Category: falling balls

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.

How is there inertia on earth? I though that inertia was just in space.

Inertia is everywhere. Left to itself, an object will obey inertia and travel at constant velocity. In deep space, far from any planet or star that exerts significant gravity, an object will exhibit this inertial motion. But on earth, the earth’s gravity introduces complications that make it harder to observe inertial motion. A ball that’s thrown up in the air still exhibits inertial effects, but its downward weight prevents the ball from following its inertia alone. Instead, the ball gradually loses its upward speed and eventually begins to descend instead. So inertia is the basic underlying principle of motion while gravity is a complicating factor.

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.

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.

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.

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.

Why is there more gravity acting on larger, more massive objects?

The fact that more massive objects also weigh more is just an observation of how the universe works. However, any other behavior would lead to some weird consequences. Suppose, for example, that an object’s weight didn’t depend on its mass, that all objects had the same weight. Then two separate balls would each weigh this standard amount. But now suppose that you glued the two balls together. If you think of them as two separate balls that are now attached, they should weigh twice the standard amount. But if you think of them as one oddly shaped object, they should weigh just the standard amount. Something wouldn’t be right. So the fact that weight is proportional to mass is a sensible situation and also the way the universe actually works.

Suppose I were to fall from an airplane that is cruising at about 30,000 feet. What would kill me, the fall itself or the sudden deceleration as I intersect with the planet? — ZE, Woodinville, WA

In effect, you would be a skydiver without a parachute and would survive up until the moment of impact with the ground. Like any skydiver who has just left a forward-moving airplane, you would initially accelerate downward (due to gravity) and backward (due to air resistance). In those first few seconds, you would lose your forward velocity and would begin traveling downward rapidly. But soon you would be traveling downward so rapidly through the air that air resistance would keep you from picking up any more speed. You would then coast downward at a constant speed and would feel your normal weight. If you closed your eyes at this point, you would feel as though you were suspended on a strong upward stream of air. Unfortunately, this situation wouldn’t last forever—you would eventually reach the ground. At that point, the ground would exert a tremendous upward force on you in order to stop you from penetrating into its surface. This upward force would cause you to decelerate very rapidly and it would also do you in.

I have read articles about research into anti-gravity. Do you think it is really possible? — JG

No, I don’t think that anti-gravity is possible. The interpretation of gravity found in Einstein’s General Theory of Relativity is as a curvature of space-time around a concentration of mass/energy. That curvature has a specific sign, leading to what can be viewed as an attractive force. There is no mechanism for reversing the sign of the curvature and creating a repulsive force—anti-gravity. I know of only one case, involving a collision between two rapidly spinning black holes, in which two objects repel one another through gravitational effects. But that bizarre case is hardly the anti-gravity that people would hope to find.