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bhopalyellow

@bhopalyellow

Senior Member

Posted: 15 years ago

am i right brother.....i don't think so.....plz tell

F=m*a

a=f/m

=2/10 2

=0.2 m/(sec) 😊

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bhopalyellow

@bhopalyellow

Senior Member

Posted: 15 years ago

Originally posted by: rajnish_here

oops i thought this que is part of game and answered. but this is for jasleen. so edited😃😃

thanks rajnish bhaiya wase i've ans. the question....so plz tell me my result...😆

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akhl

@akhl

IF-Rockerz

Posted: 15 years ago

Originally posted by: bhopalyellow
[
ans. : as u explained that the min. force which can stop the book from sliding is the required force , so the min. force here is 2N , although the book is pushed with 3N but the min. force is 2N which is enough to stop the book from sliding , so the ans. is 2 N .

Your final answer that the force of static friction =2N is correct. But the reasoning has some fallacy. Let me explain the correct reasoning.

Suppose that I apply force towards right.

Coeff. of static friction us = 0.2

Normal force Fn = 10 N

Therefore maximum value of static friction = 0.2 * 10 N = 2 N

I apply 3 N force towards right. This means that 3 N friction towards left is needed to stop the book from moving.

But static friction can be at maximum 2 N. It cannot be more than this.

So static friction = 2 N.

I apply 3 N towards right and friction is 2 N towards left. This means that friction will not be able to balance my force and the book will move towards right.

What will happen after that? The answer is that once the book starts moving, then kinetic friction takes over.

So, for a very short time, there is static friction of 2 N. Then the book starts moving and after that there is kinetic friction.

Note that us*Fn is the maximum static friction. If less than this is enough to stop the book from sliding, then static friction will be less. But static friction can never become more than us*Fn. If more friction is needed to stop the book from moving, then the book WILL MOVE and static friction will be us*Fn.

Let me know if you need further clarification.

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akhl

@akhl

IF-Rockerz

Posted: 15 years ago

Originally posted by: bhopalyellow

am i right brother.....i don't think so.....plz tell

F=m*a

a=f/m

=2/10 2

=0.2 m/(sec) 😊

Mistake you have made is that you have considered only force of friction. What about force applied by me?

I apply 3N force. Friction is 2N in opposite direction.

So net force = 3N - 2N = 1N

Mass = 10 kg

a = 1/10 = 0.1 m/s^2

As I explained in my previous post, static friction is for a very short time. This means that there is acceleration of 0.1 m/s^2 for a very short time. After that acceleration depends on kinetic friction. And kinetic friction depends on coefficient of kinetic friction, which is not given in the question. So, it is not possible to calculate kinetic friction.

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akhl

@akhl

IF-Rockerz

Posted: 15 years ago

Originally posted by: bhopalyellow

[quote=akhl]Jaslees,

I edited my explanation and in the end, have added a question. It is only for your learning. Can you tackle that? And also please try answering the following question. This is also not a part of the game but only as learning exercise.

A book is kept on the table. Both are moving towards north. Table is moving at 2 m/s and book is moving at 1 m/s. Both these values are relative to ground.

What is the direction of force of friction on the book?

[/quote]

i think.......the relative speed=speed (moving table - book)

=2-1 = 1 m/sec.

and as the friction opposes the force on the book and the force is in north direction so the friction will be in opp. i.e., south direction........😊

Correct answer is north direction. Friction opposes relative movement. We need to find direction of friction on the book. It is true that book is moving towards north. But what is important is in what direction it is moving relative to table?

Table moves at 2 m/s towards north.

Book moves at 1 m/s towards north.

So relative to the table, the book moves towards south. Therefore friction is towards north.

To give an analogy, suppose I and you both are driving our cars. We both are driving towards north. You are driving at 70 kh/h and I am driving at 50 km/h. You are driving at faster speed. So, you will see me moving towards south.

In my question, table is moving faster. So table sees book moving behind, which means towards south. Therefore friction is towards north.

Edited by akhl - 15 years ago

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343707

Inactive

Posted: 15 years ago

jasleen the question which avinash asked you are good example of question which teach your reasoning abilities as well as fundamental knowledge. most of the PMT exam paper ask only these kind of questions. they never go beyond this level. the person doing preparation of IIT may go beyond this level but for PMT students practicing these questions are only thing required. If you master these question, and a littile bit of time management I bet you can easily crack PMT exam. Anytime feel yourself free to ask any questions or doubt (except bio) to me or Avinash😊
and best of luck for your PMT exam👍🏼
Edited by rajnish_here - 15 years ago

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bhopalyellow

@bhopalyellow

Senior Member

Posted: 15 years ago

thank u very much avinash bhaiya......ur clarifications r very much useful and i did understood my mistakes.......thank u for such sweet help.....and i'll make sure not to repeat such mistakes in future.....😛

thank u to rajnish bhaiya too........i'll def. follow ur advice and pay attention to such ques. ........i'm thankful that u both will help me anytime.....i'm hopeful not to make such mistakes in future......😃

thanks for all the lucks.....i really need them.....😃

thanks to both of u ones again...😊😊😊

waiting for more ques. ........😉😃

Edited by bhopalyellow - 15 years ago

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akhl

@akhl

IF-Rockerz

Posted: 15 years ago

Yes, Jasleen, all the best.😊

So two questions are unanswerred: -

One is asked by Jasleen and another by Rajnish.

The first is of biology, so I cannot answer.

I can answer the one asked by Rajnish.

But, before that, does somebody else want to try that question?

Meanwhile let me ask one question: -

You are standing on a scale, which measures weight. The scale shows your correct weight. There is a cupboard nearby. You keep your palms on the top of the cupboard and push the cupboard down, which means that you apply downward force.

Will the scale show the same reading or less or more? Explain.

Answer this question correctly and earn points.😊

Edited by akhl - 15 years ago

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bhopalyellow

@bhopalyellow

Senior Member

Posted: 15 years ago

avinash bhaiya ...under which topic does this que. comes....plz tell

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akhl

@akhl

IF-Rockerz

Posted: 15 years ago

This question combines the following topics:-

1. Force of gravity

2. Normal force

3. Newton's laws

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