What is the difference between "in umerīs" and "in umerōs"?

Question

In chapter 6 of LLPSI, we have the following sentence

Syrus et Lēander duōs saccōs in umerīs portant

While in chapter 9, we have:

Pāstor laetus ovem in umerōs impōnit.

Why the ablative in the first sentence and the accusative in the second one?

Answer 1

In + ablative means "in/on something" while doing the verb.

In + accusative means "into/onto something", i.e. the verb involves moving/transferring something else into/onto the something.

So, Syrus and Leander carried the sacks on their shoulders (while carrying them to Iulius's villa); the shepherd placed the sheep onto his shoulders.

Answer 2

…carry sacks in their arms. (Abl. —> no motion happening with the sacks) …placed the sheep into his arms. (Acc. -> motion happening with the sheep)