictional force is acting against the object zooming down the ramp/slope. Both the weight of the object and gravity contributed to the normal force. Therefore, there is force opposing force.
Picture (b) - stationary object on a ramp or slope
As stated, the object is stationary on a ramp or slope. So why did it not slide down like the one shown in picture(c) where the weight of object and gravity combined will generate the normal force? Here static friction is at play because the normal force is not able to overcome the frictional force. Thus the object stays where it is on the ramp/slope. Therefore, there is force opposing force.
Picture (a) - stationary object on level ground
Unless the object is a ball or an object without complementary flat surface to the ground it is resting on (i.e. can be affected by factors such as wind, etc), it is viewed as an object resting motionless on the level ground. The keyword here is 'level'. The full weight of the object is being pulled down by gravity alone. There is contact sure, but no force opposing force.
Therefore, (b) and (c) is the answer.
Unfortunately, students and parents of their DCs whose answer is other than the above will likely to disagree. Conversely, those whose answer is similar to the above will agree. I suppose this will always be the case even when we obtained the official answer. This is life, and perhaps life of a kiasu parent.
The problem with so many answers being given are due to how frictional force is being defined in various textbooks and how the teachers/tutors have explained.
To say frictional force exists only when there is motion is partially right. Frictional force is a force that comes about to oppose motion when there is tendency of motion. It does not mean the body has to move before there is frictional force. This means friction occurs in (b) and (c).
It is also not 100% correct to say that friction exists when there is contact. In the case of (a) where the ground is level, there is no tendency of motion at all and hence there is no friction.
In case you would like a more mathematical explanation to the above answers, it is easy to draw the force diagram for (a). The only forces acting on the object are its weight (downwards) and a normal contact force (acting upwards). In equilibrium, these 2 forces will cancel each other out. There is no horizontal force and hence there is no frictional force to cancel the horizontal force.
For (b), its weight can be resolved to 2 forces, namely a force perperdicularly into the slope(W1) and another force down along the slope (W2). The normal contact force is acting perpendicularly up to cancel W1. Friction comes about to oppose W2. There is no motion because friction has not built up to a maximum value that is equal to frictional coeff x W1.
For (c), the slope is tilted more and W2 has increased to a point where it is greater than the maximum frictional force as explained above.
I hope this helps explain the answer. For P6 level, the mathematical explanation is way beyond them. However, the explanation of friction existence to oppose tendency of motion is a concept that they can understand only if the teachers/tutors have explained appropriately. -
Adding on to MeKiasuLeh's reply,
MeKiasuLeh:
This setup is not in equilibirum. Therefore, if you draw a http://en.wikipedia.org/wiki/Free_body_diagram, u will have one force acting normally on the object by the slope, another (gravitational) force pointing vertically down from centre of the object, and another one parallel to the slope, pointing from the object to the end of the slope. As there is no info on whether the slope is frictionless or not, we may or may not have a (frictional) force component acting upwards along the slope due to the object accelerating down the slope.
Picture (c) - object moving down a ramp or slope
As stated, the object is in motion. This means that kinetic friction is at play because the frictional force is acting against the object zooming down the ramp/slope. Both the weight of the object and gravity contributed to the normal force. Therefore, there is force opposing force.
So no way to tell if there is frictional force or not, unless we assume that any surfaces in contact should have some and frictionless contact does not exist.MeKiasuLeh:
SInce this is in equilibrium, the diagram will not have the component pointing downwards parallel to the slope since there is ZERO acceleration downards. Instead it will have a frictional force component pointing upwards parallel to the slope.
Picture (b) - stationary object on a ramp or slope
As stated, the object is stationary on a ramp or slope. So why did it not slide down like the one shown in picture(c) where the weight of object and gravity combined will generate the normal force? Here static friction is at play because the normal force is not able to overcome the frictional force. Thus the object stays where it is on the ramp/slope. Therefore, there is force opposing force.
The three forces, frictional, gravitaional and normal reaction will add up vectorially to ZERO.MeKiasuLeh:
Since this diagram has only 2 force components, one = gravitional force on the object, and second = equal and opposite normal reaction of surface to object, and it's in equilibrium, there is no other force component so no frictional force.
Picture (a) - stationary object on level ground
Unless the object is a ball or an object without complementary flat surface to the ground it is resting on (i.e. can be affected by factors such as wind, etc), it is viewed as an object resting motionless on the level ground. The keyword here is 'level'. The full weight of the object is being pulled down by gravity alone. There is contact sure, but no force opposing force.
Therefore, (a) is confirmed OUT, (b) is confirmed IN, (c) is 'cannot tell' unless some assumption is made. -
Nebbermind:
Trust me, this would be my last contribution to the MCQ question on friction. I will move on to provide my take on the next contentious question regarding frog/tadpole.Adding on to MeKiasuLeh's reply,
This setup is not in equilibirum. Therefore, if you draw a http://en.wikipedia.org/wiki/Free_body_diagram, u will have one force acting normally on the object by the slope, another (gravitational) force pointing vertically down from centre of the object, and another one parallel to the slope, pointing from the object to the end of the slope. As there is no info on whether the slope is frictionless or not, we may or may not have a (frictional) force component acting upwards along the slope due to the object accelerating down the slope.MeKiasuLeh:
Picture (c) - object moving down a ramp or slope
As stated, the object is in motion. This means that kinetic friction is at play because the frictional force is acting against the object zooming down the ramp/slope. Both the weight of the object and gravity contributed to the normal force. Therefore, there is force opposing force.
So no way to tell if there is frictional force or not, unless we assume that any surfaces in contact should have some and frictionless contact does not exist.
Go to this link http://en.wikipedia.org/wiki/Frictionless_plane to understand more about frictionless plane (surface), a concept from Galileo for predicting the motion of an object moving down an inclined plane. I understand this is beyond PSLE, and I have tried extremely hard not to cloud the discussion with topics way beyond P6 Science; no jargons and no formulas. But I thought it useful that I present this URL so that interested people will find this statement:
\"There are no frictionless planes anywhere to be found; therefore (typo error found!), by definition, cannot be\".
One can interpret however one wants to see it (this definition, this question, etc), but I will close my case. -
creamyhorror:
Absolutely agreed.
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. åå»ļ¼åÆåÆļ¼å¤å), 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.
Of all the answers, I saw all with the exception of atutor2001 were articulated with a big dose of secondary school physics. I can never expect a p6 to articulate his reason coherently using the little bit of simplified concepts he is thought.
And Oh gosh, I will spend top money for the teacher who can help my kid to answer this type of question with that little bit concept... :rotflmao:
Something for Mr Heng to think about since he is revamping PSLE :?
Starting with Science
Great Job! -
I have a little parody on the question on friction.
A teacher is teaching students John and Mary how to answer a question on friction. (Reference a,b,c).
Teacher: In c, the object is moving down the slope because the gravitational force is greater than friction. Whereas, in b, the gravitional force is less than friction so it is not moving.
John: But teacher, I thought gravitational force is the same no matter how steep is the slope. Does it mean that there is greater gravitational force when we stand on the top of the stairs?
But the teacher cannot teach vectorisation of forces.
Teacher: No, gravitational force is the same on top of the stairs as below. But on the slope, it is greater. TRUST ME. This is PSLE science WANTS you to know. Remember you slide down easier when the slope is steeper.
Teacher: Now in a, there is no movement so there is no friction.
John: But in b, there is also no movement so there is no friction.
Teacher: In b, the gravitational force is greater than on b. TRUST ME. This is PSLE.
During PSLE, the same question came out. (Note this is only a hypothetical play).
John asks Mary: Did you get the question on friction correct? I got confused because I start asking the same questions again.
Mary: Yes, I got it right with a lot TRUST. -
creamyhorror:
Off topic just discussion on language and interpretation.
....atutor2001:
...
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.
\"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.
I have come to my conclusion because of the words \"arises\" and \"occurs\" that were used in the definitions. These words are in simple present tense.
When simple present tense is used without any \"time qualification\" such as \"everyday\", \"now\", \"fortnightly\", as in the above definitions, it means that the information is \"always true\" or \"generally true\". Therefore, the requirement of \"whenever or every time\" is implicitly implied through the use of \"simple present tense\" for the verb.
Definitions in science are specific so I don't think it intends to mean \"generally true\" when the definition uses simple present tense. Otherwise, there will be qualifications like \"when there is an applied force to start motion\" in the definition to highlight conditions whereby the situation is not true.
Just to clarify on my interpretation. -
IMHO, @verykiasumummy and @atutor2001 answers to the frog/tadpole question would readily be acceptable. I did not read all threads so there might be more worthy ones.
This is a booklet B question, and I believe most questions in booklet B require students to demonstrate their knowledge of Science, it's application, as well as reasoning skill. And for this reason, PLSE markers will be asked to mark \"holistically\", a terminology used widely in the answer key found in RGPS exams marking scheme. Probably RGPS uses the same format (answer key) as MOE or vice versa. I also believe, like in most schools, each paper is marked then verified by another before the final score is set in stone.
How do one tackle such a question? First, we know its biology we are dealing with and one of the first things that should come to mind is that a tadpole is the young of a frog and a frog is an amphibian, both of which are living things that need a conducive habitat to survive in. Next, we ask ourselves a series of questions e.g. Do they live on their own? Does one need to know about life cycle of a frog? Where is the food source of a frog and tadpole? Also, do we need to consider the stages that a tadpole has to go through before being an adult? Articulating the answer would be the challenge but I believe keeping within these premise should be safe and it is best to keep the answer within a natural setting (i.e. avoid throwing disasters, like fire, drought, etc into the mix).
I am inclined to answer this way .....
A frog is an amphibian which lay eggs on water, leaving their young, the tadpole, to survive on their own in its life cycle. Frogs feed mainly on insects and other small creatures that can be found mostly on land whereas tadpoles feed primarily on plants, mainly aquatic plants which can be found mostly in water. Frogs are more agile on land than on water, and this helps them avoid predators and find food better, thus ensuring their survival. On the other hand, tadpoles, which do not resemble their adults particularly in their initial stage of development, has no or ineffective limbs. Therefore, tadpoles are inherently adaptable to water than on land, allowing them to move better to find food and avoid predators. These are the advantages for frog to live on land and for tadpoles to develop into adulthood in water.
How much to write? I will use the space provided in the booklet as a guide. Now, I truly do not know what the right answer is, or will my proposed answer be acceptable. It might just managed to elicit a sly smile from the marker but no marks! But so long as a student keep it within a logical framework and mentioned key things like \"survival\" and \"Food source\" and relate these to land and water and demonstrate what habitat means, I believe he/she should be safe.
For those die die must know 'the right answer' then I suggest he/she tap on VitaMindZ. I heard the folks there used to set PSLE Science questions and the top PSLE girl last year had her tuition in that centre. But for me, I believe a student should practice using his/her own intuition, learn to perform own research, interact with fellow students and consult teachers/parents as much as they can for the real challenge is after P6.
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