Changing speed is different for different things depending on their current speed and size, or their mass.  To understand this, we use something called momentum. Have you ever been on a roller coaster? They start of slow. You can move around and lean forward in your seat with almost no energy, but when they start to speed up, like at the first drop, you get pushed into the back of your seat until you’ve stopped speeding up. That change in velocity over time is acceleration.  These are our next stops in our series on introductory physics! Check out more here