Q&A - PSLE Science

[verykiasumummy]

creamyhorror, i think u r right to say both balls have the same acceleration, but due to the gravity acting on the heavier ball is higher, the heavier ball should reach the ground first. 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…


pri students are supposed to assume that way… heavier one will reach ground first… they are not supposed to assume no friction or no air resistance, because that cannot be practically done.

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.

i cannot say about the equations that u all have mentioned, whether they are right or wrong, i long forgotten about them though i was good in sci during my days… i deleted those memory to learn all about pri sci so as to coach my dc along… hope i didnt say anything wrong…

[Nebbermind]

verykiasumummy:

alamak all the grandmasters here so chim...


this is PSLE sci ya?? need not consider acceleration & velocity right??

i cannot say i'm good in sci, i must say i make every effort to read and remember the texts and the many different sci guides in my hse...

the 2 objects, 1 heavier than the other, when released from the same height, they possess the same GPE as in gravitational potential energy. in pri sci, GPE is a type of potential energy due to the increased in height that the object is placed off the ground. in this case, both balls are at the same height, hence possess equal GPE. - (1)

however gravitational force, or gravity as all u mean, acting on the heavier ball will be higher due to its greater mass, hence the heavier ball will reach the ground first. (2)

correct me if i miss out any keywords.. but there is a difference in the time that each of the ball takes to reach the ground..

(1) If they have different mass and all else remains the same, the GPE (which I presume is defined by 'mgh') will be different.

(2) As shown above, final velocity of the object is not dependent on mass but on 'g' & 'h' which are constants if the diff objects are at the same height. So all objects will travel at the same V and reach the ground at the same time.

[Nebbermind]

verykiasumummy:

creamyhorror, i think u r right to say both balls have the same acceleration, but due to the gravity acting on the heavier ball is higher, the heavier ball should reach the ground first. 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..


pri students are supposed to assume that way.. heavier one will reach ground first... they are not supposed to assume no friction or no air resistance, because that cannot be practically done.

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.

i cannot say about the equations that u all have mentioned, whether they are right or wrong, i long forgotten about them though i was good in sci during my days... i deleted those memory to learn all about pri sci so as to coach my dc along... hope i didnt say anything wrong...

Pls do not confuse FORCE with ENERGY.

[Way2GO]

creamyhorror:

change in GPE = change in KE
mgh = 1/2 mv^2
gh = 1/2 v^2
v^2 = 2gh
v = sqrt(2gh)

meaning that the final velocity v is a function of the distance fallen h, and not mass. Mass doesn't affect the final velocity of the object.

u nail it with dis working. :salute:
can use dis to explain to kids fr conservation of energy POV.
:udaman:

[Way2GO]

Nebbermind:

verykiasumummy:

the 2 objects, 1 heavier than the other, when released from the same height, they possess the same GPE as in gravitational potential energy. in pri sci, GPE is a type of potential energy due to the increased in height that the object is placed off the ground. in this case, both balls are at the same height, hence possess equal GPE. - (1)

however gravitational force, or gravity as all u mean, acting on the heavier ball will be higher due to its greater mass, hence the heavier ball will reach the ground first. (2)

correct me if i miss out any keywords.. but there is a difference in the time that each of the ball takes to reach the ground..

(1) If they have different mass and all else remains the same, the GPE (which I presume is defined by 'mgh') will be different.

(2) As shown above, final velocity of the object is not dependent on mass but on 'g' & 'h' which are constants if the diff objects are at the same height. So all objects will travel at the same V and reach the ground at the same time.

verykiasumummy, Nebbermind is correct
GPE = mgh
therefore, d greater d mass or height, d higher d GPE.
g is a constant.

[verykiasumummy]

but pri sci dun learn velocity and acceleration.

[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.

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.

[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…