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  #1  
Old July 10th, 2002, 07:21 AM
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A physics question - Why do dust particles fall of when you beat a rug?

When you clean a rug/mat , you beat it with an object(like a stick or brush) or against a wall so that you can beat the dust particles out of the rug or mat. Can someone please explain in terms of newtons laws why that happens
i.e when we beat the rug with an object or even shake it, the dust particles fall of. Perhaps the question you might want to ask is WHY the dust particles fall of.
Happy solving
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Old July 10th, 2002, 07:28 AM
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Newton's First Law :

A translation of Newton's first law is " Every body continues in a state of rest or in uniform motion in straight line unless it is compelled to change that state by forces impressed upon it ." This law is often called the law of inertia because 'inertia' means resistance to change and a law states that an object naturally tends to maintain whatever velocity it happens to have (including zero velocity). If an object is in a state of rest or uniform motion in a straight line then its acceleration is zero. Thus, first law can be stated as ' if no external forces are exerted on an object, then objects acceleration is zero'. The word ' external' specifies that the force must act from outside the body on a system of bodies to produce a change in its velocity. The internal forces occurring in pairs of equal and opposite forces cancel each other. By pushing a carriage from within we cannot set it in motion even if air resistance and frictional forces are zero.



Concept Of Inertia :

Inertia is an inherent property of the body by virtue of which it resists change in its state of rest or uniform velocity or it is the resistance of an object to have its velocity changed. We know from everyday experience that some objects have greater inertia as compared to others. Thus, the acceleration produced in a body in addition to force applied depends on inertia. The more massive the body is, the greater is the quantity of matter it possess and hence greater is its inertia. Thus, we can say that mass is the measure of inertia of body.



Inertia of Rest :

Inertia of rest is property by virtue of which body is unable to change from its state of rest . For eg.

(1) A passenger standing in a train falls backwards when the train suddenly starts moving. The reason that his feet share the motion of the train while the upper part of his body is still at rest.

(2) A coin placed on cardboard covering a glass beaker falls into the beaker if cardboard sheet is pulled to one side.

(3) When we beat a coat with stick, the dust particles lying between the threads of coat fall off. This happens because coat is suddenly set in motion, the dust particles tend to remain at rest due to inertia of rest.



Inertia of Motion :

Inertia of motion is the inability of body to change its state of uniform motion. For e.g.

(1) A dry ice disc when given a little push on a clean horizontal glass plate moves with uniform velocity.

(2) When a horse running at a high speed suddenly stops, the rider if not carefully seated, is thrown forwards.



Inertia of Direction :

It is the property by virtue of which a body cannot changes its direction of motion by itself. For e.g.

(1) When sharpening a knife, the sparks coming from grindstone, fly off tangentially due to the property of directional inertia.

(2) A stone is being whirled in a circle flies off tangentially when the string suddenly stops.
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Old July 10th, 2002, 07:38 AM
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long time !!

i think electrostatic is what is happening!!
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Old July 10th, 2002, 09:09 AM
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I think it's a combination of the First law (intertia) and third law (action-reaction).

Fourth law ('To and fro motion, produces white lotion') and Fifth law ('A pole and a hole combine to form either a pole or a hole') are related to questions regarding mattresses and cushions, but not directly related to the question at hand, so lets skip em for the while.

OK. When the mattress/carpet is hung and it's beaten by a sideways strike, the first law comes into play. The strike causes the carpet to move away, while the dust particles, by virtue of intertia, stay where they are. This causes them to get seperated. But in the case where the carpet or cushion are lying on the floor and we hit them hard, we can actually see the dust particles fly high above their original positions. This is the action-reaction thingie.

Gossipmonger, you should clean up your room more regularly. Then you won't have questions like these troubling you.
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Old July 10th, 2002, 09:23 AM
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There are small pores in human skin that trap dirt. Can we clean them by beating a person with a stick ?
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Old July 10th, 2002, 09:23 AM
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LAW:

"The more you Slap it from back the more it comes from front"

!!! jeeeez simple as that!!
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Old July 10th, 2002, 09:24 AM
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abay costermonger , yeh 8th class kay bacchon vaalay sawaal kyu pooch rahaa hai ? Tuson padaataa hai kyaa school kay baccho ko .

Abhi tu poochegaa ke if action and reaction are equal in magnitude , opposite in direction then how the FU(k is motion possible
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Old July 10th, 2002, 09:25 AM
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*sigh* .......This is the first time someone has found a connection between Indian Politics and Newtonian Physics ......
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Old July 10th, 2002, 09:25 AM
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Gospel baba

jis site say copy paste maaraa , uskaa URL dena
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Old July 10th, 2002, 09:26 AM
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Quote:
Originally posted by SlyBoots
Tuson padaataa hai kyaa school kay baccho ko .

Abhi tu poochegaa ,
padaaati and poocheegi!! samjhe narci!!
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Old July 10th, 2002, 09:30 AM
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Quote:
Originally posted by Big-G
Fifth law ('A pole and a hole combine to form either a pole or a hole')
this law does not explain cyberian
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Old July 10th, 2002, 09:34 AM
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Quote:
Originally posted by CyberKhiladi


padaaati and poocheegi!! samjhe narci!!
yaar Khillu , mujhay nahin maalum thaa kee Aunty ji hai .I thot kee koi unkil hai .

Sorry GossipMonger - No offence intended dearie :love and Kisses:
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Old July 10th, 2002, 09:38 AM
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yah the inertia answer is correct. This question was part of an 9th standard exam in an indian school. 90%of the kids answered it was due to the third law. A close friend of mine who happens to be a teacher marked it incorrect. It is because of the inertia funda. HOWEVER three other teachers of the same school marked it correct . The kids complained and the principal interfered telling him he was wrong. He tried to prove it with the classic example of 'the person in the train'. Some were convinced but basically he might be fired because of this incident.More so because the kids complained that he didn't teach out of the book. I am really mortified because I studied in that same school for 3 years and I recollect how ludicrous it seemed to me that questions used to come from the exercises at the end of each chapter.
I know how people condemn the british style of teaching ( zambian schools follow the cambridge syllabus or atleast emulate it) but hey ...I never mugged the answers. I had to understand what the hell was going on because none of the questions came from within the exercises.

I guess the real question is ...do we spoonfeed the kids because of the volume of information that they have to learn ...if so what the **** are they learning ?????
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Old July 10th, 2002, 09:46 AM
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Quote:
Originally posted by gossipmonger
yah the inertia answer is correct. This question was part of an 9th standard exam in an indian school. 90%of the kids answered it was due to the third law. A close friend of mine who happens to be a teacher marked it incorrect. It is because of the inertia funda.
Not completely. Like I said, it is because of both intertia as well as action-reaction. Inertia funda doesn't explain why the dust particles would jump to a height two times from the height they were initially.

If the kids didn't get it right, the teacher too was a pedantic dumbass.
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Old July 10th, 2002, 10:03 AM
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Re: A physics question

Quote:
Originally posted by gossipmonger
When you clean a rug/mat , you beat it with an object(like a stick or brush) or against a wall so that you can beat the dust particles out of the rug or mat. Can someone please explain in terms of newtons laws why that happens
i.e when we beat the rug with an object or even shake it, the dust particles fall of. Perhaps the question you might want to ask is WHY the dust particles fall of.
Happy solving
I remember that this was a question which our Science teacher in 9th standard asked us once.

Well, the answer is simple and I think GpeL has already explained.

In short - When you strike a rug/mattress with a stick or even give it a strong jerk, the dust particles on the rug remain in their place due to inertia. The rug has moved away but the particles remain at their place. Now gravity comes into play. The dust particles are attracted towards thge ground due to inertia.

You might notice some particles flying around before they fall to the ground. One more force comes into play here. When the rugh moves suddenly from its initial resting place there is a body of wind which is displaced, and this causes the particles to fly away for a brief while and then they finally fall down due to gravity.
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