Force and motion is a great fifth grade topic and there are soooo many possible investigations you can do. I've pasted a good website below. It is important that you pick something to do that really interests you. It could be about friction for example, or one of the six simple machines and how they combine to make more complex machines such as a bicycle. Or it could be about how things work differently here on earth vs in space or on the moon. Or you can investigate real-world examples of Newton's Laws of Motion. The important thing is that you pick something that you want to learn more about. Don't let some adult (like me) tell you what to investigate. Good luck!