Q&A - PSLE Science

[verykiasumummy]

Robertpattinsonidle:

Robertpattinsonidle:

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why is answer 3? why not 4?

someone pls answer this qsn asap. tmrw i sitting for psle. thanks

hi,

A) though adil increased higher breathing rate, but it does not mean she used more energy because the increased breathing rate depends on the amount of muscles a person has. since u dun know their mass, u cant deduce that.

B) as similar to above, having same increase in breathing rate doesnt mean using same energy.

hope it helps... good luck for tmr..

[creamyhorror]

Good luck to you kids, and to you mummies/daddies who are cheering for them. You should be in bed already!

[NY1969]

Yes should be in bed now. Need very careful thinking and clear mindset for tomorrow’s battle. A lot of application skills need to be applied.

[Way2GO]

creamyhorror:

charsen:

[quote=\"vaidyanathan padmini\"]I just have a doubt.please help......

Two balls are placed on top of a ramp.ball A has a greater mass than ball B.which ball reaches the ground first?

Thanks in advance

Ball A with greater mass will reach the ground first.
Objects with greater mass will possess higher kinectic energy and hence A will travel faster than B.

In general, if you ignore air friction, objects dropped from the same height fall with the same acceleration, regardless of their mass. This means that objects speed up at the same rate as they fall. A more massive ball will therefore reach the ground at the same time as a lighter ball dropped from the same height. This extends to objects rolling down a slope/ramp, if you ignore friction. So I'd say that both balls reach the ground at the same time.

(The gravitational potential energy (gpe) of the more massive object is higher, and after x seconds of falling it possesses more kinetic energy than the lighter object -- but that doesn't mean it's travelling faster than the lighter object. Kinetic energy is not the same as velocity.)

Let me know if this does not agree with your textbook or teachers


For parents/advanced students: http://www.batesville.k12.in.us/physics/phynet/mechanics/RotMechanics/fall_slide_roll.htm giving the evidence. The summary quote is as follows:[quote]This result says that the linear acceleration of the object down the ramp does not depend on the object's radius or mass, but it does depend on how the mass is distributed. Therefore, all spheres have the same acceleration on the ramp[/quote]Of course, at the primary school level it's unnecessary to go into the details. Just learn that objects with different masses fall through the air and roll down ramps with the same acceleration.[/quote]creamyhorror,
me think u mixed up acceleration with velocity here. d acceleration u refer to in ur post is gravity?
If u meant gravity, it is a given dat d same gravitational force acts on all objects.
If one looks at d question fr a P6 level understanding,
fr a purely conservation of energy point of view, GPE = KE
most P6s will likely reason dat d heavier ball possess greater KE n thus must travel faster or further.
(Since KE is a function of velocity squared, n d heavier ball possesses greater KE, where does d additional KE translate to, if not greater velocity or further distance travelled?)

d answer looks simple on d surface but d explanation requires a much deeper understanding of physics, which is really too much to ask of a P6.
Since it's a ball rolling down a slope, d rotational aspect of each ball must be taken into account. d rotational aspect is what u alluded to in 'distribution of mass'.

It is not a well thought-out question.
Don't u all like to hear d explanation/reasoning fr d teacher who set dis question - for d benefit of students who will come after d current batch?

[Nebbermind]

Way2GO:

creamyhorror,
me think u mixed up acceleration with velocity here. d acceleration u refer to in ur post is gravity?
If u meant gravity, it is a given dat d same gravitational force acts on all objects.
If one looks at d question fr a P6 level understanding,
fr a purely conservation of energy point of view, GPE = KE
most P6s will likely reason dat d heavier ball possess greater KE n thus must travel faster or further.
(Since KE is a function of velocity squared, n d heavier ball possesses greater KE, where does d additional KE translate to, if not greater velocity or further distance travelled?)

d answer looks simple on d surface but d explanation requires a much deeper understanding of physics, which is really too much to ask of a P6.
Since it's a ball rolling down a slope, d rotational aspect of each ball must be taken into account. d rotational aspect is what u alluded to in 'distribution of mass'.

It is not a well thought-out question.
Don't u all like to hear d explanation/reasoning fr d teacher who set dis question - for d benefit of students who will come after d current batch?

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.

[creamyhorror]

Nebbermind:

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.

Good explanation, Nebbermind. I actually considered showing that equation but decided it might just end up confusing some students.

@Way2GO: In a sense, the more massive ball only has more kinetic energy because it has more mass. It doesn't speed up faster because it has more KE - it has more mass, so it gains KE faster. The cause is acceleration, the result is KE.

From a P6 point of view, all that students need to remember is: objects tend to fall with the same acceleration toward the Earth, regardless of their mass, when air resistance is ignored. This means that both heavy and light objects take the same time to fall to the ground, as demonstrated by Galileo's famous thought experiment about http://www.juliantrubin.com/bigten/galileofallingbodies.html. When this experiment was done on the Moon by an astronaut in 1972, a feather and a much heavier hammer fell to the ground at exactly the same time.

This is secondary-level knowledge, but it doesn't hurt to know it.

[Way2GO]

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.

Arh Nebbermind,
IMHO, u made a fundamental error to assume mass is d same in d two balls in ur deduction.
Let’s denote d heavier ball as 1, n lighter ball as 2.
For heavier ball, m1gh = ½ m1v1^2
For lighter ball, m2gh = ½ m2v2^2
g is a constant, h is d same, n d mass in each equation cancels each other out.
However, m1 > m2 n we know for certain KE1 > KE2, therefore we cannot simply conclude dat v1 = v2.

Dis problem lents itself easily to experiment for kids to visualized without all d theoretical mambo jumbo which I thot is beyond them at dis level n too much to expect of them to explain. Dat’s why I was interested for whoever came up with d question to explain the solution to d kids at their level fr a conservation of energy perspective or r hv d kids been taught dat acceleration due to gravity is independent of mass.

Let’s just discuss it for our understanding.
For a rolling object on an inclined plane, there is an additional rotational aspect day we need to include in d equation.
Dis aspect takes into account d moment of inertia and angular velocity. For simplicity, let’s denote dis as M.
D moment of inertia is different for different shape, size n mass distribution referred to by creamyhorror.
It was for dis reason dat I said d question was not well set-up, coz it just mentioned two balls but fail to specify dat d balls r of d same size n dat they r both solid or spherical shells, but it was left to d child’s assumption dat they must both be solid spherical balls of d same size, n friction is not at play.
If d two balls are exactly d same in size, shape except mass, we can drop dis term in our evaluation.
Dis is prolly wat creamyhorror meant when he said d radius n mass terms do not matter when an object rolls down a slope.
But these terms will matter in d calculation of M if d objects r of different shapes, sizes n mass distribution.
Otherwise d equation reduces to simply mgh = ½ mv2
At d top of d slope, d heavier ball will start with greater inertia. Both balls will gather speed as they roll down d slope. If u plot d velocity vs time graph, it will not be a straight line, n d graphs for d two balls will not be exactly d same. Since it is not a straight line, it means there is acceleration (not g, which is a constant) as d ball rolls along d slope.

[Way2GO]

creamyhorror:

Good explanation, Nebbermind. I actually considered showing that equation but decided it might just end up confusing some students.

:celebrate: Gud for u.
I did d same by withholding dis discussion till after d Science paper so as not to confuse d kids.
However, I did ask my DS2 last nite for his answer, he said d heavier ball will reach d end of d slope first fr his understanding of conservation of energy.
He checked with his frenz in class today, two of them mentioned same speed but they didn't explain why.

creamyhorror:

@Way2GO: In a sense, the more massive ball only has more kinetic energy because it has more mass. It doesn't speed up faster because it has more KE - it has more mass, so it gains KE faster. The cause is acceleration, the result is KE.

d acceleration u mentioned here is due to gravitational force, g? But dat is a constant for both balls.
In ur analysis, d velocity in d equation KE = 1/2 mv^2 has no effect on KE?

creamyhorror:

From a P6 point of view, all that students need to remember is: objects tend to fall with the same acceleration toward the Earth, regardless of their mass, when air resistance is ignored. This means that both heavy and light objects take the same time to fall to the ground, as demonstrated by Galileo's famous thought experiment about http://www.juliantrubin.com/bigten/galileofallingbodies.html. When this experiment was done on the Moon by an astronaut in 1972, a feather and a much heavier hammer fell to the ground at exactly the same time.

This is secondary-level knowledge, but it doesn't hurt to know it.

d example u cite on free-falling objects in a vacuum is well known n its simpler to understand mathematically compared to a ball rolling down an inclined plane.
d observable evidence in free-falling objects having different masses dropped fr d same height is dat d object with heavier mass will hit d ground first.
Thus kids can be easily swayed by wat they hv experienced than an experimental result dat they hv not been previously exposed to.
d bone I was picking with d one setting d question is d kids hv to make many assumptions (eg no air friction, no difference in moment of inertia) to arrive at d answer dat both balls will reach d bottom of d slope at d same time AND IF they hv been taught dat acceleration due to gravity is independent of mass.

[Nebbermind]

W2G

What u very the chim leh. Once u go into rotational motion, I have to concede defeat!!

But hor…if the ball roll on the inclined plane, then there must be friction…yet another component to take care of.
Assume no friction…balls slide down…no moments…can?

[Way2GO]

Nebbermind:

W2G

What u very the chim leh. Once u go into rotational motion, I have to concede defeat!!

But hor...if the ball roll on the inclined plane, then there must be friction...yet another component to take care of.
Assume no friction...balls slide down...no moments...can?

Bro where got chim.
Question setter expecting kids to make too many assumptions to arrive at correct answer, tio bo?

u r rite one cannot ignore friction - without friction, ball cannot roll.
kids will be more confused! :slapshead:
if on one hand, assuming no friction to be in agreement with falling objects of different masses fall at d same rate,
n on d other, no friction - no roll. :slapshead: :slapshead: