PSLE 2012 - Science

[fantasy1]

I just got this information from yahoo website.

‘A book resting on a flat table, the frictional force is zero.There is no force trying to move the book across the table,so there is no need for a frictional force because there is nothing for the frictional force to oppose.’

[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

[creamyhorror]

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

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*?


* 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.

[fantasy1]

I think we shld look at this qn within the text of P6 syllabus.

[paranoid]

creamyhorror:

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

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*?


* 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.

I'm a P6 student that took PSLE this year.
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)

[creamyhorror]

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.

[paranoid]

creamyhorror:

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.

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.
Now that it seems that we are going into very advanced science, it seems rather surreal that this is a PSLE question... :shock: :sad:

[me-is-kiasu]

so is there friction when the block is resting? Static friction thought only in secondary school then learn

[me-is-kiasu]

Congratulations to those who had finished their PSLE Papers. Sincerely, wish everyone get good results.

:congrats:

[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.


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.