Bridges can vibrate when wind, traffic or people provide repeated forces. Every structure has natural frequencies determined by its mass, stiffness and shape. If a repeated driving force matches or approaches a natural frequency, resonance can increase the amplitude of the oscillation. Engineers therefore analyse vibration carefully during design. Damping removes energy from oscillations and prevents amplitudes from growing too large. Dampers, tuned masses and changes in structure can all help. A wobbling bridge is not automatically about to collapse, but uncontrolled resonance can become dangerous if it is not considered.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

The motion can be understood by adding the forces as vectors. If they balance, the resultant force is zero and the object stays at rest or continues at constant velocity; if they do not, it accelerates. Friction and drag transfer energy to the surroundings, usually as heating, so maintaining a steady speed can still require continuous work even when the speed itself is not changing.