I don’t understand work done without any acceleration. Since F=ma and a=0, F=0 a…

Lou Bloomfield Leave a comment

I don’t understand work done without any acceleration. Since F=ma and a=0, F=0 and thus W=0.

You are merging two equations out of context. The force you exert on an object can be non-zero without causing that object to accelerate. For example, if someone else is pushing back on the object, the object may not accelerate. If the object moves away from you as you push on it, then you’ll be doing work on the object even though it’s not accelerating. The only context in which you can merge those two equations (Force=mass x acceleration and Work=Force x distance) is when you are exerting the only force on the object. In that case, your force is the one that determines the object’s acceleration and your force is the one involved in doing work. In that special case, if the object doesn’t accelerate, then you do no work because you exert no force on the object! If someone else is pushing the object, then the force causing it to accelerate is the net force and not just your force on the object. As you can see, there are many forces around and you have to be careful tacking formulae together without thinking carefully about the context in which they exist.

What effects do forces acting on an object which are not from the same pair have…

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What effects do forces acting on an object which are not from the same pair have on one another? i.e. the force pulling the egg downward and the potential force of the table? Are they equal upon impact and there a pair?

Different forces acting on a single object are not official pairs; not the pairs associated with Newton’s third law of action-reaction. While it is possible for an object to experience two different forces that happen to be exactly equal in magnitude (amount) but opposite in direction, that doesn’t have to be the case. When an egg falls and hits a table, the egg’s downward weight and the table’s upward support force on the egg are equal in magnitude only for a fleeting instant during the collision. That’s because the table’s support force starts at zero while the egg is falling and then increases rapidly as the egg begins to push against the table’s surface. For just an instant the table pushes upward on the egg with a force equal in magnitude to the egg’s weight. But the upward support force continues to increase in strength and eventually pushes a hole in the egg’s bottom.

If there is an upward force on the egg when it hits the table, why doesn’t it bo…

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If there is an upward force on the egg when it hits the table, why doesn’t it bounce upward?

The enormous upward force on the egg when it hits the table does cause the egg to accelerate upward briefly. The egg loses all of its downward velocity during this upward acceleration. But the egg breaks before it has a chance to acquire any upward velocity and, having broken, it wastes all of its energy ripping itself apart into a mess. If the egg had survived the impact and stored its energy, it probably would have bounced, at least a little. But the upward force from the table diminished abruptly when the egg broke and the egg never began to head upward for a real bounce.

How does the egg (sitting on a table) hold up the table? If the “weight vs. sup…

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How does the egg (sitting on a table) hold up the table? If the “weight vs. support force of table” is not always an equal pair then how is the “support force of the egg vs. the table” an equal pair?

When an egg is sitting on a table, each object is exerting a support force on the other object. Those two support forces are equal in magnitude (amount) but opposite in direction. To be specific, the table is pushing upward on the egg with a support force and the egg is pushing downward on the table with a support force. Both forces have the same magnitude—both are equal in magnitude to the egg’s weight. The fact that the egg is pushing downward on the table with a “support” force shows that not all support forces actually “support” the object they are exert on. The egg isn’t supporting the table at all. But a name is a name and on many occasions, support forces do support the objects they’re exerted on.

When people are able to bend spoons or move tables with their minds (if this is …

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When people are able to bend spoons or move tables with their minds (if this is actually possible and not just a hoax), what sort of force is being exerted on the object? Is it possible to create forces with the mind?

I’m afraid that spoon bending is simply a hoax. While there are electrochemical processes going on in the mind that exert detectable forces on special probes located outside the head, these forces are so small that they are incapable of doing anything as demanding as bending a spoon. Spoon bending and all other forms of telekinesis are simply tricks played on gullible audiences.

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

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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.

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

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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…

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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.

When you drop a small rubber ball and a large rubber ball simultaneously, why do…

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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.