A shuffleboard disk slides to a stop on a rough lane. Flor looks for the disk's kinetic energy afterward. Where is it?
A car brakes to a stop at a red light. The speed is gone, and the brake discs, the tires, and the road are all slightly warm.
You have seen where the energy went. The new question: can it ever come back?
You have seen that friction and air resistance convert mechanical energy into thermal energy that spreads into the moving object, the surfaces, and the surroundings. That process is called dissipation.
The car's kinetic energy is now thermal energy, spread thin through the discs, the tires, the road, and the air.
Energy was not destroyed. Every joule is still out there.
Yet the car will never gather that energy back and re-spin its wheels on its own. The spread-out energy never collects itself.
A process that never runs backwards on its own is called irreversible.
Real processes all dissipate at least a little, so real processes are irreversible.
A film of the stop, played backwards, shows warmth gathering out of the road into the tires while the car speeds away with no engine. Nobody has ever seen that happen outside a film run in reverse.
A playground swing, set moving once, swings lower and lower and comes to rest. Its energy has spread as thermal energy into the air and the hinges. Can that energy gather itself back and restart the swinging on its own?
The swing's energy is now spread through the air and the hinges.
Once energy and matter have spread out in a real process, the process never runs backwards on its own.
No. The dying-away of the swing is irreversible.
Hot bathwater has cooled overnight, and its energy has spread into the bathroom. Could the tub water reheat itself while the bathroom cools back down?
Reheating would need the spread energy to gather itself back into the water.
Spread-out energy never collects itself; the process never runs backwards on its own.
No. The overnight cooling is irreversible.
A playground merry-go-round, set spinning once, slows and stops as its energy spreads into the bearings and the air. Can that spread energy gather itself back and re-spin it, on its own?
Harriet says: 'The thermal energy that spread out when my bike's brakes stopped the wheel will never collect itself back and re-spin the wheel.' Is she correct?
A hot bowl of ramen cools in a food court, and its thermal energy spreads into the table and the air. Which of the following statements is correct?
A video shows a swimming pool: droplets rise out of the splash, join back into one smooth column of water, and a diver lifts feet-first back onto the board. How can you tell the video is playing backwards?
Which of the following statements about real processes is correct?
Once its energy and matter have spread out, a real process never runs backwards on its own. What are such processes called?