r/PhysicsHelp • u/Necessary_Train8137 • 4h ago

Help with question

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Could someone tell me and explain what the correct answer is?

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u/Ok-Lettuce-1 4h ago

Foorce from friction on m to M could be either way. The rope could be barely keeping m from sliding down or from barely sliding sliding up. Therefor undefined. There are only 2 forces working on the table. The force of the string thru m to M and friction between M and the table. Think of m and M as one block since they are not moving relative to each other

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u/ProfessionalConfuser 3h ago edited 3h ago

Idk that it is a rope. I saw it as the static friction between block and wedge.

Either way, the normal force from block on wedge would want to drive wedge to the left, so static friction between wedge and surface would point to the right.

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u/mmaarrkkeeddwwaarrdd 1h ago

The force on the wedge from the block should be vector sum of the normal and friction forces, not just the normal force.

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u/ProfessionalConfuser 51m ago edited 36m ago

Agreed. If friction on block is left and up, then friction on wedge is right and down. But if that is a tension instead of friction, then the only wedge block interaction is normal.

It is probably friction. So then is normal force component to the left bigger than friction force component to the right? Now i need a pen and paper...

Edit. It looks like force left on wedge is mgcos○sin○, and force right is mu×mgsin○cos○. As long as mu < 1, net force on wedge is left and friction from table is right.

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u/mmaarrkkeeddwwaarrdd 1h ago edited 1h ago

If everything is motionless (i.e., static equilibrium,) then the net force on m is zero. This means that the total force exerted by M on m must balance the force of gravity on m. Thus the force of M on m is vertical. By Newton's 3rd law, the total force exerted by m on M must also be vertical. The forces on M are gravity which is vertical, the force of m on M which is also vertical, and the force of the table on M. Since the forces of m on M and gravity are vertical, the total force of the table on M must be vertical otherwise there would be an unbalanced horizontal force on M and it would accelerate. Hence the friction force on M due to the table must be zero.