In static equilibrium, all forces balance so the net force is zero: \$\sum \mathbf{F}=0\$.
In dynamic equilibrium, the object moves at constant speed, so the net force is still zero but kinetic energy remains constant.
Think of a book resting on a table – the weight of the book is balanced by the table’s normal force. No motion, no net force.
When a force does no net work, energy is conserved in the system.
Example: A box sliding on a frictionless surface. The only horizontal force is the applied push. If the push stops, the box keeps moving – kinetic energy remains constant because no net work is done after the push.
Power measures the rate of energy transfer: \$P = \frac{dE}{dt}\$.
For a constant force moving at constant speed: \$P = \mathbf{F}\cdot\mathbf{v}\$.
Analogy: Power is like the speed of a river – it tells you how quickly water (energy) flows downstream.
⚡️ Quick calculation: A 10 N force pushes a box at 2 m s⁻¹ → \$P = 10 \times 2 = 20\$ W.
🔍 Remember:
Two teams pull on a rope with equal forces of 500 N each, but in opposite directions.
Net force = 0 → static equilibrium. No acceleration.
Work done by each team is zero because the rope does not move. Energy stays in the system as stored elastic potential energy in the rope.
⚙️ Power? Since there is no displacement, power is 0 W.
| Concept | Formula | Units |
|---|---|---|
| Work | \$W = \mathbf{F}\cdot\mathbf{d}\$ | J (joule) |
| Power | \$P = \frac{dE}{dt}\$ or \$P = \mathbf{F}\cdot\mathbf{v}\$ | W (watt) |
| Kinetic Energy | \$K = \frac{1}{2}mv^2\$ | J |
Equilibrium is the balance act of forces, and understanding how work, energy and power fit into this picture helps you predict and explain real‑world situations – from a parked car to a roller‑coaster.
Keep practising force diagrams and energy calculations, and you’ll ace those exam questions! 🚀