Q&A - PSLE Science

[meimeitan]

Another challenging question:


A group of boys were competing to see whose car can travel the greatest .
The car is flung forward using a elastic band like a captapult on the ground.

If you were one of them, what can you do to increase the chances of winning?

1. extend the elastic band as far as possible
2. use a thicker rubber band
3. use a lighter toy car
4. put some powder on the surface the car is travelling on

thanks!! :? :? :?: :?: :?: :?:


meimeitan

[Nebbermind]

I wld choose (1) & (3)


(2) cannot tell…what if it is so thick that u cannot even exted it?
(4) if car is gliding on the surface, then prob go further (like carrm puck). If rolling, then it’ll slip and will not go further.

[meimeitan]

Hi! Nebbermind


Thanks a lot! However, the answer on the exercise book is a, b, c. Could it be that the powder makes the surface the car is moving on more slippery? I think the answer in the exercise book is wrong! I really don't know! Need to check with others!! Once again, thanks!!

meimeitan

[meimeitan]

Hi! Nebbermind,


I think option 2 is correct! I think the elastic band can be stretched, if not it will not be called an elastic band!
For option 4, ...I don't know!! :?: :?: :?: :? :? :?

meimeitan

[tianzhu]

Hi


When a pulling force is applied, the elastic band is stretched. Upon releasing, the elastic spring force will cause the elastic band to return to its original position. The extension of the spring is the result of the pulling force exerted on it.

Extend the elastic band as far as possible and using a thicker rubber band will cause the car to travel a longer distance. This is because a greater force is acting on the car.

The same force exerted on a lighter car will move it further.

In P6 science, students learnt that friction helps to reduce slipping and sliding. Friction between the soles of the shoes and the ground allows us to walk and run. For wheels rolling on a road, friction (traction) prevents them from slipping. Therefore, applying powder on the surface may cause the car to move a shorter distance as it causes slipping.

Hence option (4) is out.

Best wishes

[atutor2001]

tianzhu:

........


In P6 science, students learnt that friction helps to reduce slipping and sliding. Friction between the soles of the shoes and the ground allows us to walk and run. For wheels rolling on a road, friction (traction) prevents them from slipping. Therefore, applying powder on the surface may cause the car to move a shorter distance as it causes slipping.

Hence option (4) is out.

Best wishes

Hi

My view differ from yours. If a surface is frictionless, the car will move indefinitely (i.e. there is no retarding force). All moving object comes to a stop because of friction. Without friction, a moving will continue moving indefinitely. A car skidding on snow surface can continue moving for a long distance. However, the driver has no control of the direction.

Friction is purposely created in shoes and wheels for \"control\". If we are able to control e.g. ice skating, we can move effortlessly for great distance with lesser effort.

[tianzhu]

meimeitan:

1) A jar with grasshoppers in it in field from 8 am to 5 pm. Which of these factors of the environment would affect their survival?
a availability of food
b presence of predators
c inappropriate temperature

Hi

The jar is placed in the field from 8 am to 5 pm.We may infer that it is under direct sunlight for some parts of the day.

In P6 science, I think greenhouse effect is taught in the topic on global warming. Carbon dioxide is one of the green house gases and all green house gases trap heat.

The jar allows heat from the sun to be trapped, similar to that of the greenhouse.Carbon dioxide is produced when the grasshoppers respire.Hence the temperature increases makes the living condition inappropriate.

The grasshoppers are likely to survive without food from 8 am to 5 pm. Also, the jar protects the grasshoppers from their predators.

Best wishes

[mew22]

atutor2001:

tianzhu:

........


In P6 science, students learnt that friction helps to reduce slipping and sliding. Friction between the soles of the shoes and the ground allows us to walk and run. For wheels rolling on a road, friction (traction) prevents them from slipping. Therefore, applying powder on the surface may cause the car to move a shorter distance as it causes slipping.

Hence option (4) is out.

Best wishes

Hi

My view differ from yours. If a surface is frictionless, the car will move indefinitely (i.e. there is no retarding force). All moving object comes to a stop because of friction. Without friction, a moving will continue moving indefinitely. A car skidding on snow surface can continue moving for a long distance. However, the driver has no control of the direction.

Friction is purposely created in shoes and wheels for \"control\". If we are able to control e.g. ice skating, we can move effortlessly for great distance with lesser effort.

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?

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

[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