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General Strategy for Applications in Kinetics

Key Concept: Euler's Laws govern the motion of rigid bodies in plane motion. The key step

is the construction of a complete and accurate free body diagram (FBD) to identify all forces

and moments (couples) acting on each body. The table below details the basic strategy.

1.	Draw a FBD of one or more parts to identify the forces (and possibly moments) that act on each part. This is the key step. If incorrect, then all remaining calculations will be for naught.
2.	Write equations of motion for each part. i.e. ∑ F = ma_c, ∑ M_C= I_C α
3.	Identify all unknowns in the set of equations in step 2.
4.	If the number of unknowns equals the number of equations, then proceed to solve for the unknowns. Note: For plane motion there are 3 independent equations of motion for each part ˗ x˗component, y˗component, and rotation.
5.	If the number of unknowns is greater than the number of equations, then you need more information.
6.	Additional information comes in three forms ˗ kinematics that relates the accelerations such as the acceleration of the center of mass to the angular acceleration under a condition of rolling, maximum friction when sliding occurs, or perhaps location of the normal force when tipping impends or occurs.
7.	It may be that the conditions for rolling, sliding, or tipping are unknown. Then you need to make assumptions for admissible cases and proceed to solve the set of equations based on your assumptions. Your analysis ends once you confirm by checking that your assumptions were correct.
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