I'm new to physics and i just started studying energy and i have a question:
- Consider an object (mass m=2kg) dropped freely from a height h = 2m (there is no friction between object and air).
1. If potential energy mgh only belongs to object, so its energy is
E = PE + KE + U
according to the law of conservation of energy, we have:
ΔE = ΔPE + ΔKE + ΔU = Wg + Q
but Wg = 2*10*2 = 40(J) and Q = 0 (because there is no friction between air and object) and ΔPE + ΔKE = 0
so ΔU = 40(J), so its internal energy changed (?) but I know this is wrong because the object volume and its tempurature don't change and U = f(V,T) 
Therefore, potential energy should belongs to system "object-earth"
2. If potential energy mgh belongs to system "object-earth", so why textbook often say that mgh is potential energy only belongs to object ?
I am so confused about this problem. Hope that you can help me solve this problem