A ball is thrown vertically upwards, reaches its highest point, then drops down to the ground, bounces once,reaches a height of 1.2 m, then falls again and finally rests on the grass. At which one or more times is the gravitational force on the ball zero?

Question

BlackTom AI · Accepted Answer

To determine when the gravitational force on the ball is zero, we need to recall a basic fact: gravity acts on the ball at all times unless the ball has no mass or is in a zero-gravity environment. In this scenario, the ball is under Earth's gravity the entire motion: rising, stopping, descending, bouncing, and resting.

Option a: At the highest point it reached. At the top the ball has zero vertical velocity, but gravity is still pulling downward. Therefore the gravitational force is not zero here.

Option b: When the ball had bounced up to a height of 1.2 m. During the ascent after the bounce, gravity continues to act downward; the height at which the velocity may be momentarily small does not eliminate gravity.

Option c: The ball always experiences the force of gravity. This aligns with Newtonian physics: gravity acts continuously on a mass near the Earth, regardless of its motion phase. This is a true statement.

Option d: Whenever it was moving down. While the ball is moving downward, gravity remains downward; the force is not zero.

Option e: At the exact instant the ball was thrown up. The motion may involve a change in velocity, but gravity still acts downward at that instant, so the force is not zero.

Option f: When the ball rested on the grass. Even at rest, gravity pulls the ball downward toward the ground; the force is still present.

Option g: At the instant the ball hit the ground for the first time. Contact forces (normal reaction) may be present, but gravity is still acting downward as well; the gravitational force is not zero.

Option h: Before the ball was thrown. Prior to release, gravity already acts on the ball just as it does during any other time near Earth.

Option i: Whenever the ball was moving up. While moving upward, gravity pulls downward; the force is not zero.

In summary, the only correct statement is that gravity always acts on the ball throughout its motion; none of the other listed moments render the gravitational force zero.

A ball is thrown vertically upwards, reaches its highest point, then drops down to the ground, bounces once,reaches a height of 1.2 m, then falls again and finally rests on the grass. At which one or more times is the gravitational force on the ball zero?多项选择题

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A ball is thrown vertically upwards, reaches its highest point, then drops down to the ground, bounces once,reaches a height of 1.2 m, then falls again and finally rests on the grass. At which one or more times is the gravitational force on the ball zero?

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