Monday, February 13, 2006

An Interesting Situation



Got this picture while surfing the internet for images of skateboards. The question is simple. If the power cord is connected to a plug and the fan is turned on, where does the skateboard go? Towards left or towards right? Ok lemme make it simpler. Whether the skateboard move or not? Assume that both the screen and the fan are fixed on the board. Once again, think and answer. :-)


18 comments:

silverine said...
This comment has been removed by a blog administrator.
silverine said...

Just saw your 'actual' question. Will be back with an answer later!

Sreejith Narayanan said...
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Sreejith Narayanan said...

It won't move jithu ettaa.. Newton's first Law ;)

>|' ; '| said...

hehehe! tryin to fool me :(

if the fans powerful enuf, it will blow tht screen down n if its still powerful, it gona move to the right?

i am wrong..aint i? :D

Geo said...

Don’t tell me you were browsing for an online tutorial to learn skating... ;_)

silverine said...

My two bit. For every action there is an equal and opposite reaction. The objects i.e. the turning fan exerts a force on the white board and the board repels the force. The direction of the force on the first object is opposite to the direction of the force on the second object hence the skateboard won't move. Newton's third law I think.

Sreejith Narayanan said...

@ silverine.. i think its the first law, by which both are internal forces, and hence cant change the state of rest.

MS said...

I think,oh I never think (nice question.it reminded me of that)
;))

silverine said...

@Sreejith: Maybe you are right. I am just taking a wild guess here!Besides I was not a science student and took a colleagues help to come to this conclusion.

Geo said...

@Silverine: you cheat a lot ;_))
(not just here, even for xlation)
:_D

tangy said...

hey! dats quite cool :)

MadV said...

How will it move?

silverine said...

@Geo: The first comment I deleted explained what would happen if the fan or board were removed. I got that from my brother. Later realised in office that the question Jithu asked was why the board wouldn't move. Like I told Sreejith, I took some colleagues help. Sent this out to couple of colleagues and one of their answers I have cut and pasted here. This has generated a debate in my office and all work has come to a stand still :))
@Jithu: Great question. I see why Paheli was so interesting and hard to crack! :)

Sreeram's World said...

I guess, it will move to the right!!
If the fan is rotating in the opposite direction (exhaust action)it will create a vacuum between itself and the cardboard. The air from behind will push the cardboard forward and along with it the skateboard will move to the right.....

(Patting myself...;p)

Sreejith Kumar said...

The answer to this question is that the skateboard will not move even when the fan is switched on. It is the application of Newton's first law of motiion. All the forces here are internal and hence the body (skateboard) will continue to be in the state of rest. If the working fan is removed, then the skateboard will move in the forward direction. That is; to the left of the picture. Here the fan would act as an external force. Here also Newton's first law is valid. If the white coloured board is removed, then the skateboard will move in the backward direction. That is; to the right of the picture. This is an application of Newton's third law if motion. That is; there is an equal and opposite reaction to every action.l This is just like the movement of a boat with the help of an oar. :-)

Jithu said...

ideal case
as most of you correctly put it across, the skateboard wont move because all the force generating bodies (fan as well as the whiteboard) are internal objects and hence wont change the state of the system (fan, whiteboard, skateboard) as a whole (Newton's first law). now how it happens. the fan pushes air from its back to the front. as a result it generates a reactive force towards the right (think of an aircraft propeller). since the fan is connected to the skate, the skate tends to move towards the right. now the air that the fan pushes to the left (which is equal to the reactive force which pushes the skate towards the right) will fall on the white board and pushes it towards the left. since the white board is connected to the skate, the skate experiences a force towards the left which is equal in magnitude to the aforementioned force which pushes it to the right. hence these forces cancel each other and the skate wont move.

practical case
now the frictional forces exerted by air between the fan and the whiteboard, wheels of the skate and the ground, wheel shafts etc tend to reduce these forces at different instants. depending on the variations of these frictional forces and a subsequent resultant force may move the skateboard towards the left or towards the right. so when we do this experiment practically, the skateboard MAY move towards the left OR towards the right. but ideally it shudnt move.

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