# Identifying third-law pairs in two masses connected by a string

Consider two masses connected by a string over a pulley, like so:

Where mass $$M$$ is held in place by a hand.

I'm asked to draw free-body diagrams for mass $$m$$ and $$M$$ individually, and identify third-law pairs in the two diagrams. I'm then asked to consider when the hand moves and mass $$m$$ begins accelerating downwards.

We can assume the surface to be frictionless and the pulley to be massless.

It doesn't seem to me that there are any third-law pairs. The only third-law pairs I can identify in the diagram are the normal forces between the hand and the block (in the first case).

• yes, you are wrong, cant you think of any other forces acting on each mass?
– user65081
Oct 29 '20 at 22:41
• @Wolphramjonny In what way? Third-law pairs must a) act on two objects and b) be of the same type and magnitude. I see no such pairs here? Oct 29 '20 at 22:42
• each force you can identify will have a pair. Can you find any other forces other than the ones between the hand and the mass on the table?
– user65081
Oct 29 '20 at 22:43
• Mass $m$ will have two forces, the tension $T$ of the rope, and the force of gravity $F_g$. Mass $M$ will have the normal force from the hand, $F_g$ and the normal force between the surface and the mass, and $T$ the tension. Oct 29 '20 at 22:48
• good!!! now find the reaction (pair) of each one, which is the force that the masses do to the objects that make a force on them
– user65081
Oct 29 '20 at 23:44