1ok anyway... but still, the 3rd law is valid for all cases...
According to Newton's Third Law "Every action has an equal and opposite reaction". The question is: A block of mass m is kept on a horizontal table. What is the "reaction" to the force mg experienced by the block?
Options are:
(a)Frictional force on the block by the table
(b)Normal force on the block by the table
(c)None of these
People who already know this question, please don't spill the beans early!
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27 Answers
1no ur fbd is nt wrong bt action reaction forces acts on two diff bodies
24ohhh my GOD..........u were all thinking I am answering that question and I am fighting for that..........I was always writing to clear my statement............
Of course answer to ur ques is (c)
whatever I wrote in post#3 was not for ur question...........it was just that I was defining third law as a hint for others for ur question
1u're right, but that is NOT asked in the question... i've asked for the "reaction" to the force mg experienced by the block in the context of newton's 3rd law... so what's ur answer now? normal still??
1@eureka normal reaction is numerically equal to d mg as block is not accelerated but it s not d reaction force to mg
24i dont know anything else....i just feel that block experiences normal force is equal to mg
1what?? i'm not gettin ur point... and finally tell me what's ur answer to the actual question plz...
11are bhai ur fbd is absolutely correct but that is not the answer for wat has been asked?
Read the question carefully u'll get it.......
24what action reaction are u saying my dear friend....................
ok answer these questions then..........IN yes or no only..........
Q1 Wont N act on the block?
Q2 Wont N decrease if block were to accelrate downward
106i think it is (C) none of these...
this is because . action-reaction pairs act on different bodies and as 'mg' acts on block so its reation shud not act on block.. so (a) (b) are wrong... the reaction is actually the gravitational attraction by the block on the earth... if u consider earth's FDB, there u will get that reaction force of 'mg'
1No your FBD is not wrong, your perception of the third law is flawed.
1@eureka
okay... eureka, u're making exactly the mistake this question is made for... so finally someone fell into the trap :)... then acc to u the answer shud be (b) right? u r forgettin that the "action" and "reaction" that are mentioned in the 3rd law act on different bodies, never on the same body. e.g.a block of mass m is lying on a rough horizontal surface, with the coefficient of kinetic friction being μ. A constant horizontal force F starts acting on the body at t=0, giving it an acceleration 'a'. Then, the floor applies a force μmg on the block backward, and the block applies equal force on the ground forward. You must hv encountered this situation many times in friction based questions, and applied the 3rd law, perhaps without realizing it. And another thing, the answer is (c), since the "reaction" is force mg on the earth by the block. The normal on the block by the table is the "reaction" not to mg on the block, it is the reaction to the normal applied by the block on the table. Probably Nishant sir can explain better than me. Apart from that, i did not understand your situation #2 at all, sorry but can u explain it further?
1bt why r u trying to equate N and mg they r not action reaction pair na
1a freely falling body is accelerated but mg on earth due to body n on body due to earth r action reaction pair vicky is100%correct
24NOOOOOOOOOOOOOOOOO
If action and reaction would be equal then net force in that direction would be zero and hence accelration would be zero....................
I still believe I am 100% corrrect.............
1Yes asish u r right.
@eureka
the 3rd law is valid in all cases, even in accelerating systems. u urself must hv used it hundreds of times in solving problems of friction on accelerating blocks, as an example.
The purpose of this question wz just to give u a check for ur basics. This question has a long story behind it at my coaching classes: My phy teacher(head of the institute) asks dis question to all ppl who apply for the post of teaching phy n, surprisingly, only a handful of those "teachers" r able to answer it. Its really shocking wen he tells us all dat most of them answer (b) to dis question, n even after dey r told the answer and explanations r given, dey try to defend themselves wid illogical arguments.
106no eureka ... action and reaction are always equal and opposite be it with acceleration or without....
24Every action has an equal and opposite reaction if and only if body is not accelrating.......This word makes third law complete.....otherwise it is a waste.......