According to the law of conservation of energy, when a ball falls (no air resistance), its potential energy is:

Transformed into kinetic energy

A box slides to a stop on a rough floor. Where does its kinetic energy go?

It becomes thermal energy (heat) and a little sound

Some input energy is always transformed into unwanted thermal energy

12questions. Pick an answer and you'll see why right away.

A force of $20$ N moves an object $5.0$ m in the direction of the force. How much work is done?
What is power?
A $3.0$ kg object moves at $2.0$ m/s. What is its kinetic energy?
If the speed of a moving object doubles, its kinetic energy:
A $4.0$ kg object is lifted $2.0$ m. What is the gravitational potential energy gained? (Use $g = 10$ m/s squared.)
According to the law of conservation of energy, when a ball falls (no air resistance), its potential energy is:
A box slides to a stop on a rough floor. Where does its kinetic energy go?
Two like magnetic poles are pushed together against their repulsion. What happens to the energy stored in the magnetic field?
By the second law of thermodynamics, heat flows spontaneously:
How much heat is needed to raise $1.0$ kg of water by $10$ degrees C? (Specific heat of water $= 4200$ J per kg per degree C.)
A motor takes in $400$ J of electrical energy and does $300$ J of useful work. What is its efficiency?
Why is no real energy conversion device 100 percent efficient?