When to use energy methods:

• Relating speeds at different positions
• Variable forces (avoid complex integration of F=ma)
• Path doesn't matter (conservative forces)
• Collision problems (combined with momentum)

When NOT to use energy:

• Need acceleration or force at a specific point (use F=ma)
• Need time information (energy is time-independent)
• Need trajectory details

Step-by-step:

1. Identify initial and final states
2. Choose PE reference (usually lowest point = 0)
3. Check if only conservative forces act
4. If yes: $KE_1$ + $PE_1$ = $KE_2$ + $PE_2$
5. If friction: subtract friction work from one side
6. For collisions: momentum first, then energy
7. For power: P = Fv or P = $\frac{W}{t}$

Common traps to avoid:

• Using conservation when friction is present
• Forgetting that PE reference is arbitrary
• Using $\frac{1}{2}$k($delta_x$)^2 instead of $\frac{1}{2}$k(x_2^{2-x}_1^2) for spring work
• Confusing KE loss fraction in collisions
• Assuming constant acceleration under constant power