Final answer:
A force that decreases the ball's speed from 15 m/s to 7 m/s does negative work. The force acts in the opposite direction to the ball's motion, causing a loss in kinetic energy.
Step-by-step explanation:
When a force acts upon a ball, causing its speed to decrease from 15 m/s to 7 m/s, the force has done negative work on the ball. This means that the direction of the force is opposite to the direction of the ball's motion. In physics, work is defined as the product of the force applied to an object and the distance over which that force is applied, in the direction of the force. Since the ball's speed has decreased, the force was applied in the opposite direction of the ball's initial motion.
In the example of a ball thrown upwards, gravitational force, which is a constant force, acts over a period of time to change the ball's momentum. Similar is the case with the ball whose speed decreases: the initial force may vary, but if we consider the average force, it shows that the force was acting against the direction of the ball's motion, resulting in a loss of kinetic energy and the performance of negative work.