A car skids to a stop on a dry road.
Its 200000 J of kinetic energy leaves the motion, and 200000 J of thermal energy appears: in the tires, the road surface, and the air.
Mechanical energy on its own stayed constant only while friction stayed negligible. Counting one more store, is there a total that holds in every real process?
Count three stores side by side: kinetic, potential, and thermal.
For the skid: before, 200000 J kinetic and 0 J thermal. After, 0 J kinetic and 200000 J thermal. The sum is 200000 J both times.
The one condition: no energy enters or leaves the counted system. The skid's count covers the car, the road, and the nearby air.
Dissipation moves energy between the columns, from kinetic or potential into thermal. It never changes the total.
A car skids to a stop. Before the skid it holds 200000 J of kinetic energy; after, none. What happened to the energy, and what stayed constant?
Friction between the tires and the road converted the kinetic energy into thermal energy.
The thermal energy spread into the tires, the road surface, and the air.
Count the car, the road, and the nearby air together: no energy enters or leaves that system.
Before: 200000 J kinetic, 0 J thermal. After: 0 J kinetic, 200000 J thermal.
The kinetic energy became thermal energy, and the total energy of the car, the road, and the air stayed 200000 J throughout.
A freight cart rolls into a long gravel arrester bed on level ground. Its kinetic energy drops from 90000 J to 18000 J. No energy enters or leaves the cart, the gravel, and the air. Enter the thermal energy that has appeared, in J.
A ball that bounces lower each time obeys the same account: the missing height's energy sits as thermal energy in the ball, the floor, and the air, and the three columns still total the same.
A library book cart rolls to a stop on a flat aisle. Gael says: 'Some of the cart's energy was destroyed as it stopped.' Is he correct?
A tennis ball bounces lower each time on a patio. Edda says: 'Counting the thermal energy in the ball, the patio, and the air, the total energy after a bounce equals the total before it.' Is she correct?
Mechanical energy conservation was the special case: with friction and air resistance negligible, the thermal column stays at zero, so the mechanical total alone holds.
In every real process, energy changes form. It is never created, and it is never destroyed.
A rolling cart's account reads: earlier, 500 J kinetic, 120 J gravitational potential, 0 J thermal. Later, 310 J kinetic and 60 J gravitational potential. No energy enters or leaves. Find the thermal energy at the later snapshot.
Write down the earlier total:
500 + 120 = 620. The total is 620 J.
The total stays constant, so the later stores must also total 620 J.
310 + 60 = 370
620 - 370 = 250
E_thermal = 250 J at the later snapshot.
A garden wagon rolls along a sloping path. At the start it holds 900 J of kinetic energy and 260 J of gravitational potential energy, and no thermal energy has appeared. Later it holds 410 J of kinetic energy and 330 J of gravitational potential energy. No energy enters or leaves the counted system. Enter the thermal energy that has appeared, in J.
A loose boxcar rolls to a stop on a long, level siding. Which statement is correct?
Counting kinetic, potential, and thermal energy, under which condition does a system's total energy stay constant?