PhysicsMediumClass 11

Power — P=Fv, average power

Work, Energy & Power

JEE Qs

Hard

min

Clearly distinguish between instantaneous and average power, and always apply the vector dot product for P = F · v to avoid directional errors.

🧮 Key Formulas

P_avg = W_total / Δt_total

P_inst = dW/dt

P_inst = F · v

η = P_out / P_in

1Instantaneous power (P = F · v) is the rate at which work is being done by a force at a particular instant. Remember to use the dot product as F and v are vectors.
2Average power (P_avg = W_total / Δt_total) is the total work done over the total time interval.
3Power can be positive, negative, or zero depending on the angle between the force and velocity vectors (cosθ term in the dot product). Positive power means force is doing work, negative means work is done against the force.
4Efficiency (η) is the ratio of output power to input power, a crucial concept in problems involving engines, motors, or systems with energy losses.
5For problems with constant power, remember that force will vary with velocity (F = P/v), often leading to non-uniform acceleration.

✕Mistaking average power for instantaneous power, or vice versa, in problem-solving.
✕Forgetting the vector nature of P = F · v and simply multiplying magnitudes, especially when F and v are not parallel.
✕Incorrectly calculating the total work done when finding average power, often missing non-conservative work or initial/final kinetic/potential energies.