Force Body Diagrams of object sliding down ramp So I think I am confusing myself about forces. I know that when something is sliding down a ramp that it will have the force of gravity, normal force and force of friction acting on it. 
But for an example, what if I had a toy cart and added a chocolate bar to it. When it is sliding down the ramp, the forces acting on it will be the force of gravity, normal force, friction but would there be an applied force on the toy cart from the chocolate bar? 
Or would I just treat this still as "one system" and make it so that when I am calculating the forces I increase the mass to take into account the chocolate bar? Or would I have to put an applied force downwards? 
 A: You can treat it as one system. Since the weight of the bar and the car are downwards, you can add them.
A: You can do either.
If the chocolate bar is not moving relative to the cart and you don't care how much force is between the cart and the chocolate bar, then you can treat them as a single system.
If you do care about the forces or the chocolate bar is sliding around within the cart, then you'd have to model the system with two separate independent masses. The cart would have a normal and fiction force from the ramp, a normal and friction force from the chocolate, and gravity. The chocolate would have the normal and friction force from the cart (equal and opposite to the cart's from the chocolate as per newtons third law) and gravity.
This is true in general for multiple bodies, as long as they move together and you don't care about internal forces you can model it as one solid body. That's why you don't have to model each half of the cart or even each molecule of the cart separately. If however you were worried about the cart breaking then you'd want to model those internal forces to see if it would break.
A: For sure the object you will put in cart will act as one body until and unless they both have same velocity or I say that they both move relatively and this is possible if both have contact with each other. And if you want to take internal forces then you have to work on that but besides everything of what you ask the answer is simple that you have to take that as single body.
Example-
Let us say one block sliding down and for the sake of question and imagination let us take one small section of (red portion in image) as chocolate bar. So by seeing the FBD we can recognize that it will act as one body (as the red section is already a part of that block) will work as one system and it will act just same in your Cart example due to which the mass of the system increases.

