Definition of Work

What is work?  Work (W) is the energy change of a system due to an external force.

When work is done on a system, W > 0.
When the system does work on the external environment, W < 0.

How do we calculate work?  There are three situations to consider:

  1. Constant net force colinear  with the displacement;
  2. Constant net force non-colinear  with the displacement;
  3. Variable  net force.

1. Constant colinear force:    W = F x    x = distance traveled.

2. Constant non-colinear force:    W = F Δr = F Δr cos θ = Fx Δx + Fy Δy + Fz Δz
See transparencies for definition of dot product.

Note that:   W > 0 when the force is in the same direction as the displacement;
W < 0 when these are in opposite directions;
W = 0 when FΔr;
W is maximized when F || Δr.

3. Variable force:   dW = F(r) • dr → W = ab F(r) • dr

Justification: From Newtons' 2nd law:

Fs = m as = m dvs/dt = m (dvs/ds) (ds/dt) = m vs (dvs/ds).

Multiply by ds:   Fs ds = m vs dvs.

Integrating gives:   ab Fs ds = vivf m vs dvs= ½ mvf2 - ½ mvi2

→ W = Δ K