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.

Momentum is what we get when we multiply the mass of an object by its velocity.

A large semi-truck travelling at 60 mph will have more momentum than a small car at the same speed.  A person walking will have less momentum than a person running. It will take more energy for a person to slow down when they are running compared to when they are walking because they have a greater momentum.

Either way, we call this change in velocity acceleration.

Acceleration is the change in velocity over time, just like speed is the change in distance over time. 

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. 

Some cars are often advertised based on how fast they can accelerate. You might hear the term 0 to 60 in 5 seconds. This means the car can go from 0 mph to 60 mph in just 5 seconds. But that’s just a change in speed. We have to add a direction to make it velocity.