Final answer:
A ball thrown upwards is an example of vertical motion with a positive velocity and a negative acceleration. As it rises, gravity acts downwards (negative acceleration) and slows the ball down until it momentarily stops at the peak of its trajectory before falling back down.
Step-by-step explanation:
The example of a vertical motion with a positive velocity and a negative acceleration is 1) A ball thrown upwards. When the ball is thrown upwards, it initially has a positive velocity as it moves away from the ground (upwards), but the acceleration due to gravity is always acting downwards, which is negative in this context. Therefore, as the ball rises, gravity slows it down, and eventually, the ball comes to a temporary stop at its peak before falling back down, all while having negative acceleration.
Projectile motion of this kind is an example of one-dimensional motion involving gravity, where the only acceleration is due to gravity (negative), which is constantly g, or approximately 9.8 m/s² downward.
Signs are essential in this context. Positive velocity indicates motion upwards, and the negative acceleration (due to gravity) indicates a force opposing the upward motion, slowing the ball down. This is an important distinction as, for example, an upward thrown ball will slow down due to negative acceleration, even though its velocity is still directed upwards until it reaches the peak of its trajectory.