Definition
A shock absorber is a key part of a car's suspension that uses the friction of oil to quickly calm the shaking and vibration caused by bumpy roads.
A spring alone can't stop the bouncing
When you drive fast over a speed bump, the big springs under your car softly take in the jolt coming up from the road. But a spring stores elastic energy as it compresses and springs back. Left alone, it keeps bouncing up and down like a rubber ball.
If a car had only springs, it would keep rocking up and down for a long time after a single speed bump. Passengers would feel seasick, like on a boat in rough waves. The wheels would also keep lifting off the road, which makes steering shaky.
So something has to catch the bouncing spring and stop it. A shock absorber takes over the energy the spring has soaked up. It acts like a reliable safety belt that quickly wears down the back-and-forth wobble.
The power of oil squeezing through tiny holes
Fill a squirt gun with thick cooking oil or shampoo instead of water, and the plunger gets hard to push. That's because a liquid forced through a tiny hole creates a lot of fluid resistance (friction).
The cylinder inside a shock absorber works just like that syringe or squirt gun. It is filled with thick, sticky special oil. A piston plate with very small holes (valves) slides up and down through the oil.
When the car jolts and the piston moves, the oil has to squeeze out through those tiny gaps. The sticky resistance of the oil this creates grabs hold of the car body's motion and holds it back hard.
Along the way, the energy of the bouncing turns into a little frictional heat that drifts off into the air. The car loses its wobble and settles back into its calm, steady position.
To be more precise: it tames vibration, not the impact itself
Translated literally, "shock absorber" means "a thing that absorbs shock." But to be more precise, it is not the main part that stops the impact itself. The real star that takes the first jolt from the road is the spring.
In physics, the way a shaking object loses energy to resistance and comes to rest is called "damped oscillation." Once the spring softens the first jolt, the shock absorber takes charge of the aftershocks. It is the damper that wipes out the endless leftover vibration.
Without this perfect pair of spring and shock absorber, a car can't drive properly. A shock absorber doesn't only make the ride comfortable. When you brake hard or take a sharp curve, it keeps the wheels from hopping, helping the tires stay in contact with the road and keeping your drive safe.
Common misconceptions
A shock absorber is just another name for the spring that directly absorbs a car's jolts.
The spring is what takes the first hit. The shock absorber is a separate damping device that uses oil friction to stop the spring from bouncing on and on.
Where you meet it
After you thump over a speed bump, the car settles back to level after just one or two wobbles.
When a washing machine spins at high speed, a damper at the bottom calms the shaking so the drum doesn't thrash itself apart.
In one sentence
The spring softly takes the impact, and then the shock absorber uses the friction of oil to quickly stop the vibration that would otherwise keep going.