Final answer:
When you suddenly stop the wagon while pulling it, your friend will continue to move forward due to inertia.
Step-by-step explanation:
When you suddenly stop the wagon while pulling it, your friend will continue to move forward due to inertia. Inertia is the tendency of an object to resist changes in its state of motion. Since your friend was initially moving with the wagon, they will keep moving forward after the wagon stops until an external force (such as friction) acts upon them and brings them to a stop as well.
This can be seen in the concept of Newton's first law of motion, which states that an object at rest will stay at rest, and an object in motion will stay in motion with the same speed and direction, unless acted upon by an external force.
So, when you suddenly stop the wagon, your friend will continue moving forward until friction or another force stops them as well.