Q&A - PSLE Science

[ChiefKiasu]

tianzhu:

... For solution 3, moment refers to the turning effect of a force.How does increase in moment help the wooden disc to fly higher?Please explain...

The downward torque generated by the metal ball is equal to the product of weight of the ball and its distance from the fulcrum (longer now). This creates an counter-force which is the product of the weight of the wooden block and its distance from the fulcrum (shorter now). Since the weight of the wooden block remains constant, the greater upward counter torque on the block minus the weight of the wooden block will propel the block to greater heights.

tianzhu:

... Please explain why a higher fulcrum help in solution 4...

A taller fulcrum will bring the block closer to its objective, as well as increase the torque due to the greater degree of freedom offered by the download force.

tianzhu:

... For solution 5, a lighter wooden disc may fly further under the principle of conservation of energy, but not sure if it makes the same dome sound when it hits the drum...

The question did not state the requirement for attaining a particular pitch or volume of \"dome\" sound.

tianzhu:

... For solution 6, does it helps if this experiment is done on the moon, taking into consideration the reduced gravitational force?

I was partially kidding when I put down that answer. But it is true that the reduced gravitional force and air friction (in a vacuum) will allow the block to traverse greater distances when propelled by an external force. The problem is sound does not travel in space, so no \"dome\" sound :lol: .

Sorry... that's all I remembered from my Air-level Physics

[ksmama]

Theme park question:

yes, agree with a) and b) but not c).
I would perhaps start P at a level higher than the peak of the loop. At a higher level, the same car would have a greater gravitational energy. As the car descends, this energy is converted to kinetic energy. Even as some energy is converted to heat and maybe even sound energy, there would be enough kinetic energy to propel the car up the loop and pass the peak. At a certain point past the peak of the loop, gravitational energy in the car would turn to kinetic E as to descends to reach X.

[Dharmaratnam]

tianzhu:

Thank you for your time.


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One way I can think of is to increase the height of the metal stand carrying the ball.Another way is to use a bigger metal ball(increased mass)
But, how about moving the fulcrum closer to the wooden disc.Does it helps?

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

[Dharma]

tianzhu:

An interesting question concerning roller coaster.What's your view about the answer?


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The answer in the worksheet is
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Hi Friends,

This is my suggested answer.

a) Gravitational potential energy

b) Some of the kinetic energy of the moving car will be used to overcome the gravitational force and its remaining kinetic energy will be converted to heat and sound energy.

c) Thanks Ksmama for your clear explanation.
# When we draw the new track, we must ensure that the starting point P is the highest point of the track and we must also draw a smaller loop; i.e. the highest point of the loop must be lower than P. By this, we will not compromise on the track design.


1.The original track design has a loop with its highest point that is higher than the initial starting point.
2.We must bear in mind that the engine of the car has been switched off at point P.

The car will definately not be able to reach the top of the slope of the loop...if it does then its gravitational potential energy at the top of the loop has to be higher that its gravitational potential energy when it started off at point P. This is impossible as its maximum kinetic that it attains at the lowest point of the slope before ascending the loop will be used up overcome the gravitational pull when it tries to go up the loop. Furthermore, some of the original potential energy of the car will be converted to sound and heat energy.(Remember that the car's engine is switched off ...no chemical potential energy from the engine of the car)

The car is likely slip downwards at some point between the bottom of the loop and top of the loop.

# The model answer by the publisher is unbelievable!!! ...

[Dharma]

tianzhu:

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Hi Tianzhu,

Noticed that this qn you posted was not answered. Hope the reply below makes sense.

(a) When you pedal, Gear A and Gear B moves in the same direction as they are linked by a chain. Pedalling will drive Gear A to turn and Gear B to follow to turn in the same direction. Gear B is connected to an axle that turns and causes the wheel to turn in the same direction causing the bicycle to move forward.

(b) To make cycling easier, Gear A has to have a larger diameter than Gear B. When Gear A's diameter is larger than that of Gear B, it has to move a longer distance to make 1 revolution thus reducing the effort required to pedal.

[Dharma]

tianzhu:

Original Title: Upper Primary Science


Respiratory Systems

Under respiratory systems, there are two popular types of questions concerning the increase in breathing rates and heartbeat while doing exercises. What is the most appropriate information to put in while answering these types of questions?

Q1) After playing basketball with his friends, Tom checked his breathing rate again. He found that his breathing rate was 20 breaths per minute. Why was his breathing rate higher?

Q2) Jerry ran around the track for 20 minutes.
(a) What would happen to his heartbeat after 20 minutes? (b)Why is this so?

Hi Tianzhu,

a) When Tom played basketball, his cells burn oxygen faster to produce more energy, which creates more CO2 as a waste product. The brain detects this level of CO2 and signals for a higher breathing rate to provide more oxygen for the cells.

b) When Jerry ran round the track, his heartbeat will increase in order to supply his muscles with more oxygen to produce extra energy.

BTW...there are 2 ways the heart can meet the body's need for oxygen during exercise. It can either beat faster or it can beat harder, moving more blood each time it pumps. However, it can only beat harder if the heart has been strengthened through regular excercise. This is evident if you notice the heartbeats of marathon runners and athletes who are very fit have very low heartbeats. Their heart is able to pump more blood with each beat resulting in a slower heartbeat.

[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.