PSLE 2012 - Science
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I think we shld look at this qn within the text of P6 syllabus.
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creamyhorror:
I'm a P6 student that took PSLE this year.
The friction acting on the stationary box and surface is 0. This is actually quite clear, since there's no horizontal force acting on the box (meaning that there's no force for friction to oppose). The only controversial part IMO is, should P6 students be aware that a lack of force being applied (e.g. no one pushing the box sideways) means that there's no friction*?Monsterparent:
My dd's answer was A,B and C for the frictional force question, though I think at their level, stationary object should be taken as no friction because by definition, friction opposes motion.
Few websites mentioned about static and kinetic frictions exist for stationary and moving objects.
http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/ben_townsend/staticandkineticfriction.htm
I wonder if this is a fair and good question to ask a P6 student if it can cause controversy
* Which isn't actually true. If you consider a box sitting on a surface which is accelerating, then there is friction acting on the box, even though there's no horizontal force being applied besides friction. That shows that a lack of force applied to the box doesn't mean no friction is acting on the box.
My take on the question is that in all 3 surfaces there is frictional force acting on them. In selected websites, they mention about \"sliding friction\" and \"static friction\". Although this is not in the PSLE syllabus, consider the parallel example below:
Suppose a block is resting on a flat surface parallel to the ground. The block needs to overcome frictional force in order to move. If there is frictional force, the block cannot move.
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.
Do not get confused by other types of friction. Based on PSLE syllabus, I reason that the answer is A,B and C (Option 4)
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paranoid:
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.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.
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. -
creamyhorror:
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. There is no such thing as a insulator of heat, only very poor conductors of heat. Believe me but the so called insulators of electricity do conduct the tiniest amount of electricity, and a plant placed in green light's rate of photosynthesis may not be a complete zero.
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.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.
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.
Now that it seems that we are going into very advanced science, it seems rather surreal that this is a PSLE question... :shock: :sad: -
so is there friction when the block is resting? Static friction thought only in secondary school then learn
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Congratulations to those who had finished their PSLE Papers. Sincerely, wish everyone get good results.
:congrats:
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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.
In university you will assume there is rotatinf earth centrifugal and centripetal forces at once acting on stationary objects for any mass. Therefore be definition Fs > 0. -
Written in one if the PSLE science supplementary book : friction is force due to motion.
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I chose A,B&C for the friction one.stationary objects also have static friction in them!
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paranoid:
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.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.
(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:
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.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.
[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.
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