Physics Uncovered on Instagram: "Newton’s first law of motion is also called the law of inertia. It says that an object will stay at rest, or keep moving in a straight line at the same speed, unless an outside force acts on it. In other words, things don’t change their motion by themselves — they only change when something pushes or pulls on them. For example, a ball rolling on the ground would keep rolling forever if there were no friction or other forces slowing it down. Similarly, if a book i