When observing the motion of a gazelle, which statement is true for Alex and Harry?

The statement that Alex measures the same acceleration for the gazelle as Harry is correct based on the principles of physics regarding motion and acceleration. Acceleration is defined as the rate of change of velocity of an object over time and is a scalar quantity as long as both observers are measuring it over the same duration of time and under the same conditions.

In this scenario, if Alex and Harry are both observing the same gazelle, they both are seeing the same events unfolding in terms of motion, assuming they are relatively close to each other and not in significantly different frames of reference or contexts (like one being on a vehicle or at a vastly different angle). As such, both observers will calculate the gazelle's acceleration using the same formula: the change in velocity divided by the time during which that change occurs. Therefore, their measurements should be consistent.

The other statements suggest differences in perception or measurement that are not inherently rooted in the laws of physics as they relate to non-relativistic speeds and comparable frames of reference.