CP PHYSICS
Particle in Equilibrium Model: How to draw a Free Body Diagram
A force diagram, or a free body diagram, is a picture of all the forces acting on an object.
To construct a force diagram, ask the following questions:
1. Is the object near the earth or another very massive body, like a star?
NO For us, the answer will always be yes
YES If the answer is yes, draw an arrow from the object pointing toward the earth or other body. WeThe call the force of the earth on the object gravity, and label the vector as FEarth on object.
2. Is the object on a surface?
NO Proceed to question 3.
YES Then the surface exerts a force on the object. This force, Fsurface supporting object, is perpendicular to the surface.
3. Is something pulling or pushing the object right now?
NO Proceed to question 4.
YES Then draw an arrow to show this force. It could be labeled, for example, Frope on object. If the object is being pulled by a cable or rope, the force needs to be drawn in the same direction as the cable or rope.
Careful—if something was previously pushing or pulling the object, but no longer does, it does not exert a force on the object.
4. Might friction be acting on the object?
NO Then the object is 1) on a frictionless surface or 2) at rest with no forces except FN and Fg.
YES If the object is moving, friction (F friction on object )acts in the direction opposite of the motion.
For a ball rolling along on the floor, the answer to question 3 can be no, but the answer to 4 can be yes.
How to decide how long the vectors should be
Key rule. Galileo divided motion into two categories: 1. “Natural motion:” an object at rest or moving with constant velocity; and 2) accelerated motion.
Accelerations require an unbalanced force. If an object is accelerating to the right, the forces pointing to the right must be longer than those pointing to the left.
If an object is not accelerating, then the forces pointing left should be equal to those pointing right, and the forces pointing up should be equal to the forces pointing down.
MHS Physics Group Free Body Diagram Reading