Divide, split

Tack, as for horses

Not the word "tack" but the category of horse tack.

Machine

Fast

Disc, disk, disco, discus

Round disk, or dancing to round disks, or round things to throw in the Olympics.

Unite, united, union, unison, unit

"Uno!"
Any word about making things one, with that Latin root "uni-".

Uniform

Specific parts of uniforms, when it's talking about it as a uniform, would be fine.

Swim

Rope

Clearly rope-related references are okay too, like noose or lasso.

Us

(The word "we" was played another time, but think of a song with the word "us.")