The reason why things move faster if they are going down a hill is down to gravity. Everyone knows that gravity is the reason why people should never attempt to fly unless they wish to fall to the ground and be very sore the next morning. But less people actually know what gravity exactly IS. So let's think about it....
So this guy Isaac Newton was sitting under a tree minding his own business when an apple fell and his head. Now, while most of us would have rubbed our sore head and then probably eaten the apple and forgotten the whole event, Newton was not like most of us. He went on to try and explain exactly how gravity works and did a pretty good job of it too.
Basically, gravity is a force of attraction that exists between any two masses, any two bodies, or any two particles. Gravity exists between all objects, but you don't notice it in most cases. So when you are standing next to a friend, both of your bodies are in fact being pulled towards each other. But luckily not enough to make you crash into people all of the time!
The reason why we don't notice all of these attractive forces between objects is down to their mass. If you have ever attempted to push a car or an extremely large person, you will have noticed how the heavier something is, the harder it is to move. This means that a bigger pushing force is needed to move something big. It can work the other way too, with a big person generally being able to generate a bigger force than a little person. A similar sort of thing is happening with gravity. When something is very big, it has a bigger gravitational pull on anything close by. Because the Earth is pretty massive, its gravitational pull on other objects is huge. In fact, the gravity on Earth is so strong that is keeps the atmosphere, oceans, and everything living on our planet from drifting into space, and it keeps pulling on the moon making it stay in orbit around our the Earth.
When you fall off something, you may have noticed that you unfortunately fall towards the ground - this is due to gravity. If you were to fall on the moon, you would fall slower due to the smaller size of the moon making its gravity less strong. This is where the difference between mass and weight gets involved. Mass can be thought of as a measure of how much matter an object has whereas weight depends on the mass and gravity. For example, an object will have the same mass on the Earth and the moon but will weigh less on the moon due to there being less gravity. So you would weigh a lot less if you were on the moon despite still being made up of the same amount of matter, and you would feel very light and be able to jump much higher than on the Earth! Gravity is roughly constant all over the Earth (it equals about 9.8 meters per second if you are curious).
Now imagine you fall out of a plane. You will gather speed, or accelerate downwards, the longer you fall. This acceleration will depend on your mass and the force pulling you downwards (this force, as we know, is gravity). Now, gravity also has the same pull on anything that is already on the ground, only you can't get pulled down any further as the ground pushes back and stops you. So a car sat on flat ground is being pulled downwards by gravity and doesn't move forwards unless another force is put on the car. However, if you put a car on a slope, gravity still pulls it downwards towards the center of the Earth.
If you look at the pictures below you can see that in the first one, only the force from the cars engine is pulling the car forwards. However, in the second, gravity is also pulling the car in the direction it is going. The more force on something, the more it will gain speed and the faster it can go. Depending on how steep the hill is, gravity will have a bigger effect on pulling the car downwards. This is turn increases the acceleration (gain of speed) of the car and it goes faster.