Final answer:
The adult exerts a force on the child equal in magnitude to the force the child exerts on the adult during their collision, as per Newton's third law, resulting in different effects on their motions due to their mass difference.
Step-by-step explanation:
When a small child collides with an adult, according to Newton's third law of motion, the force that the adult exerts on the child is equal in magnitude and opposite in direction to the force that the child exerts on the adult. This is true regardless of the initial states of motion of the two individuals. Since the adult was initially at rest and the child was moving, we can apply this principle to understand the interaction between them during the collision. Both the child and the adult experience forces of the same strength during the collision, but the effects on their motions are different due to their differing masses—the child may experience a greater acceleration or change in motion compared to the adult.
In the scenario where a small object (like the child) collides with a larger object at rest (the adult), and assuming a perfectly elastic collision, the smaller object often bounces back while the larger object begins to move forward at a lower speed. The internal kinetic energy and total momentum of the system remain unchanged, demonstrating the conservation of energy and momentum.