[My] friend asked me if [you're] able to pick up police signals from an [aircraft's] radio. [Could] you tell me [I] don't like not knowing things about aviation.

Good god! Use spell checker. No, you cannot pick up police signals from the aircraft COMM radio. The 2 are a whole different range of frequencies. The only way that you can do this (I do this occassionally when im flying the 182RG) is to bring a scanner and listen to it. You will get amazing audio quality because you are above the horizion with no obstacles. Anyways the short is answer is no, you cannot.

Kind of off topic, but I'll say it anyway.
I can pick up police audio through my freeview box on my T.V. They're only faint but if you turn the volume up, you can just make out what they are saying.

Strange but true