Help with Friction Related Problem

[physixguru]

A block of mass 0.1 kg is held against a wall with a horizontal force of 5N.The coeff. of friction between the wall and the block is 0.5.WHat is the magnitude of frictional force acting on the block??There is no friction between floor and block.

Pls provide the complete logic...

HELP WILL BE HIGHLY APPRECIATED>>>

 

[Doc Al]

Show your work and tell us where you got stuck.

Hint: Identify all forces acting on the block. Does the block move?

 

[physixguru]

exactly...the block does not move ..

this implies that the force applied by static friction equals the applied force..

but how do i estimate the magnitude of contact force between wall and the block...

i solved it the following way..

the force applied by man is equal and opposite to the force by the wall on the block.[NEWTON'S 3RD LAW]..this is why the block does not move...

5+mg = frictional force+normal force by wall.
>>5+.98= frictional force+5
>>frictional force=.98

the answer is correct but pls elaborate the logic.

 

[Doc Al]

exactly...the block does not move ..

this implies that the force applied by static friction equals the applied force..

No it doesn't. It implies that the net force on the block is zero in all direction.

Realize that the friction force acts vertically while the applied force is horizontal--they act in different directions!

Instead: Analyze the vertical forces acting on the block. They must add to zero.

(Realize that it's friction that holds the block up. What's the maximum value of static friction given the normal force pushing the block against the wall? Is that enough to support the weight?)

but how do i estimate the magnitude of contact force between wall and the block...

i solved it the following way..

the force applied by man is equal and opposite to the force by the wall on the block.[NEWTON'S 3RD LAW]..this is why the block does not move...

Careful here: The reason that the applied force of man on block must equal the force of the wall on the block is NOT Newton's 3rd law. It's because we know the block does not move horizontally.