Final answer:
The object would strike the water in 4 seconds whether it is thrown with a horizontal velocity of 30 m/s or 60 m/s, as horizontal velocity does not affect the time taken to fall in projectile motion.
Step-by-step explanation:
If an object is thrown from a cliff with a horizontal velocity and strikes the water some time later, this is an example of projectile motion, a key concept in physics. Specifically, the question at hand involves an object thrown horizontally from a height, which is a characteristic of projectile motion problems, commonly studied in courses at the high school level.
When the object was thrown with a horizontal velocity of 30 m/s, it struck the water after 4 seconds. This time duration is dependent on the vertical motion only, as horizontal and vertical motions are independent in projectile motion under the assumption of no air resistance. Thus, for the object thrown with a horizontal velocity of 60 m/s, the time to hit the water will remain 4 seconds because the horizontal velocity does not affect the time taken to fall the same vertical distance due to gravity.
It is important to understand that the horizontal distance covered will change with the horizontal velocity, but the vertical distance (height of the cliff) and the acceleration due to gravity are constant, so the time it takes to hit the water remains the same regardless of the horizontal velocity, assuming an ideal projectile motion without air resistance.