Q&A - PSLE Science

[atutor2001]

mew22:

Hi

if u try cycling over a pile of sand, u can feel this effect
can u cycle easier over a pile of sand or normal road surface?
powder same ya?

however if the object had no wheels, like a box, and with just a few grain of sand
the sand wud have acted like ball bearings to make it move further.
if the box had to move over a pile of sand, then it would move lesser

same same for powder, ya?

When an object is set in motion we need to consider the \"type of force\" applied.

For cycling, a ROTATIONAL force is applied. If there is no friction, the wheel will just keep rotating on the spot and the bicycle will not move forward

However, for this question, the force applied is a horizontal force (not rotational). Without the wheels, the object will move forward. With the wheels, it will move forward further as the area of contact with the surface is smaller. If the surface is very rough, the wheel will grip the surface and rotate, causing the object to move forward. If the roughness is in between, the wheels will both rotate and skid forward. If the surface is very smooth, the object will simply slide forward without the wheel rotating at all. Which way will the object move further? Not possible to tell.

mew22:

also
for option 3, where car is made lighter
if car is made of paper, then it is useless for distance

so maybe it is not a very good question?

maybe

I agree with your view on changing the weight of the car. Though it is beyond Pr level to understand drag force, but kids will know that it is impossible to throw/shoot a styrofoam ball further away than a rubber ball (with both balls of the same size).

[Nebbermind]

OK, after consdering what others have said,


1. extend the elastic band as far as possible
extending the band will increase the EPE

2. use a thicker rubber band
if the band can be extended equally far, then the EPE will be greater as it'll take more effort to extend it

3. use a lighter toy car
if we only consider the context of KE, reducing the mass will increase the velocity. Given that or all other factors remains the same, the car shd go further

4. put some powder on the surface the car is travelling on
since the body of the car has a initial V when released and not really dependent on the rotation of the wheel, it can still glide/slide along the surface (like kicking a ball, it'll slide more than it'll roll). That being the case, it'll go further when the surface friction is decreased (likie ball on wet grass)

juz my 2c

[atutor2001]

I think I understand the question's intention. The question is to test the student's understanding of the requirement, something related to \"experimentation\" - not so much on the concept of energy conversion (it is only there to confuse).


Like what Nebbermind has reasoned, all 4 options will make the car go further (if we exclude factors that are beyond Pr science syllabus).

However, the question is \"what can you do to increase the chances of winning\"

The setter probably means that \"putting powder on the surface\" will be applied to all the other cars too - since they are rolling on the same surface. That is, the other cars will also go further as they are subjected to the new condition and as a result, the chance of winning will not be increased. However, I can also interpret the option as \"putting powder for my car ONLY\". So mew22 conclusion that it is not a well set question is valid.

[meimeitan]

Hi! All,


Then, may I know if the thing, let’s say a small metal block, put on the top of a cupboard has gravitational potential energy?

thanks.

[starlight1968sg]

meimeitan:

Hi! All,


Then, may I know if the thing, let's say a small metal block, put on the top of a cupboard has gravitational potential energy?

thanks.

Yes.

[meimeitan]

Hi! TianZhu and Starlight1968sg


Thanks for previous explanation. However, i quite confuse about the term of gravitational force. If a man lying on the ground, there should be no gravitational force acting on him. But, how come i cannot lying on the ground when i am on the moon. As i dun hv clear concept, then i dun know how to answer the following question.

Which of the following object(s) do(es) not have any gravitational force acting on it?

(a) a bird flying in the sky
(b) a man lying on the floor
a balloon rising into the air

Many thanks for your kind help.

[fristrom]

It has been a long time that I do physics… so please correct me if I am wrong.


From recollection, gravitational force is an universal attractive force between two objects, so all 3 cases would have gravitational force acting on the object.

"how come i cannot lying on the ground when i am on the moon" … because gravitational force is proportional to the mass of the object. As moon is much lighter than earth, the gravitational force is much smaller.

[meimeitan]

Hi Firstrom


thanks

[Muffins]

and by the way, meimeitan, you can lie down on the moon. The gravitational pull is ⅙ that of the Earth's, so you can still lie down on the moon....

[meimeitan]

Hi!


Ha ha ha .... :lol: Learning is with fun. Hope one day I can lie down on the moon.