CMake - Weird Include Directories - c++

Question

I have created a project which consists of a main.cpp file and another class (named Physical) that is broken up into a Physical.hpp file and Physical.cpp file.
The project file structure looks like this:
main.cpp
header/Physical.hpp
src/Physical.cpp
The project compiles fine, but only if I include Physical.hpp in different ways depending on whether or not I'm including from the Physical.cpp or main.cpp file.
From the main file, I have to use:
#include "header/Physical.hpp"
Whereas, from the Physical.cpp file I have to use:
#include "../header/Physical.hpp"
Why is there this discrepancy? I expect it has something to do with my cmake configuration file, although I'm really not sure. Below is my cmake config file.
cmake_minimum_required(VERSION 2.8.4)
project(gravity_simulator ${PROJECT_BINARY_DIR})
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
set(SOURCE_FILES main.cpp src/Physical.cpp)
include_directories("${PROJECT_BINARY_DIR}")
add_executable(gravity_simulator ${SOURCE_FILES})
# Detect and add SFML
set(CMAKE_MODULE_PATH "/usr/share/SFML/cmake/Modules/" ${CMAKE_MODULE_PATH})
#Find any version 2.X of SFML
#See the FindSFML.cmake file for additional details and instructions
find_package(SFML 2 REQUIRED system window graphics network audio)
if(SFML_FOUND)
include_directories(${SFML_INCLUDE_DIR})
target_link_libraries(gravity_simulator ${SFML_LIBRARIES})
endif()

Answer 1

Everything works as expected.
For your main.cpp file the header is located at header/Physical.hpp while for your Physical.cpp the header is located at ../header/Physical.hpp.
The mistake here is, that you think the relative path would always start from a specific directly, while instead every source file (*.cpp) gets compiled independently and thus the header search is happening for each file independently.
Personally if I don't write a library, I simply put all my source and header files in a src directory and everything will be fine. However if I'm writing a library, I use the <header.hpp> style and add the header directory to the include directory (include_directories("header")). The whole reason why one usually splits up things into header and source files is, so when one ships a library one can easily just ship the headers and doesn't have manually filter them out between the source files.
tl;dr When writing an application put everything a source directory. When writing a library, use the <> for the inclusion add the header directory to the include directories.