IIT JEE , AIEEE Forum - Physics , Mechanics

hm.india

The man is trying to pull the block on which he is standing throgh a rope connected through two fixed pulleys to the block.

The surface between the man's feet and the block is frictionless.

There is friction between the block and the ground.

utkarshsingh_91

Well i think the whole ques depends on the coefficient of friction between the block and ground
if there is no friction between the block and ground then the force exerted by man will cause acceleration of the combined mass of (the man & block.)
else u have to subtract the friction

kmmankad

No,as when the man pulls the string,his feet shall have a sliding tendency forward(torque) and since no friction opposes this tendency,he shall fall over......But it does depend on the torque he exerts and also the Coefficient Of Friction as said above.....

Pls Rate if u find this appropriate...

bvsatyaram

As stated by kmmankad, the man has a tendency to topple.
So, lets neglect the height of the man (a lilliput). So, no toppling!!

Now the question asks if the man can pull the block. The max possible friction is
f= (mu)*(M+m)*g, where mu is coeff of friction, m and m are masses of block and man respectively. So, if the man is sufficiently strong and can apply this much force on the srting (the same force will be transmitted to block in the form of tension), then he can definitely pull the block!!

nishant13940

THERE IS NO FRICTION BETWEEN THE MANS FOOT AND PLANK.
WHEN MAN WILL PULL THE BOX THEN ACTION-REACTION LAW CAN BE APPLIED.
NOW THERE IS NO FRICTION B/W MAN AND BLOCK WHILE BLOCK &GROUND EXERT FRICTION. THEN FORCE OF STRING WILL BE EFFECTIVE AND IT WILL PULL THE MAN TOWARDS IT SELF.
HENCE MAN WILL SLIDE AND SO HE CAN'T PULL THE BLOCK.

rajvarun

lets try to solve it mathematically then we can pu tu=0

lower block -

since there is reative motion friction will try to oppose it

t should be reater than uN

N= mg or normal force between he man and the block

where m is the mass of the man

the friction will act in an opposite direction on the man and in equal magnitude

f plus un should be greater than t where fis the force exerted by the man

therefore f should be greater than un

which is equal to 0

therfore if the force is even 1 newton the man will be able to pull it

abhi.sharma

As i think what ever may be the force man cannot pull the block whether the friction is present or not.

we can consider man and the platform as a system

as internal forces cannot do any work .resultant force acting on system is 0.

please kindly correct me if iam wrong anywhere.

rooney

Satya Sir is right.

@abhi.sharma

Here, the energy of the system is coming from the muscular energy of the man. Total energy is same. Just that the stored muscular energy got converted into kinetic energy.

And external force on system is not zero. The ground is exerting frictional force on the system.

@raj varun
If force applied is less than the friction exerted by the ground, the block will not move but the man will move and the string will become slack since the man will slide even if he applies infinitely small force. But the Force applied needs to overcome the frictional force to move the block.

Ask & Discuss Questions with Community & Experts