Air resistance. You throw a ball straight up and measure the velocity as it passes you on its way down. Will the velocity be larger, the same, or smaller if you did the same experiment in vacuum?

Air resistance is a force that acts against the motion of an object as it travels through the air. It's like trying to run through water; the water pushes against you, slowing you down. This force is especially noticeable with objects moving at high speeds. For a ball thrown into the air, air resistance will slow it down both when it is rising and when it is falling. This means the ball won't go as high as it would without air resistance and it won't come down as fast either. Think of it as nature's way of putting the brakes on anything that moves through the air.

Velocity refers to the speed of an object in a particular direction. In the case of our ball, when you throw it straight up, it has an initial upward velocity. As it climbs, this velocity decreases due to the opposing forces of gravity and air resistance. At its peak, the velocity is zero for an instant before the ball starts to fall back down. On its way down, gravity accelerates the ball, increasing its velocity, but air resistance acts in the opposite direction, reducing this increase. In a vacuum, where there is no air resistance, the ball would fall back with exactly the same speed it had when it was thrown up. The presence or absence of air resistance makes a big difference in the ball’s velocity.

A vacuum is a space completely devoid of matter, including air. In a vacuum, there is no air resistance, meaning the only force acting on a falling object is gravity. If the ball is thrown up in a vacuum, it will slow down at a constant rate due to gravity alone, and then speed up at the same rate while falling. This means the ball would pass you on its way down at the same speed it had when thrown up, but in the opposite direction. Because air resistance isn't there to slow it down, the ball's velocity in a vacuum is greater when it falls back past your hand than it would be in regular air.

Gravity is a force that attracts objects toward the center of the Earth. It's what makes us stay grounded and what causes objects to fall when dropped. When you throw the ball up, gravity works against the ball’s initial upward motion, slowing it down until it stops and begins to fall back. When the ball is falling, gravity accelerates it, increasing its velocity. This effect of gravity is constant and applies whether the ball is in the air or in a vacuum. The crucial difference, though, is that without air resistance, gravity acts unopposed in a vacuum, allowing the ball to accelerate faster and maintain a higher velocity as it descends. For our experiment, this means the ball in a vacuum will always pass you on its way down faster than it would in the presence of air.