After watching this video, Blake, a student in an introductory physics class, makes the following claim: The acceleration and velocity of the glider are both momentarily zero when the glider changes direction. The velocity of the glider must be zero for an instant when the glider changes direction. Because the velocity is zero, the acceleration must also be zero. 1. Respond to Blake's claim. Which parts, if any do you agree with, and which parts do you not agree with

Any time that an object changes direction (from leftward to rightwward, or from upward to downward) the velocity must be zero just for one instant, when is on the verge of changing the direction.
This is needed because velocity changes as a continuous function of time, so it needs to cross the t-axis when passing from positive to negative or vice versa.
However, the claim that in the moment that velocity is zero, the acceleration is also zero, is false.
Due to acceleration is the rate of change of velocity, and velocity is a vector, this means at any time there is a change of direction, there is an acceleration that is non-zero.
For example, when an object that has been thrown upward, reaches to its maximum height, just one instant before starting to fall, the velocity becomes zero, but the acceleration (which causes the object to fall) is non-zero, due to it's the acceleration due to gravity.