Final answer:
Newton's first law of motion, the law of inertia, best describes the situation where a rider is thrown forward when their bike hits a crack because the outside force of the crack halts the bike but not the rider's momentum.
Step-by-step explanation:
The scenario described involves a bike coming to an abrupt stop when it hits a crack in the sidewalk, which results in the rider being thrown over the handlebars. This is a practical demonstration of Newton's first law of motion, often referred to as the law of inertia. Newton's first law states that an object will remain at rest or move at a constant speed in a straight line unless acted upon by an outside force. In this case, the outside force is the crack in the sidewalk that stops the bike. However, the rider's body continues to move forward due to inertia until some external force stops it, which could be gravity pulling them down to the ground or coming into contact with an object, such as the ground or another structure.