Q&A - PSLE Science

[creamyhorror]

verykiasumummy:

u can do a simple experiment by placing a cola can with drink inside and the other empty... the one with drink which is heavier will reach the ground first..

Will it? I think they would reach the ground at about the same time...
[quote]pri students are supposed to assume that way.. heavier one will reach ground first...[/quote]I don't think primary students are supposed to assume that heavy things fall faster than light things. I've not seen that in any textbook or guidebook. In fact, the principle that heavy things and light things fall at the same speed is a very fundamental concept in science, so I doubt any book would try to teach otherwise. It's more likely the books say nothing about it at the primary level, and only teach it at secondary level.
[quote]my dd's sci tutor came with an explanation to explain this some time ago. she said for example, a helium balloon can lift up and carry with it maximum 10g, so we say that to lift up 100g, we need 10 helium balloons. if the object gets heavier, say 200g, we need more balloons to lift the object up which is the same as the amount of force applied to move things, the heavier it is, the more force required to move or stop it.[/quote]This is correct. Gravity is a force. The heavier (more massive) the object, the greater the gravitational force acting on it. However, the object also has more mass, and takes more force to move. It turns out that the ratio of gravitational force to mass (force/mass) is a constant.

force = mass * acceleration (Newton's second law)
force/mass = acceleration

When you calculate (force/mass) for different objects, you find that the figure is a constant. Example: a 10g mass takes about 100N force to hold in place. 100N/10g = ~10 m/s^2. This acceleration is g, the constant acceleration due to gravity.

Sorry that I resorted to equations again. At the primary school level, the equations don't matter - what matters is that students are taught the correct principles. They don't need to learn about acceleration or velocity - they only need to remember that heavy and light things take the same time to fall/roll to the ground. It's good to introduce such possibly surprising facts early and get kids used to them. That will give them the right intuition to handle the problems when they get to secondary school physics.

Thanks for the discussion!

[creamyhorror]

Way2GO:

creamyhorror:

I think it's fair to assume that the balls have the same shape and are similar in mass distribution. Therefore, they will roll down the slopes with the same acceleration, even though their sizes/radii/moments of inertia are different. Solid balls of different masses will roll down ramps at the same rate, if other conditions are equal.

In dis problem, d assumption has to be made dat d balls hv similar mass distribution n r of d same size, for ur stated conclusion to be correct.
Coz for a spherical solid ball rolling down an inclined palne,
GPE = KE + M (where M is a function of moment of inertia * angular velocity)
For a solid sphere, moment of inertia = 2/5 * mr^2 where r is d radius of d ball.
Therefore size n radius matters.
if d problem was changed to one with balls of different mass distribution, eg a solid spherical ball vs a ball with just a spherical shell, d solution wld be different.

According to http://www.batesville.k12.in.us/physics/phynet/mechanics/RotMechanics/fall_slide_roll.htm, rotational acceleration = torque / moment of inertia. It turns out that both mass and radius cancel out in the (torque/MI) calculation, meaning that the rotational acceleration (rolling rate) is independent of mass and radius. This means that two marbles will roll down a ramp at the same rate, even if one is giant and one is tiny. Surprising, eh?

Way2GO:

creamyhorror:

I wouldn't overthink the problem. The important idea/law here is that mass itself does not affect acceleration due to gravity, or acceleration down a ramp due to gravity. Students can learn about additional considerations like air resistance and mass distribution at a more advanced stage, especially through experimentation, but the basic principle should be given the most weight.

(I like teaching physics, maybe I should be a tutor...)

Agree.
Why not? u r gud.

Heh, thanks. Maybe secondary/JC tuition, primary schoolers can't be taught too much.

[verykiasumummy]

based on the above theory, any objects, regardless of their mass, will fall at the same speed and land on the ground at the same time…


i’m quite sure pri sci was not taught like tat…

[verykiasumummy]

i’m open for discussion cos i dun want to teach my dc the wrong thing also…

[creamyhorror]

verykiasumummy:

based on the above theory, any objects, regardless of their mass, will fall at the same speed and land on the ground at the same time..


i'm quite sure pri sci was not taught like tat..

I just went to look in my brother's PSLE Ultimate Science Guide (from EPH) and found a small \"Watch Out!\" box covering this issue:
[quote]False: Heavy objects fall faster than light objects.
True: Take a heavy object and a light object that are about the same shape and drop them from the same height at the same time. Note that the heavy object and the light object will reach the ground at the same time. Many of us have the misconception that heavier objects fall faster than lighter ones because that is what we have always observed. In the real world, there is air resistance and that explains how the misconception comes about. Every object on this Earth, independent of size, experiences the same gravitational pull and so all falling objects fall at the same rate.[/quote]There's also an \"Applying the Concept\" section that covers an experiment with two bags of marbles, one with twice the number of marbles as the other. The experiment shows that the bags hit the ground at the same time when they're dropped from the same height. (It's at the very end of Chapter 23, Forces.)

So I guess the principle appears in at least some primary science guidebooks. It shows that it's important to focus on the application sections of guidebooks, not just the main notes.

[verykiasumummy]

thks for highlighting…


let me go thru the books again…

just asked my ds1, (psle last yr), and he said he thought mine was correct but after 2nd thoughts, he said ur concept was not wrong either… let me sort it out and come back…

[verykiasumummy]

i’m sorry… creamyhorror, way2go, nebbermind…


my mistake… i had mistaken and mixed up with impact of the object…

thks to creamyhorror, i have that same book as u and i saw that quote too. i was still thinking that the GPE on both object is the same, but the impact of the object landing on the ground will be different. the heavier object will have a greater impact, hence i mix it up that the heavier object will also fall faster…

sorry, my mistake…

the question my ds1 had was about 2 boxes, one with 5kg, and the other 20kg, similar size and material. when the 5kg was dropped from a certain height, the box did not tear apart, but the 20kg box was torn apart when it landed on the ground due to the greater impact it had because of a greater mass.

i’m really getting old…

[atutor2001]

Nebbermind:

W2G

Donch quite get u.

PE = mgh, KE = ½ mv²

Assuming the balls begin with v=0, and h=0 at the end of the slope, then

mgh = ½ mv²
=> gh = ½ v²

Since g and h are constant, v must be the same.

Lets put in frictional losses to the equation :

mgh = ½ mv² (without friction)
mgh = ½ mV2² + heat + sound (with friction)

Reduction in speed = v² - V2² = 2(heat + sound)/m

So the reduction in speed is inversely proportional to the mass of the object if there is friction.

That is, the reduction in speed will be smaller if the mass is big and vice versa.

For a \"free falling object\" the (heat + sound) is negligible as it is due to air resistance, so the final speed is about the same.

However, for a ball rolling down a ramp, the (heat + sound) depends on the surface texture and the length of the ramp.

If it is a smooth short ramp then the final velocity should be about the same. However, if the ramp is long and rough, frictional loss is quite substantial. Therefore, the ball with a bigger mass will reach the bottom first because the reduction in velocity of the bigger mass is less.

[Ares23]

all these equations are ‘O’ level physics/sec 3 IP syllabus…

[atutor2001]

Ares23:

all these equations are 'O' level physics/sec 3 IP syllabus...

The problem with Pr Science is that they like to question concepts that are quite chim. Like this question, there will be different outcome for different conditions. We need these equations to understand better. However, Pr school kids will not learn these equations until much later. So how to make them learn with understanding when they are not even well verse with the basics?

By the way, those equations are only for kiasu parents' discussion. Not meant for the children to learn.