Will was riding his bike when a dog ran out in front of him. he slammed on his brakes. during this quick stop, some of the mechanical energy (his motion) was changed into

Answer is: heat energy.

Braking is the process of controlling the velocity of an object (in this example bicycle) by inhibiting its motion.

Mechanical energy can be either kinetic or potential energy and has to do with the energy of position and motion of an object.

Kinetic energy is energy of the speed of an object and how much mass it has; basically how the object is moving.

Potential energy is type of energy is stored energy; it can be increased by getting into a higher position and/or stretching an object such as a rubber band.

A mechanical brake applies a friction force to convert the kinetic energy of the bicycle into heat energy which then dissipates into the atmosphere.