Work vs Power Mistakes (Joules vs Watts)
Work is energy transferred (joules). Power is how fast that happens (watts). Exams punish mixing them.
W = F d · P = W/t · P = F v — 1 W = 1 J/s · convert minutes → seconds
Trap 1: Calling power “work” because both use W
Wrong: P = 50 W described as “50 joules of work.”
Right: 50 W means 50 joules every second. Work would need a time: W = P t.
Fix: Check the unit: J → work/energy; W (watt) → power.
Trap 2: Leaving time in minutes for P = W/t
Wrong: 600 J in 2 minutes → P = 600/2 = 300 W.
Right: t = 120 s → P = 600/120 = 5 W.
Fix: Always convert minutes × 60 before dividing for watts.
Trap 3: Using P = Fv when force is not along velocity
Wrong: Any F and any v multiplied, even when force is perpendicular to motion.
Right: Use the component of force along the velocity: P = F v cosθ (or F_parallel × v).
Fix: P = Fv is the special case when force and velocity are parallel.
Trap 4: Mixing mechanical power with electric P = VI
Wrong: A lifting problem answered with P = VI because “power is VI.”
Right: Mechanical: P = W/t or P = Fv. Electric circuits: P = VI (or I²R, V²/R).
Fix: Match the formula to the context — motors can involve both, but the given data tells you which.
Trap 5: Same work, assuming same power
Wrong: Two lifts of 100 J must both be “the same power.”
Right: Same work in less time means more power: 100 J in 2 s → 50 W; in 10 s → 10 W.
Fix: Power always needs a time (or a speed with force).
Bottom line
Joules are work; watts are joules per second. Convert time to seconds, and only use P = Fv when force lines up with velocity.
FAQ
What is the difference between work and power?
Work (J) is energy transferred. Power (W) is the rate of that transfer: P = W/t.
How many watts in one horsepower?
About 746 W for mechanical horsepower in most school conversions.
Work Calculator · Power Calculator · Horsepower Calculator · Electric Power Calculator