The most difficult physics question!! please help?

Question

Plz help on this---physics question?

Which of the following should be included in a free body diagram

A. active forces

B. external forces

C. internal forces

D. forces with unknown magnitudes or distances

E. labels of the forces included

F. angles or directions showing the line of action of each force

G. reactive forces

pplease do not direct me to google or any other site!!

Answer 1

A:Yes- Only forces acting should be included

B: Yes- Only external forces need to be supported

C: No - You don't need to support internal forces

D: No (?) - Can't consider what you don't know except those you are solving for.  If this is what you mean here then Yes.

E: Maybe - Helps if you do this but the labels are not required.  You "should" but you don't have to.

F: Yes - Need these to do the Trig to solve the unknown forces

G: Yes - These are what you are solving for.  You need the magnitude and direction these forces act.  

Answer 2

By definition, a free body diagram should contain only external forces working on you. For example, a FBD of a block of metal on top of a table. It should have at least 2 external forces working on it. 1 is the gravitational force working on it by the eart (in simple words its called: WEIGHT). 2. is the normal force exerted by contact between table and metal.

Depending on the system of interest, other external forces in various forms might work on the block. Say, electromagnetic forces (if the metal block was inside a electromagnetic field), or friction and inertial forces should the metal be moving. These external forces that works on the systems of interest are the things you should identify first and then put on your FBD. Since force is a vector off course you need to also know the angles and line of action of each force, this should also be put on your FBD.