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

The scenario involves a ball undergoing vertical motion under gravity, with moments of contact on the ground or after bounces. We assess each time or condition listed to see whether the gravitational force could be zero.

Option a: At the highest point it reached. At the top of the trajectory, if the ball is in free motion just under gravity, gravity is still acting downward. There is no state here where gravity cancels to zero, since the only forces present are gravity and possibly a normal force from contact, but at the apex there is no contact. Therefore, gravity is not zero here.

Option b: Before the ball was thrown. Before release, the ball is at rest on the ground, typically held in place. Even then, gravity acts downward on the ball; the weight is present regardless of motion. So gravity is not zero.

Option c: At the instant the ball hit the ground for the first time. Upon impact, the ball experiences a large impulse from the ground, producing a nonzero contact force. The gravitational force is still present downward; it does not become zero. Hence this is not a time when gravity is zero.

Option d: When the ball rested on the grass. At rest on the grass, the weight acts downward and a normal force from the grass acts upward, balancing but not eliminating gravity. The gravitational force itself remains nonzero. So gravity is not zero here either.

Option e: The ball always experiences the force of gravity. This statement asserts that gravity is present at all times. Indeed, gravity acts constantly downward on the ball, throughout ascent, descent, contact, bounce, and rest (modulated by other forces but never vanishes). This option correctly describes the universal presence of gravitational force.

Option f: Whenever the ball was moving up. While moving upward, gravity is still acting downward, altering the velocity. However, there is no moment where gravity becomes zero during the upward motion. So this is incorrect because gravity is not zero during upward motion.

Option g: When the ball had bounced up to a height of 1.2 m. Reaching 1.2 m on the bounce means the ball is in flight under gravity (and perhaps a normal force only during contact). Gravity is still acting downward; it does not disappear at that height. Therefore this is not a time when gravity is zero.

Option h: At the exact instant the ball was thrown up. At throw, the hand applies a force to launch the ball, but gravity is still acting downward, and the question concerns the gravitational force on the ball itself. The weight remains present; it does not vanish at the moment of release. So this is incorrect.

Option i: Whenever it was moving down. Similar to upward motion, during downward motion gravity continues to act downward. There is no instant where gravity is zero during descent. So this option is also incorrect.

MUF0121 Physics Unit 1 - Semester 2, 2025 5.8 Application of forces

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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