Internal Energy!!!

[peacefulheart]

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Which of the following cases does the internal energy of described system increase?

I. A clay ball is dropped and sticks to the ground.
II.Hydrogen and oxygen react exothermically to form water
III.A car is driven at constant speed (consider the effect of air resistance)

the answer is I and III.

Do not understand why. Need help. Thanks a lot!

 

[Entadus]

For part III, the car travels at constant speed so its Kinetic Energy does not change.

However, the air drag (particles of gas striking the front of the car) will increase the temperature of the car, thus increasing the Internal Energy. (Collisions between car particles and air particles will make the car particles vibrate faster)

EDIT: I am also curious about part I. As the ball falls, its gravitational Potential Energy should decrease. The collision with the floor seems to be completely inelastic, meaning all of the kinetic energy is lost. I'm not sure how the PE would increase....

 

[Pre-Med Village]

The answer is none of the above. Entadus has correct reasoning regarding choice III, but choice I also represents a case of decreasing internal energy. Energy is constantly being expended by the car's engine to maintain constant speed, represented in the work performed by the friction force (drag). This is a flawed question.

 

[Captain Sisko]

The answer is none of the above. Entadus has correct reasoning regarding choice III, but choice I also represents a case of decreasing internal energy. Energy is constantly being expended by the car's engine to maintain constant speed, represented in the work performed by the friction force (drag). This is a flawed question.

+1

Not only is the engine converting chemical energy to mechanical, the car is working on the air too. The ball loses energy when it hits the ground and stops, but that energy is gained by the earth! Question needs to define what each system is comprised of. If the system is the universe, energy is constant.