Final answer:
The student's question about kicking a ball is answered by explaining that hitting the top of the ball won't provide lift and addresses a series of misconceptions about the behavior of objects in motion, such as the energies involved when a rock is thrown. It also explains why bending knees when jumping is vital for reducing impact force.
Step-by-step explanation:
The statement in the student's question, "the reason why you did not kick the ball in the air was because your kicking foot hit on top of the ball not below," relates to the dynamics of a soccer ball when kicked. For the ball to achieve an upward trajectory into the air, the kicking foot must contact the ball below its center of mass, thus creating an upward force component. If the foot strikes the top of the ball, it would not be lifted into the air but instead pressed against the ground, possibly causing it to roll along the surface.
For your reference, the statement "If a rock is thrown into the air, the increase in the height would increase the rock's kinetic energy, and then the increase in the velocity as it falls to the ground would increase its potential energy," is false. When a rock is thrown upwards, its kinetic energy is converted to potential energy as it rises. At its peak, the kinetic energy is at a minimum and potential energy is at a maximum. As the rock falls, the potential energy decreases, and the kinetic energy increases, not the other way around.
In the soccer scenario given, assuming the balls have equal mass and are kicked with the same speed, the initial kinetic energy for both ball A and ball B would be the same because kinetic energy is dependent on mass and speed only, not the angle of kick. However, the change in gravitational potential energy would likely differ because it depends on the height gained, which is influenced by the angle of the kick.
Lastly, the advice to bend your knees when jumping is wise because bending your knees increases the time of the impact when landing. According to the impulse-momentum theorem, increasing the time of impact lowers the force experienced by your body, making it less stressful on your joints and bones. Therefore, the correct answer is (a): Bending your knees increases the time of the impact, thus decreasing the force.