The prepositional phrase in this sentence is "difficult to understand." It is an adjectival phrase because it gives information about a noun ("physics"), the subject of the sentence.
A prepositional phrase is either used to give information about a verb or a noun in the sentence. It must contain a preposition (like <em>in</em>, <em>to</em>, <em>of</em>, <em>about</em>, <em>at</em>, <em>before</em>, <em>by</em>, <em>during</em>, <em>for</em>, <em>from</em>, <em>over</em>, <em>with</em>, etc.), the object of the preposition, and possibly, words which modify the object.
- It is an adjectival phrase if it modifies a noun (<em>I can't reach the toy </em><em><u>on the highest shelf</u></em><em>.</em>)
- It is an adverbial phrase if it modifies a verb (<em>He approached </em><em><u>with hesitation</u></em><em>.</em>)
- It is a noun phrase if it revolves around a subject, like a noun or pronoun (<em>We come </em><u><em>from various different backgrounds</em></u><em>.</em>)
