Tag: falling balls

How can an object in space “fall”?

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How can an object in space “fall”?

Gravity still acts on objects, even though they are in space. No matter how far you get from the earth, it still pulls on you, albeit less strongly than it does when you are nearby. Thus if you were to take a ball billions of miles from the earth and let go, it would slowly but surely accelerate toward the earth (assuming that there were no other celestial objects around to attract the ball—which isn’t actually the case). As long is nothing else deflected it en route, the ball would eventually crash into the earth’s surface. Even objects that are “in orbit” are falling; they just keep missing one another because they have large sideways velocities. For example, the moon is orbiting the earth because, although it is perpetually falling toward the earth, it is moving sideways so fast that it keeps missing.

Is there a fixed amount of force in the universe?

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Is there a fixed amount of force in the universe?

No, forces generally depend on the distances between objects, so that two objects that are moving together or apart will experience different amounts of force as they move about. As a result, the total amount of force anywhere can change freely. But there are quantities that have fixed totals for the universe. The most important of these so-called “conserved” quantities is energy.

I can accept that weight is a force, but it doesn’t seem to follow common sense …

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I can accept that weight is a force, but it doesn’t seem to follow common sense to me.

It would seem like a force if you had to lift yourself up ladder. Imagine carrying a friend up the ladder; you’d have to pull up on your friend the whole way. That’s because some other force (your friend’s weight) is pulling down on your friend. But when you think of weight as a measure of how much of you there is, then it doesn’t seem like a force. That’s where the relationship between mass and weight comes into play. Mass really is a measure of how much of you there is and, because mass and weight are proportional to one another, measuring weight is equivalent to measuring mass.

Isn’t there “some” acceleration at the very start and very end of an elevator …

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Isn’t there “some” acceleration at the very start and very end of an elevator ride? Why does one’s stomach take a flop when the elevator stops and not when it starts?

Yes, there is acceleration at the start and stop of an elevator ride. As the car starts, it accelerates toward the destination and as the car starts, it accelerates in the opposite direction. Your stomach takes a flop whenever you feel particularly light, as when you are falling or otherwise accelerating downward. As you accelerate downward, your body doesn’t have to support your stomach as much as normal and you feel strange. In fact, you feel somewhat weightless. You have this feeling whenever the elevator starts to move downward (and therefore accelerates downward) or stops moving upward (and there accelerates downward).

I don’t understand the horizontal component of a ball thrown downfield. Does it …

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I don’t understand the horizontal component of a ball thrown downfield. Does it have constant velocity and/or acceleration, even at the start?

Until you let go of the ball, you are in control of its velocity and acceleration. During that time, it does accelerate and its velocity isn’t constant. But as soon as you let go of the ball, everything changes. The ball’s motion in flight can be broken up into two parts: its vertical motion and its horizontal motion. Horizontally, the ball travels at a constant speed because there is nothing pushing or pulling on it horizontally (neglecting air resistance). Vertically, the ball accelerates downward at a constant rate because gravity is pulling down on it. Thus the ball travels steadily forward in the horizontal direction as it fall in the vertical direction. Of course, falling can begin with upward motion, which gradually diminishes and is replaced by downward motion.

What is the difference between mass and weight?

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What is the difference between mass and weight?

Mass is the measure of an object’s inertia. You have more mass than a book, meaning that you are harder to accelerate than a book. If you and the book were each inside boxes, mounted on wheels, I could quickly determine which box you were in. I would simply push on both boxes and see which one accelerated most easily. That box would contain the book and you would be in the box that’s hard to accelerate. Weight, on the other hand, is the amount of force that gravity (usually the earth’s gravity) exerts on an object. You weigh more than a book, meaning that the earth pulls downward on you harder than it does on the book. Again, I could figure out which box you were in by weighing the two boxes. You’d be in the heavier box. So mass and weight refer to very different characteristics of objects. They don’t even have the same units (mass is measured in kilograms, while weight is measured in newtons. But fortunately, there is a wonderful relationship between mass and weight: an object’s weight is exactly proportional to its mass. Because of this relationship, all objects fall at the same rate. Also, you can use a measurement of weight to determine an object’s mass. That’s what you do when you weigh yourself on a bathroom spring scale; you are trying to determine how much of you there is-your mass-but you are doing it by measuring how hard gravity is pulling on you—your weight.

I don’t understand the relationship between mass, acceleration, and force in New…

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I don’t understand the relationship between mass, acceleration, and force in Newton’s second law.

First off, force causes acceleration. The stronger that force, the more the acceleration. In fact, the two are exactly proportional to one another: double the force and you double the acceleration. Secondly, mass resists acceleration. The more mass an object has, the less it accelerates. The two are exactly inversely proportional to one another: double the mass and you halve the acceleration. These two ideas can be combined into one observation: the force you exert on an object is equal to the product of its mass times the acceleration it experiences. Look at that relationship: if you double the force you exert on an object, you double its acceleration, so that part checks out. If you double the object’s mass and leave the force unchanged, then the acceleration must be halved, so that part checks out. Thus Newton’s second law is simply a sensible relationship between the force you exert on an object, its mass, and its acceleration.

When you pushed the baseball and bowling ball with an equal force, the baseball …

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When you pushed the baseball and bowling ball with an equal force, the baseball went farther on the table because it has a smaller mass. If gravity also exerts an equal force on the 2 balls, like your push, then why do they fall at equal speeds?

The answer is that gravity doesn’t exert equal forces on the 2 balls! It pulls down harder on the bowling ball than it does on the baseball. Suppose the bowling ball has 10 times the mass of the baseball. Then gravity will also exert 10 times the force on the bowling ball that it exerts on the baseball. The result is that the bowling ball is able to keep up with the baseball! The bowling ball may resist acceleration more than the baseball, but the increased gravitational force the bowling ball experience exactly compensates.

If a projectile released or hit at a 45° angle above horizontal should go th…

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If a projectile released or hit at a 45° angle above horizontal should go the farthest, then why, in the game of golf, does the three iron (20° loft) hit a golf ball so much farther in the air than, say, a seven iron (approximately 45° loft) if the same technique and force are produced by the golfer? Is it backspin, shaft length, etc.?

It’s backspin! Air pushes the spinning ball upward and it flies downfield in much the same way as a glider. When you throw a glider for distance, you concentrate your efforts on making it move horizontally because the air will help to keep the glider from hitting the ground too soon. Similarly, the air holds the spinning golf ball up for a remarkably long time so that giving the ball lots of downfield speed is most important for its distance. That’s why a low-loft club like a three iron sends the ball so far.