Cadet Competition Written Practice Exam

What causes the quantity of motion to slowly decrease for a hockey puck sliding over the ice?

1. Gravity

2. Friction

3. Momentum

4. Air resistance

The correct answer is: Friction

The quantity of motion for a hockey puck sliding over the ice decreases primarily because of friction. When a puck moves across the ice, it experiences a friction force between the puck and the ice surface. This force acts in the opposite direction to the puck's motion, gradually slowing it down. While gravity does influence the puck by pulling it downwards, it does not directly contribute to the reduction of the puck's horizontal motion on a flat ice surface. Momentum is a property of the puck itself and describes its mass and velocity; it doesn't account for the forces acting on it that reduce its speed. Air resistance also plays a role but is relatively small compared to the friction between the puck and the ice. In this scenario, the most significant factor causing the decrease in the puck's motion is the frictional force, which effectively converts some of the puck's kinetic energy into heat, leading to a gradual loss of speed until it eventually comes to a stop.