Q&A - PSLE Science

[tianzhu]

Dharmaratnam:

My answer will be as follows..
(a) Move the fulcrum closer to the load (wooden disc)

(b) The metal ball drops on the wooden plank with the same force but a greater moment is created due to the increase in the effort arm. Since the load arm is shorter now, the wooden disc will be thrown up a higher distance with a greater force. It will then be able to hit and sound the drum

Hi Dharmaratnam
Thank you for your reply.

In P5 Simple Machines, students are taught that moving the fulcrum towards the load leads to a lesser effort. From the Law of Conservation of Energy, moving the fulcrum may not help much. The potential energy of the metal ball at the initial stage still remain very much the same and this is transferred to the wooden disc. (m1gh1=m2gh2).
I agree that a greater moment (not in PSLE’s syllabus ) is achieved if the fulcrum is moved, but the load at the other arm still remain the same. I am still figuring out why the wooden disc is moved a higher distance given this scenario.

[tianzhu]

ksmama:

Btw, which school is this with this question? Interesting to discuss but no fun to see this in an exam...

Hi ksmama
Thank you for your reply.
This question is from Nanyang SA2 Science 2008.
http://www.wendykoh.com/08/primary6-nanyangsa2-science.pdf

[tianzhu]

Hi Dharma and Dharmaratnam


Thank you for your reply.Your participation in this thread is like a breath of fresh air.

[Dharma]

tianzhu:

Dharmaratnam:

My answer will be as follows..
(a) Move the fulcrum closer to the load (wooden disc)

(b) The metal ball drops on the wooden plank with the same force but a greater moment is created due to the increase in the effort arm. Since the load arm is shorter now, the wooden disc will be thrown up a higher distance with a greater force. It will then be able to hit and sound the drum

Hi Dharmaratnam
Thank you for your reply.

In P5 Simple Machines, students are taught that moving the fulcrum towards the load leads to a lesser effort. From the Law of Conservation of Energy, moving the fulcrum may not help much. The potential energy of the metal ball at the initial stage still remain very much the same and this is transferred to the wooden disc. (m1gh1=m2gh2).
I agree that a greater moment (not in PSLE’s syllabus ) is achieved if the fulcrum is moved, but the load at the other arm still remain the same. I am still figuring out why the wooden disc is moved a higher distance given this scenario.

Hi Tianzhu,

I am absolutely sorry for leading you on the wrong path ...my thoughts were somewhat distorted then !!! You're definately right ...the qn should be answered in terms of the concepts of simple machine ...in this case a lever.

I think the right answer should be ...

a) [b]Move the fulcrum to the left, nearer to the point the metal ball will land on the wooden plank.

b) Since the effort arm is shorter than the load arm, more effort is required to move the load but the the trade-off is that load will move a greater distance than the effort. This will result in the load, the wooden disc to move higher to hit the drum and create the sound.

[Dharma]

tianzhu:

Hi Dharma and Dharmaratnam


Thank you for your reply.Your participation in this thread is like a breath of fresh air.

Hi Tianzhu,

Thanks for the compliment. Actually I am learning from you guys.When you post something that tickles the mind, it gives me an opportunity to share new knowledge that I learn with my daughter who is P6 this year. I find the PSLE Science requires the child to have strong concepts that requires a lot of application to real life situations....especially for Section B -Open Ended Qns.

I managed to familiarize a bit on maths and science when my older daughter sat for her PSLE 2 years ago. Fortunately, she did well enough to get into her dream school.

I must thank you Tianzhu for keeping this science thread going!!

[ChiefKiasu]

Dharma:

...I think the right answer should be ...


a) Move the fulcrum to the left, nearer to the point the metal ball will land on the wooden plank.

I have to disagree. If the weight of the ball and its distance from the lever the remains constant, the total torque generated by the downward force FD is actually lowered if you move the fulcrum to the left, since FD = (Force generated by conversion of potential to kinetic energy) * distance from fulcrum. This means that the upward torque (FU) on the other side of the lever is effectively increased since its distance from the fulcrum is lengthened even though the weight of the wooden block remains constant. So you will need to apply more force on the shorter arm to move a smaller weight on the longer arm!

[Dharma]

ChiefKiasu:

Dharma:

...I think the right answer should be ...


a) Move the fulcrum to the left, nearer to the point the metal ball will land on the wooden plank.

I have to disagree. If the weight of the ball and its distance from the lever the remains constant, the total torque generated by the downward force FD is actually lowered if you move the fulcrum to the left, since FD = (Force generated by conversion of potential to kinetic energy) * distance from fulcrum. This means that the upward torque (FU) on the other side of the lever is effectively increased since its distance from the fulcrum is lengthened even though the weight of the wooden block remains constant. So you will need to apply more force on the shorter arm to move a smaller weight on the longer arm!

You are right Chief Kiasu. Maybe increasing the drop height of the metal will help to increase the torque.

[tianzhu]

Another question about gears in bicycle(Q38).

http://www.wendykoh.com/08/primary6-acssa1-science.pdf

b) How does the relative sizes of the two gears affect the eventual/final speed of the movement of the back wheel?
What's your view on the answer in the worksheet?(click at link to see)

[Dharma]

tianzhu:

Another question about gears in bicycle(Q38).

http://www.wendykoh.com/08/primary6-acssa1-science.pdf

b) How does the relative sizes of the two gears affect the eventual/final speed of the movement of the back wheel?
What's your view on the answer in the worksheet?(click at link to see)

b) The ratio of size of Gear A : Gear B will determine the speed of rotation of the rear/back wheel.

Size of Gear A/ Size of Gear B = No. of Teeth in Gear A/No. of Teeth in Gear B

For example ....
No. of teeth in Gear A = 16
No. of teeth in Gear B = 8

Gear Ratio = 16/8 = 2

This means that everytime the cyclist pedals to turn Gear A to make 1 revolution, Gear B has to make 2 revolutions. Gear B turns faster than Gear A.

The answer should be as follows
b) The greater the size of Gear A as compared to Gear B, the faster the rear/back wheel will turn.

# The answer given by the publisher is too \"chim\"for me to comprehend

[tianzhu]

Dharma:

b) The greater the size of Gear A as compared to Gear B, the faster the rear/back wheel will turn.

# The answer given by the publisher is too \"chim\"for me to comprehend

Thank you for your answer,Dharma
The answer given in the WS is correct to state that gear B would turn faster when there is increase of speed in gear A.However, it's incomplete as it did not compare the relative sizes of the two gears. In P5 Simple Machines, students are taught that smaller gear turns faster than bigger gear.

As long as the answer goes along this principle, I think it should be fine.