The Kids Discover website is a great resource for teachers and is FREE to sign up for a basic use. This specific section focuses on force, motion, and friction. It answers questions that I am sure many young kids would be wondering. There are simple definitions given for the terms and they are explained briefly. The examples that are given are easy to relate to for young children, and easy to understand for people who are unfamiliar with the concept.
Source: Force Motion & Friction