"slow down" VS "slow up" What is the difference between these phrasal verbs when we talk about moving or doing something more slowly? To be honest, I can't grasp the necessity of having two verbs with the OPPOSITE adverbial particles, which at the end form two synonymical phrasal verbs... How on Earth? Can they be used interchangeably? Or is there any slight difference? For example, may I use them interchangeably in the following sentences? 1a The car slowed up as it approached the gate. 1b The car slowed down as it approached the gate. 2a Computing problems slowed us up a bit. 2b Computing problems slowed us down a bit.