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This is the question, and the answer is given as 2).

Now my basic doubt is, are they asking us the direction of the total contact force on the object by the floor? Or, are they asking the direction of the static friction on the block by the floor? What is it? Because, the normal reaction between any two surfaces in contact is always perpendicular to the two surfaces.


Your doubt is well found. It is actually the book which is wrong here in choice of words.

Basically, according to the term "reaction", it should mean that the answer should be (3), but the book is trying to ask only about friction which it fails to ask properly by using a poor choice of words.

So, both (2) & (3) are correct in their own context, however, (3) is more correct, since this is what anybody would understand the question to be.

But since the book says (2) is correct, it has failed to ask the question properly & if this is asked in an exam or something, you can blindly go for (3) as (2) won't be termed as correct there.

So the book is wrong here .

Hope that helps.

  • $\begingroup$ ok, but how is it 3? the normal reaction is just along +ve y axis $\endgroup$ – Saurabh Raje Jun 18 '13 at 4:39
  • $\begingroup$ @SaurabhRaje: Because there is friction too, not only normal force. $\endgroup$ – Abhimanyu Pallavi Sudhir Jun 18 '13 at 6:04
  • $\begingroup$ @dimension10, Exactly!! But they are Just asking you for the direction of the Normal Reaction. $\endgroup$ – Saurabh Raje Jun 18 '13 at 8:10
  • $\begingroup$ If they are asking for the direction of the normal reaction, then it should be along the +Y axis right? $\endgroup$ – Saurabh Raje Jun 18 '13 at 8:12
  • $\begingroup$ @SaurabhRaje: If it were the normal force, then by definition, yes. But I doubt the question is asking for that. As Iota pointed out, since the given answer is (2), they are probably only asking for friction (which is something really foolish to ask for, in my opinion). But when they say "Reaction", they cannot mean just friction OR just the normal force, but both. This is why if the question is taken literally, it should be (3). $\endgroup$ – Abhimanyu Pallavi Sudhir Jun 18 '13 at 8:24

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