Author: Lou Bloomfield

When you drop a small rubber ball and a large rubber ball simultaneously, why do they both hit the floor at the same time?

The fact that both balls fall together is the result of a remarkable balancing effect. Although the larger ball is more massive than the smaller ball, making the larger ball harder to start or stop, the larger ball is also heavier than the smaller ball, meaning that gravity pulls downward more on the larger ball. The larger ball’s greater weight exactly compensates for its greater mass, so that it is able to keep up with the smaller ball as the two objects fall to the ground. In the absence of air resistance, the two balls will move exactly together-the larger ball with its greater mass and greater weight will keep up with the smaller ball.

How does the floor exert a force?

When you stand on the floor, the floor exerts two different kinds of forces on you—an upward support force that balances your downward weight and horizontal frictional forces that prevent you from sliding across the floor. Ultimately, both forces involve electromagnetic forces between the charged particles in the floor and the charged particles in your feet. The support force develops as the atoms in the floor act to prevent the atoms in your feet from overlapping with them. The frictional forces have a similar origin, although they involve microscopic structure in the surfaces.

When you drop a baseball and a bowling ball, you say that its velocity acts faster and faster as it falls. How can you say that the acceleration is constant at 9.8 m/s2? If it is falling faster and faster wouldn’t the acceleration change also until the object reaches terminal velocity and then it would be accelerating at 9.8 m/s2?

It’s very important to distinguish velocity from acceleration. Acceleration is caused only by forces, so while a ball is falling freely it is accelerating according to gravity alone. In that case it accelerates downward at 9.8 m/s2 throughout its fall (neglecting air resistance). But while the ball’s acceleration is constant, its velocity isn’t. Instead, the ball’s velocity gradually increases in the downward direction, which is to say that the ball accelerates in the downward direction. Velocity doesn’t “act”—only forces “act.” Instead, a ball’s velocity shifts more and more toward the downward direction as it falls.

About terminal velocity: when an object descends very rapidly through the air, it experiences a large upward force of air resistance. This new upward force becomes stronger as the downward speed of the object becomes greater. Eventually this upward air resistance force balances the object’s downward weight and the object stops accelerating downward. It then descends at a constant velocity—obeying its inertia alone. This special downward speed is known as “terminal velocity.” An object’s terminal velocity depends on the strength of gravity, the shape and other characteristics of the object, and the density and other characteristics of the air.

Why does the bigger ball have more gravity pulling on it? Because it weighs more? Which causes which?

The force that gravity exerts on an object is that object’s weight. An object that has more gravity pulling on it weighs more and vice versa.

Why is it that when people jump, they don’t bounce up?

A ball bounces because its surface is elastic and it stores energy during the brief period of collision when the ball and floor are pushing very hard against one another. Much of this stored energy is released in a rebound that tosses the ball back upward for another bounce. But people don’t store energy well during a collision and they don’t rebound much. The energy that we should store is instead converted into thermal energy—we get hot rather than bouncing back upward.

When you throw a ball upward and claim that there is no upward force on it as it rises, why don’t you count your hand? The ball was thrown up, so there was an upward force on it! I’m confused.

While you are throwing the ball upward, you are pushing it upward and there is an upward force on the ball. But as soon as the ball leaves your hand, that upward force vanishes and the ball travels upward due to its inertia alone. In the discussion of that upward flight, I always said “after the ball leaves your hand,” to exclude the time when you are pushing upward on the ball. Starting and stopping demonstrations are often tricky and I meant you to pay attention only to the period when the ball was in free fall.