How Everything Works
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Page 2 of 4 (37 Questions and Answers)

47. When a falling egg hits a table and breaks, did it fail to push equally on the table?
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No. It pushed hard against the table and the table pushed hard against it. The forces exerted were exactly equal but in exactly the opposite directions. Each object experienced a strong push from the other object. But as they say, "whether the rock hits the pitcher or the pitcher hits the rock, it's bound to bad for the pitcher." The egg couldn't take the push and it broke.

48. When a person bumps into something or has something dropped on them and a bruise forms, does it form because of the object hitting the person or from the person exerting a force on the object to keep that object from pass through their skin?
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The bruise forms because of the force exerted on the person by the object. When an object hits you, it's obvious that the object pushes on you. But the object also pushes on you when you hit it. In fact, it's a matter of perspective which is hitting which. To a person standing next to you when you're hit by a ball, the ball hit you. To a person running along with the ball, you hit the ball. In each case, the ball pushes on you and gives you a bruise. You also push on the ball, causing it to accelerate away from you.

49. When you drop a glass on a hard floor, why does it sometimes break and sometimes not?
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When the glass hits the floor, the floor exerts all of its force on the part of the glass that actually touches the floor. That small part of the glass accelerates upward quickly and comes to rest. The remainder of the glass isn't supported by the floor and continues downward. However the glass is relatively rigid and parts of it begin to exert forces on one another in order to stop the whole glass from bending. These internal forces can be enormous and they can rip the glass apart. Glass is a remarkable material; it never dents, it only breaks. As the glass tries to come to a stop, the internal forces may bend it significantly. It will either tolerate those bends and later return to its original shape or it will tear into pieces. Which of the two will occur depends critically on the precise locations and amounts of the forces. If the forces act on a defect on the glass's surface, it will crack and tear and the glass is history. If the forces all act on strong parts of the glass, it may survive without damage.

50. When you push up on an object, are you creating thermal energy or does that only occur when something does work on you?
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When you lift a heavy object, you do work on that object. After all, you exert an upward force on it and it moves in the direction of that force. However your muscles are inefficient and you consume more food energy (calories) during the lifting process than you actually transfer to the heavy object. Whatever energy you consume that doesn't go into the object remains in you as thermal energy. Any time you tighten your muscles, whether you do work on something, it does work on you, or neither does work on the other, you end up wasting some food energy as thermal energy.

51. Why doesn't an egg break when it falls into a pile of feathers? Isn't the pile of feathers exerting the same force on it (perhaps 1000 newtons) that a table would if it were to hit that table?
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The egg doesn't break because the feathers exert a much smaller force on the egg than the table would. The feathers can move so when the egg first hits them, the feathers don't have to stop the egg so quickly. To keep the egg from penetrating into the table, the table has to stop the egg's descent in about a thousandth of a second. That required a huge upward force on the egg of perhaps 1000 N. This large upward force, exerted on one small point of the egg, breaks the egg. But when the egg hits the feathers, the feathers can stop the egg's descent leisurely in about a tenth of a second. They only have to push upward on the egg with a smaller force of perhaps 10 N. This modest force, exerted on many points of the egg, shouldn't break the egg. During this tenth of a second, the feathers and the egg will both move downward and the egg will come to a stop well below the place at which it first touched the feathers.

727. Without gravity in space, what would happen to the recoil if a gun were shot off? — DZ, Illinois
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Even in the depths of space, so far from any planet that gravity is virtually absent, a gun will have its normal recoil. When you push on something, it pushes back on you just as hard as you push on it. That rule, known as Newton's third law of motion, is as true in empty space as it is on earth. Thus when the gun pushes the bullet forward, the bullet pushes the gun backward equally hard and you feel the gun itself jump backward as result. This recoil effect is the very basis for rocket propulsion—the rocket pushes its exhaust backward and the exhaust pushes the rocket forward. That's why rockets can work outside the earth's atmosphere and away from any celestial objects—the rocket only has to push on its exhaust in order to obtain a push forward.

762. If forces are always equal but opposite, how can a hammer drive a nail into a wall? Don't the forces on the nail cancel?
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Although forces always appear in equal but oppositely directed pairs, the two forces in each pair act on different objects. The nail and hammer experience one of these force pairs—the hammer pushes on the nail just as hard as the nail pushes on the hammer. Because the nail's force on the hammer is the only force that the hammer experiences, the hammer accelerates away from the nail and the wall. The nail and wall experience the other force pair—the wall pushes on the nail just as hard as the nail pushes on the wall. The nail thus experiences two horizontal forces: the hammer pushes it toward the wall and the wall pushes it away from the wall. As long as all the forces are gentle, the two forces on the nail cancel and it doesn't accelerate at all. But if you hit the nail hard with the hammer, the wall can't exert enough support force on the nail to prevent it from enter the wall. The two forces on the nail no longer cancel and it accelerates into the wall.

763. If the net force on an object is zero and it has no acceleration, then what causes it to have velocity? Doesn't a force give it velocity? And doesn't this make the gravitational and support forces unequal? - EH
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The great insights of Galileo and Newton were that an object doesn't need a force on it to have a non-zero velocity. Objects tend to coast along at constant velocity when they are free of forces, or when the net force on them is zero. Inertia keeps them going even though nothing pushes on them. While it takes an acceleration and thus a non-zero net force to get an object moving in the first place, it will continue to move even if the net force on it drops to zero. So while I was lifting the bowling ball upward at constant velocity, the net force on the bowling ball was truly zero—it was coasting upward because its weight and the support force from my hand were canceling one another. However, to start the bowling ball moving upward, I had to push upward on it harder than gravity pushed downward. For a short time, the bowling ball experienced an upward net force and it accelerated upward. After that, I stopped pushing extra hard and let the bowling ball coast upward at constant velocity.

764. What would it be like if Newton's third law weren't true? Can we imagine that?
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Many strange things would happen. For example, suppose that you pushed on your neighbor and your neighbor didn't push back—you wouldn't feel any force pushing against your hand so you wouldn't even notice that you were pushing on your neighbor. Your neighbor would feel you pushing on them and they would accelerate away from you.

Among the many consequences of such a change would be that energy wouldn't be conserved—you would be able to create energy out of nowhere. To see how that would be possible, imagine lifting a heavy object and suppose that as you pushed upward on it, it didn't push downward on you. As you lifted it upward, you would do work on it—you would exert an upward force on it and it would move upward. But it wouldn't do negative work on you—it would exert no force on you as your hands lifted it upward. As a result, its energy would increase but your energy wouldn't decrease. Energy would be created. In fact, you wouldn't even notice that you were lifting it because it wouldn't push on you as you lifted it.


765. Would a small-mass hammer that accelerated rapidly exert more horizontal force on a nail than a large-mass hammer that didn't accelerate very much?
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Yes. Since the only horizontal force acting on the hammer is that exerted on it by the nail, the hammer's acceleration is entirely determined by that force. The force on the hammer is equal to the hammer's mass times the hammer's acceleration (Newton's second law). If both hammers experienced the same acceleration, then the large-mass hammer would have to be experiencing the larger force from the nail and would therefore be exerting the larger force on the nail. But because the small-mass hammer is experience a larger acceleration, the force that the nail is exerting on it may be quite large. If the small-mass hammer's acceleration is large enough, the force on it may exceed the force on the large-mass hammer.

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