Tutor Niedino: Pri Science Questions and Concepts

[Tang]

yongjianrong:

Just found some confusing concepts.


1. When something is at a greater height, it has more gravitational potential energy but the gravity pull is weaker, why is that so?
No, gravity pull is constant wherever you are as it depends on the mass of Earth which is a constant.

2. Do we consider 'air resistance' as 'friction'?
Yes. The object is in contact with air which slows it down.

3. a) When duckweeds cover the surface of pond completely, do they literally mean that there is no gap between each duckweed?
Don't quite understand

b)If so, will fishes suffocate? I have one revision paper which says the fish will suffocate. However, when duckweeds photosynthesise, they will produce oxygen and the oxygen will also exit through the stomata on the underside of the leaves followed by entering the pond. So how can the fishes suffocate?
Duckweeds are floating plants, so oxygen released during photosynthesis does not necessarily be dissolved into the water.

Hi,

See my comments in bold.

[yongjianrong]

Tang:

yongjianrong:

Just found some confusing concepts.


3. a) When duckweeds cover the surface of pond completely, do they literally mean that there is no gap between each duckweed?
Don't quite understand

Hi,
See my comments in bold.

What I meant was:

When a question says 'Duckweeds cover the surface of the pond completely', does it want pupils to think that the pond is covered completely without the SLIGHTEST gap, or are there gaps for oxygen (from above water) and Sunlight (from Sun) to enter the pond?

[atutor2001]

1. When something is at a greater height, it has more gravitational potential energy but the gravity pull is weaker, why is that so?


Energy is NOT force.
Energy is the \"ability to do work\"
Force is \"something\" that can change the state of motion, shape, position of an object.

Energy = Force x Distance travelled by the object (lets not talk about heat energy)

The gravitational potential energy of an object is the amount of energy \"stored in the object due to the existence of gravitational force\". The amount of such energy that an object has depends only on its distance from the centre of the Earth (and its mass).

The gravitation force acting on an object becomes smaller when it is further away from Earth. This means that the Kinetic Energy that an object gained (which is converted from PE) when it falls through a distance of 1km at a height of 1000 km from the earth will be less than the kinetic energy gained by an object that falls through also a distance of 1m, but at a height of only 100km from the Earth. However, the total PE of the object at 1000km is definitely more than when it is at 100km above the Earth as the distance travelled by the object in the 1st case is 1000km while in the 2nd case it is only 100km.


3. a) When duckweeds cover the surface of pond completely, do they literally mean that there is no gap between each duckweed?

Realistically should not be, there will sure to be gaps.

b)If so, will fishes suffocate? I have one revision paper which says the fish will suffocate. However, when duckweeds photosynthesise, they will produce oxygen and the oxygen will also exit through the stomata on the underside of the leaves followed by entering the pond. So how can the fishes suffocate?

For floating plants like duckweeds, the stomata are found on the upperside of the leaf, not the underside as in normal leafs. Therefore, the oxygen released will not enter the water.

The fish suffocate because the RATE of introduction of dissolved oxygen into the water becomes much slower than the RATE of consumption of dissolved oxygen (which actually did not change).

Dissolved oxygen comes from oxygen entering through the water surface - which is greatly reduced because the surface is covered by duckweed

Dissolved oxygen also comes from oxygen released by submerged plants - which is also greatly reduced because most of the sunlight is blocked (i.e. reduced photosysnthesis)

However, the amount of oxygen consumed by the living organism in the pond remains the same and soon all the dissolved oxygen in the water will be depleted because OUT is faster than IN.

[atutor2001]

deleted sorry double posting

[elkniwt]

Hi,


There's this qn in the exam asking for the conditions for germination. Though it is a very basic qn, my dd got it marked wrong. Her answer is water, oxygen and warmth. Teacher says carbon dioxide is also needed hence no marks for her. When I surfed web, the websites says water, oxygen and warmth only. My dd says she's very confused cos what mummy say is different from what teacher. P3 teacher and P4 teacher also say different things.

[starlight1968sg]

elkniwt:

Hi,


There's this qn in the exam asking for the conditions for germination. Though it is a very basic qn, my dd got it marked wrong. Her answer is water, oxygen and warmth. Teacher says carbon dioxide is also needed hence no marks for her. When I surfed web, the websites says water, oxygen and warmth only. My dd says she's very confused cos what mummy say is different from what teacher. P3 teacher and P4 teacher also say different things.

During germination, the seed has no leaves and thus can't make food. If it can't make food, it does not need carbon dioxide.

[atutor2001]

elkniwt:

Hi,


There's this qn in the exam asking for the conditions for germination. Though it is a very basic qn, my dd got it marked wrong. Her answer is water, oxygen and warmth. Teacher says carbon dioxide is also needed hence no marks for her. When I surfed web, the websites says water, oxygen and warmth only. My dd says she's very confused cos what mummy say is different from what teacher. P3 teacher and P4 teacher also say different things.

God bless your dd. She gets the \"BEST\" science teacher if the question is limited to just germination.

However, sometimes they love to test \"England\" in science paper by adding a phrase \"and grow\" behind, which most students usually ignore. It there is \"grow\", then carbon dioxide, light are also needed. Maybe you would like to read the question again.

[Full.010894Cream]

hmmm..how come this thread is not maintained by the thread starter??! :?

[Tang]

elkniwt:

Hi,


There's this qn in the exam asking for the conditions for germination. Though it is a very basic qn, my dd got it marked wrong. Her answer is water, oxygen and warmth. Teacher says carbon dioxide is also needed hence no marks for her. When I surfed web, the websites says water, oxygen and warmth only. My dd says she's very confused cos what mummy say is different from what teacher. P3 teacher and P4 teacher also say different things.

Hi,

Unless the chapter 'Air' has been taught, there is no reason to state 'Oxygen' for germination.


So answer - Air, water (moist) and warmth - is the correct answer.


If diagram shows seedlings with leaves, then for the seedlings to survive, air, water and warmth will be required and not oxygen, waterr and warmth.

[elkniwt]

Hi,


Qn just says germination. I really think teacher should not mark her wrong. dd felt that she is being wronged for being specific. [Like to ask the teachers in the forum, is it wrong to write oxygen?]

I agree that writing air is good enough, but the reason teacher gives her for marking it wrong is that carbon dioxide is also needed for germination… ??