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Physics LibreTexts

01. Concepts and Principles

The Impulse-Momentum Relation

Just like the kinematic relations and Newton’s second law, the impulse-momentum relation is independently valid in any member of a set of perpendicular directions. Thus, we will typically apply the impulse-momentum relation in its component forms:

Conservation Picture 1.png

The Work-Energy Relation

From Model 1, our expression for the Work-Energy Relation, with gravitational potential energy terms, is:

Conservation Picture 2.png

It’s very important to remember that the work-energy relation is a scalar equation, meaning it can not be broken into components and “solved” separately in the x-, y-, and z-directions. This is even more important to remember now that we are working in multiple dimensions. This observation results in two important points:

  • The work-energy relation involves the actual initial and final velocities, not their components. The kinetic energy of an object does not depend on the direction of travel of the object.
  • In the expression for work, Conservation Picture 3.png, the product of the magnitude of the force and the magnitude of the displacement is multiplied by cos f, where fis defined to be the angle between the applied force and the displacement of the object. If the force and displacement are in the same direction f = 0°, and the work is positive (the object gains energy). If the force and displacement are in the opposite direction f = 180°, and the work is negative (the object loses energy). If the force and displacement are perpendicular, no work is done. Note that the actual directions of the force and the displacement are unimportant, only their directions relative to each other affect the work.