IIT JEE , AIEEE Forum - Physics , Mechanics

ramyani

A block of mass 2kg is pushed against a rough vertical wall with a force of 40N., coefficient of static friction being 0.5. another horizontal force of 15N ,is applied on the block in a direction parallel to the wall . will the block move? if yes, in which direction? if no, find the frictional force exerted by the wall on the block

catch_arnnie

since, the block is pushed against the wall with 40 N force, so normal reaction of the wall is 40N

=>limiting frictional force (f) = 0.5 * 40 = 20 N

since, f is greater than the horizontal force applied
=> the block will not move

therefore, frictional force exerted by the wall is 20 N upwards(to balance mg) & 15 N horizontally against the force(of 15N) applied..

ramyani

The ans says block will move

abhijeet_0201

how can a horizontal force be app parallel to the wall??????????

i remember this ques .there was some locha in it!!!!!!!!

ramyani

Does it mean parallel to wall ? I don't know. The book says II to wall.

catch_arnnie

the way i'm seeing it, i don't think the block will move...

catch_arnnie

i think this should be the figure...

the big black dot in center of block shows the pushing force of 40N..

ramyani

There must be something we r missing.

waterdemon

hey Ramyani I am 100% sure that the question is wrong if the answer u r saying is given that the block will move.

The force is acting in the direction parallel to the wall of 15N and that too if is acted in the direction opp to the force of 40N then there is no chance of the block to move as in

either of the cases the limiting Friction force is larger than the force that is applied II or perpendiculer to the wall.

spt_001

the block should only
move downwards

elessar_iitkgp

There are two perpendicular forces pressing the block. The reaction by the wall is constrained in a way that it can apply only a limited amount of tangential force.
N = 40 N
Now parallel to the surface of the wall there are two force 15 N and the weight mg = 20 N.
Net force parallel to the surface of the wall,
F =

15²+20² = 25 N.
Max frictional force that the wall can apply
f_max=

N = 0.5 x 40 N = 20 N
Which is less than the force F.
Hence the block moves parallel to the suface of the wall and the frictional force is
f = 20 N

ramyani

The answer says :
It will move at an angle 53 degree with the 15 N force .
How ?

nitin62225

both the question nd answer r correct!!!

soln:

F=40 N

N(normal reacn)= F=40 N

both mg anf F'(=15 N) act perpendicular to the normal reaction.

so net force exerted in perpendicular dirn. (w.r.t the N)=

((mg)²+F'²) =25 N,at an angle

tan^-1(4/3) from horizontal in downward direction.

the friction force(max.)= 20 N

so resulting force = 5 newton due to which the block moves at an angle of 53^o with horizontal in downwards dirn.

if ne doubt feel free 2 ask!

abhijeet_0201

how can a horizon. force act parallel to the wall??????????
and........the ans is 15N and ur ans 5????

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