- Six Diligence 2010

Friday, September 17, 2010 ; Questions you asked

Dear pupils,

Here's a list of questions asked by various students of 6D.

If it was a ball, then the lane goes up and down, at which point would the ball have the highest kinetic energy?

As long as the lane hasn't ended, the lowest point has the highest KE.

What if the lowest point is the point that is going upwards, like a roller coaster going up and down, what if the lowest point is going up?

If it goes up, KE would have converted to GPE, hence lower KE. So, the lowest point still has the highest KE. At the lowest point before heading up, height from the ground is the lowest. So, there is very very little GPE. That means the rest of the total energy is KE. It has not gone up yet and so GPE is still very low. KE is usually the opposite of GPE. So the lowest point has the lowest GPE and the highest KE.

In a river, what causes the water to be more acidic or have more alkaline in it?

Acidity of water is caused by pollutants. Salt can cause water to be alkali. But these are not in your syllabus, except acid rain. Burning of fuel can cause rain to become acidic and the acid rain can wash into rivers.

When liquid turns to solid, will it increase in mass? Will the volume change too? (Not in syllabus)

For the liquid become solid question, it's out of syllabus to know volume.

For your knowledge, density measures the ability to float and density = mass/volume

So when density decreases (float), volume increases. For water to become ice, the density of ice must be lower than water to float on water. So the volume of ice is higher than water in order for ice to have a lower density. Remember that mass remains unchanged.

However, for most other liquid, volume will increase when they freeze and so when they become solid, their density increases and causes it to sink.

If you put soil in a container, then some water on top, could the air between the soil escape?

There will always be space between the soil particles. Just that, when you add water to soil, clayey soil in particular, the soil particles stick together and there is very little space for air to escape from the soil.

For a setup where they hang a rubber band on a stand and add weights on it, can we can they the pull of the weights overcomes the push of the rubber bands? because for every pull there is a push. But i don't get how the rubber bands are giving the weights a push... Isn't it just elastic potential energy or something?

Gravity is the pull acting on the weights and the rubber band. At the same time, elastic spring force from the spring is pulling the weights back. As the spring is stretched, it increases in elastic potential energy. EPE is not a force. A high EPE can convert to a high KE, which can result in higher impact (force).

And for water plants, is it true that they will die on land as they can only take in dissolved oxygen during respiration? And water plants takes in dissolved oxygen, do they also take in dissolved carbon dioxide during photosynthesis and give out dissolved carbon dioxide during respiration?

Water plants take in dissolved carbon dioxide and gives out dissolved oxygen during photosynthesis. They take in dissolved oxygen only as part of respiration but respiration is a 24-hour process.

And since only water vapour can condense, isn't it wrong to say that ''tiny water droplets have formed (condensed) on the outside of the glass"? Because that was the answer given in my assesment book. I thought it should have been "Water vapour condensed on the outside of the glass to form tiny water droplets"

It's wrong to say water droplets have condensed but correct to say that water droplets have formed or "have formed after condensation".

So when the plants transpire is it like humans,--water is a by product of respiration? And during transipration it is losing water vapour but why is ''lost water'' accepted?

Unlike breathing which focuses on the gas exchange, transpiration focuses on the loss of water vapour from stomata. You're right to relate it to respiration as respiration takes place in plant cells, carbon dioxide and water are by-products of respiration and these will be released in gaseous form from the stomata. Just that the process of water vapour exiting the stomata is known as transpiration. The plant "lost water" in the form of gas through its leaves. This is a generally accepted term.

Is it true that gravity is the same regardless of height from the ground? Is it different from the concept on the heavier the mass of the object, the higher the gravity acting on it?

On gravity, both are true. Gravity is the same regardless of height because our mass does not increase as we climb higher and higher

There is a mathematical way of remembering gravity.

Gravity is a force. The mathematical formula for it is Force = mass x acceleration, F = ma
where Force is gravitational force, mass is mass of an object, acceleration is the measure of change of speed of a falling object and acceleration is the same for all objects on Earth if the object has no air resistance.

Effectively, gravity is only affected by mass and air resistance.

Height comes in when you deal with GPE as GPE = mass x acceleration of free fall x height from ground. or GPE = mgh

That's how I remember gravity as I'm more mathematical than scientific.

For friction, is it a pull or push?

Friction opposes motion. We usually push ourselves forward, so friction is pushing against us.

Do the plants xylem and phloem tube run through the fruit and flower?

Yes, they do. The cell part vacuole is made up of plant sap and primarily contains water. This is, however, not in the syllabus.

When soil erosion blows the top layer of soil away, why does the land become dry and hard and impossible to grow new plants on?

Topsoil are rich in nutrients and soil particles that are not eroded are either larger in size (pebble or sand like) or of a clayey nature. Hence without the nutrient-rich topsoil, the remaining soil is not suitable for plant growth but there'll still be highly adaptable plant that grows on them. With the remaining larger soil particles, it's easier for water to trickle deeper into ground water or evaporation on the surface to take place.

You're not required to describe why the soil is dry.

Are we required to know the different types of veins and the different things in blood like platelets and plasma?

Absolutely not. You only need to know what the blood carries, from where, and to where. Only 4 things: digested food, oxygen, carbon dioxide, waste material.

How is it there can be 2 pushed and 2 pulls. I thought theres this law saying for every pull there will be a push?

It's a matter of perspective. Let's say we said that gravity is pulling us. Actually the direction of the force or arrow is pointing away from us. To gravity, it is pulling us. But to us, we're pushing the ground. So gravity's pull is same as us pushing the ground.

While gravity is pulling us to the ground, to gravity, we're pushing the ground too.

For friction, it is a force that travels towards us in terms of direction. So, we feel a pull from friction but to friction, it is pushing us.

It's not that important to differentiate push and pull, more important to know what the directions of the forces are when they act on objects. Eg, gravity is always pointing towards the ground and friction is always pointing the opposite direction of motion. However, direction of forces, sadly, is not in primary school syllabus. Yet, it makes people understand how force works better.

You'll learn more when you study science in sec 3.

If you take two cubes, one made of sponge and one made of wood, of the same size, and throw them on the same wall at a equal distance, is it true that wood will have more KE when it hits? I was thinking that the sponge would absorb the impact so theres less KE. But i didn't think it made sense cas KE is affected by mass and speed. Or Maybe its because the sponge and wood have different masses?

Absolutely right! Given the same volume, sponge definitely has much lower mass than wood. If you apply the same force to throw it, definitely the KE of sponge is much lower than KE of wood as its mass is substantially lower. You need not consider absorption of impact for this case.

Look at absorption of impact when they compare soft and hard grounds. If it's smooth and rough grounds, they're testing on friction, not impact.

posted by Allan at 7:50 PM

0 Comments: