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Thread: Inertia

  1. #1 Inertia 
    Forum Senior bill alsept's Avatar
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    Inertia is simply the tendency of objects to resist a change in whatever state of motionthat it currently has. Put another way, inertia is the tendency of an object to"keep on doing what it is doing." Mass is a measure of an object'sinertia. The more mass which an object has, the more that its sluggish towardschange.

    This seems tobe a typical definition of inertia but I was wondering if it wascomplete or not. If you have two objects sitting on a table, say a marble and a bowling ball andboth are at rest relative to each other. According to the definition the bowling ball will resist acceleration far more than the marble. On the other hand everyone knows that when we release the two objects they accelerate the same. Experiments proves that no matter how massive an object is or how much inertia it has it will not resist being accelerated any more than other objects being accelerated toward the earth. g tends to accelerate all objects the same no matter how much inertia they have.



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    Perhaps instead of quibbling about the definition, you should just consider the mathematical description: F=ma. There is no ambiguity there.


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    Quote Originally Posted by bill alsept View Post
    Inertia is simply the tendency of objects to resist a change in whatever state of motionthat it currently has. Put another way, inertia is the tendency of an object to"keep on doing what it is doing." Mass is a measure of an object'sinertia. The more mass which an object has, the more that its sluggish towardschange.

    This seems tobe a typical definition of inertia but I was wondering if it wascomplete or not. If you have two objects sitting on a table, say a marble and a bowling ball andboth are at rest relative to each other. According to the definition the bowling ball will resist acceleration far more than the marble. On the other hand everyone knows that when we release the two objects they accelerate the same. Experiments proves that no matter how massive an object is or how much inertia it has it will not resist being accelerated any more than other objects being accelerated toward the earth. g tends to accelerate all objects the same no matter how much inertia they have.

    That's because the greater mass of the bowling ball also results in a stronger force acting on it due to gravity. Inertia my make it so many times harder to accelerate, but the gravity acting on it is that many times stronger.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    It's because the difference of both objects inertia is small and relative to Earth that difference is trivial. You have to remember that during this experiment you are pitting Earth's inertia with the objects's inertia. Another experiment that may help you to realize the implications of inertia is take two balls of equal volume, but drastically different densities, such as styrofoam and marble, and set them on a collision course. The styrofoam will obviously deflect more than the marble because it has less mass, because it resists change less.
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    Forum Senior bill alsept's Avatar
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    Quote Originally Posted by Travis Meyers View Post
    It's because the difference of both objects inertia is small and relative to Earth that difference is trivial. You have to remember that during this experiment you are pitting Earth's inertia with the objects's inertia. Another experiment that may help you to realize the implications of inertia is take two balls of equal volume, but drastically different densities, such as styrofoam and marble, and set them on a collision course. The styrofoam will obviously deflect more than the marble because it has less mass, because it resists change less.
    Thanks Travis but I don't really have an issue with inertia itself, I agree with post 2 and 3. When I posted I had a train of thought going about something else and I'm still thinking about it. As for your argument above I'm not sure I agree. It wouldn't matter how different their inertia was they will alway accelerate the same if the force is gravity. As for the styrofoam it too would accelerate the same as the marble or the bowling ball if there were no resistance. My original thought had more to do with how the inertia of a mass goes away the moment it begins to free fall.
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    Quote Originally Posted by bill alsept View Post
    My original thought had more to do with how the inertia of a mass goes away the moment it begins to free fall.
    It doesn't go away once it free falls. The free fall is directed towards the common center of gravity of the two gravitating bodies; if you try to change direction or speed of the falling body you need to apply a force, and that force is once again proportional to the body's mass. Inertia never goes away, just like mass never goes away.
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    Quote Originally Posted by bill alsept View Post
    It wouldn't matter how different their inertia was they will alway accelerate the same if the force is gravity.
    That only applies to small bodies in freefall to relatively large bodies. This allows you to neglect the difference in acceleration caused by small differences in mass. When objects start to approach a similar mass to Earth they will react to Earth's gravity much slower because their inertias are more similar.

    Quote Originally Posted by bill alsept View Post
    As for the styrofoam it too would accelerate the same as the marble or the bowling ball if there were no resistance. My original thought had more to do with how the inertia of a mass goes away the moment it begins to free fall.
    I think you misread what I wrote. I wasn't talking about the balls in freefall I was describing an experiment where the styrofoam ball and marble ball were on a collision course. Furthermore, an objects inertia never goes away.

    Quote Originally Posted by bill alsept View Post
    Experiments proves that no matter how massive an object is or how much inertia it has it will not resist being accelerated any more than other objects being accelerated toward the earth. g tends to accelerate all objects the same no matter how much inertia they have.

    This statement is blatantly wrong.
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    Forum Senior bill alsept's Avatar
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    When I say inertia goes away I mean the effect of inertia or the part we feel. As we stand here on the surface of the earth we feel it but in free fall we don't. We feel it when we accelerate but it goes away when were not.
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    We feel it when we accelerate but it goes away when were not.
    You feel the force applied to overcome inertia, but you don't feel the inertia. When you accelerate, force is being applied. When you are not accelerating, you are in free fall and no force is being applied.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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  11. #10  
    Forum Senior bill alsept's Avatar
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    Quote Originally Posted by AlexG View Post
    We feel it when we accelerate but it goes away when were not.
    You feel the force applied to overcome inertia, but you don't feel the inertia. When you accelerate, force is being applied. When you are not accelerating, you are in free fall and no force is being applied.
    I know, that's whats fun and interesting about inertia. As we free fall faster and faster it looks like we are accelerating but actually we are constantly accelerating as we stand on the ground.
    Does anyone know the best explanation of this?
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    Quote Originally Posted by Harold14370 View Post
    Perhaps instead of quibbling about the definition, you should just consider the mathematical description: F=ma. There is no ambiguity there.
    Actualy the correct definition/law is F = dp/dt. This is the general relation and holds in all cases, eve when m is not a constant. It also holds in relativity.
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    Quote Originally Posted by bill alsept View Post
    Does anyone know the best explanation of this?
    It's because you are located in a gravitational potential which is not uniform ( =has a non-vanishing gradient ). The gradient of a potential is what we experience as a force, and the force tends to push you to the point of lowest potential energy, which is at the center of the earth ( simply put ); this holds in general, for gravity as well as EM fields etc.
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    Hm, maybe there is a simpler way to view and understand inertia then how you treat your balls.

    Even light has inertia, even though it has no mass. Instead view inertia in the following sence:

    If something hit an surface, which for now we will see as an elastic collision. Then if it bounces back the surface has given twice the momentum. (say a tennisball bouncing of a wall. It hits with a certain momentum and has this same momentum but reversed when it bounces of. So the wall must have exerted twice the momentum)

    Now force is the change of momentum.

    Momentum is something an object has in a certain direction (due to several different kinds of relative aspects) In contrary to for instance weight, or speed, momentum is a very reliable and unambigously definable property of a system. Hence their use in Hamiltonians.

    A large momentum will take a large force to cause the same deceleration. That is why when I throw the tennisbal against the wall, it will bounce back. As the wall can exert twice the momentum. But when I use the wreckingball the momentum is transfered to the wall parts and their binding. In contrary to mass, and speed. Momentum is conserved.
    In the information age ignorance is a choice.
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    Well first let's define some of the terms we are using, gravity and free fall.

    Gravity is a distortion of space and time. Gravitation - Wikipedia, the free encyclopedia. Elaborating on this, picture a two dimensional graph where the axes are time (seconds squared) and distance (meters). This is convenient for us because these are the units that gravity is typically measured in (meters per seconds squared, m/s^2). A unique property of this graph is that you can change the distance between the time or distance measurements however you like. You can experiment with this and you'll notice an interesting effect. Let's say you want to change how you measure time. You make the length between your time measurements greater. As you do this you'll begin to notice that the length between the distance measurements gets shorter all on their own. Then you decide to change the distance measurements to make them further apart and all the sudden the time measurements get closer together. When you make one axis bigger it causes the other axis to get smaller by the same amount. In other words they always will maintain the same ratio with each other. If you remember from math the slope of a line is the ratio between the change in y over the change in x. Slope - Wikipedia, the free encyclopedia. If you plot a line on this graph with a 45 angle coming from the (0,0) point you'll notice that as you change time or distance it will always stay straight because the ratio between the axes stays the same. Effectively what we've just done is create a graph of gravitational acceleration where time and space can vary. One of the cool things about this is that when you are traveling through an object's gravity you are traveling in a straight line just like on the graph but your time or space is being changed. This is called a geodesic, which is a straight line on a plane this is not straight (in our case it's curved like water flowing around a sphere). Geodesic - Wikipedia, the free encyclopedia. Geodesics in general relativity - Wikipedia, the free encyclopedia.

    Now, for free fall. When you are in free fall you are simply following the geodesic of the object you are free falling towards. Free fall - Wikipedia, the free encyclopedia.

    One thing about gravity is that it's not a force. A force causes an object to deviate from it's path. Gravity doesn't cause an object to deviate from it's path. Force - Wikipedia, the free encyclopedia. Gravity bends space and time and the object follows a path that, from our perspective, seems to be accelerating. The surface of the Earth however does impart a force on people. As a sky diver free falls towards Earth they feel no force (neglecting wind resistance) until they encounter the Earth's crust which imparts a force in the opposite direction of their geodesic. This is called the Ground Reaction Force (Ground reaction force - Wikipedia, the free encyclopedia) which is really a macro manifestation of the electromagnetic force, specifically electric repulsion by the electrons surrounding the nuclei of your atoms and the ground's atoms.

    Lastly, judging from your comments, I still think you might not be getting what inertia actually means. Inertia is an objects tendency to resist a change in motion. Inertia - Wikipedia, the free encyclopedia. The key word here is change. When an object is in free fall towards Earth it accelerates at a constant rate. At this point in time the effects of inertia aren't noticed because there is no change because it has constant acceleration. While you're interacting with Earth's gravity you won't notice the effects that inertia has on your body's movement unless you can cancel the effects of gravity. Take for instance an astronaut who appears to be weightless. As the astronaut is bouncing around the space shuttle there are sometimes other objects bouncing around also, let's say a candy bar. If the astronaut and the candy bar collide they impart the same force on each other. Newton's laws of motion - Wikipedia, the free encyclopedia. What's interesting though is that the candy bar deflects more than the astronaut. If they impart the same force on each other you would figure that they both move the same distance due to the collision right? No. This is because the astronaut has more mass and his body resists changes in motion more than the candy bar. Mass - Wikipedia, the free encyclopedia. Now picture a similar scenario with gravity included. Throw a candy bar at an astronaut on Earth and the effects of inertia are far less noticeable. The candy bar simply falls to the ground. Both objects still have inertia, gravity just simply overpowers the interaction.
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