The up and down movement of the piston is changed to a __________ motion by the crankshaft.

The correct answer is rotary motion because the crankshaft is designed to convert the linear or reciprocating motion of the piston into rotational movement. In a small engine, the piston moves up and down within the cylinder as the combustion process occurs. This vertical movement is characteristic of reciprocating motion. However, as the piston moves, the crankshaft, which is connected to the piston via a connecting rod, rotates due to the alignment of its crank pins.

The primary function of the crankshaft is to transform this linear motion of the piston into rotary motion, which is essential for powering the engine's output and ultimately driving the wheels in vehicles or operating other machinery. Since rotary motion is the movement that occurs around an axis, it is integral to engine operation because it allows for continuous power generation and transfer.

Linear motion represents straight-line movement, while oscillating motion refers to repetitive swinging back and forth, neither of which accurately describes the function of the crankshaft in this context. Thus, recognizing the crankshaft's role in converting reciprocating motion to rotary motion is key to understanding engine mechanics.