PSLE 2012 - Science
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hi all, just to share my two cents worth…
in many of the sci guides, friction to a P6 student is known as a contact force, that is, when 2 surfaces are in contact, they are considered to have frictional force acting between the 2 surfaces. therefore, regardless of whether the object is stationary or moving, frictional force exists and is acting on them…
i came across a question to explain this. when u put a plastic cup on a clean and dry table, the plastic cup stays there and does not move. however when u pour some water over the table top and put the same plastic cup on the water, it moves!! this explains that frictional force is acting on the plastic cup and dry tabletop so that it does not fall off (also because no external force acting to move it). but when a lubricant such as water is used, the plastic cup moves because the frictional force between the cup and table top is reduced, hence the mass of the cup cannot overcome the frictional force and move.
i’m not good in explaining in physics terms or equations that some physicists here can interpret… i’m only looking at it as how i always explain to my dc… -
Hi , My take
Frictional force is a force that opposes motion.
Lets us say 10N force is needed to move an object on a flat surface.
Case 1:
If I exert a 1 N force on a block it will not move. Why? because of the frictional force that opposes its motion.
How much frictional force is present? It should be equal to the force exerted on it. So here the frictional force is 1N
Case 2:
If a force of 8N is exerted on the block it will not move. Why? because of the frictional force that opposes its motion.
How much frictional force is present? It should be equal to the force exerted on it. So here the frictional force is 8N
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 0verykiasumummy:
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.
i came across a question to explain this. when u put a plastic cup on a clean and dry table, the plastic cup stays there and does not move. however when u pour some water over the table top and put the same plastic cup on the water, it moves!! this explains that frictional force is acting on the plastic cup and dry tabletop so that it does not fall off (also because no external force acting to move it). but when a lubricant such as water is used, the plastic cup moves because the frictional force between the cup and table top is reduced, hence the mass of the cup cannot overcome the frictional force and 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. -
Mum of 3:
Even if the tadpoles eat different things from the frogs, they'll still affect the overall food supply. Here's one possibility: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?
- 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:
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.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
[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? -
creamyhorror:
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?
do u think those telephone operators will know how to answer?? alamak... -
Frictional force will be there as long as there is contact between 2 objects.
Frictional force is a function of the 'normal' force between the 2 surfaces, ie,
Ffriction = coefficient of friction x normal force acting on the surfaces.
Therefore it is only dependent on whether there is contact or not,or if coef = zero. -
For the question ‘xxx removed a part of the flower such that the flower stopped producing seeds’, was this in in book A? If in book A, is ‘ovules’ among one of the options to select?
My dd said she forgot already, Guess she did not have mood to discuss… -
AWSP:
It is in booklet A.For the question 'xxx removed a part of the flower such that the flower stopped producing seeds', was this in in book A? If in book A, is 'ovules' among one of the options to select?
My dd said she forgot already, Guess she did not have mood to discuss...
No, ovules is not an option.
The options are: Stigma, Anthers, Petals, and 1 more which I forgot. :l -
Regarding the toad and tadpole question, is it acceptable if the student wrote "so that the frog will not eat its own eggs and offsprings"
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Haneyi:
They are Stigma, Anthers, Petals and Sepals.
It is in booklet A.AWSP:
For the question 'xxx removed a part of the flower such that the flower stopped producing seeds', was this in in book A? If in book A, is 'ovules' among one of the options to select?
My dd said she forgot already, Guess she did not have mood to discuss...
No, ovules is not an option.
The options are: Stigma, Anthers, Petals, and 1 more which I forgot. :l
Stigma is the correct answer. :salute: -
Nebbermind:
That formula is for the maximum frictional force . That is the value of the force above which if exerted on the object it will start to move.Frictional force will be there as long as there is contact between 2 objects.
Frictional force is a function of the 'normal' force between the 2 surfaces, ie,
Ffriction = coefficient of friction x normal force acting on the surfaces.
Therefore it is only dependent on whether there is contact or not,or if coef = zero.
Please refer to the problem 1a. I think it clearly states
http://web.mit.edu/8.01x/www/probsets/sol4.pdf
In the case of the object on the slope, yes there is a frictional force acting on it since it as it opposes the gravity acting on it.
But for the object on a flat surface, the normal force acts against the gravity. Frictional force doesn't come into play.
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