Work
The work done by a force is the product of the magnitude of the displacement and the rectangular component of the force in the direction of the displacement.
When a constant force P is applied to a particle, which moves a distance d in a straight line, the work done on the particle by P is defined by the scalar product
- When the force and displacement are in the same direction, the work done by the force is positive.
- When the force and displacement are in the opposite direction, the work done by the force is negative. Work of friction is always negative, since it always opposes motion.
- When the force and displacement are perpendicular, the work done by the force is zero.
In short,
U1-2 = sum of the work done on the particle between points 1 and 2
where work terms are positive/negative if they tend to increase/decrease the kinetic energy of the particle.
Units: ft-lb, in-lb, J (same as kinetic energy)