As for the straight line, yes. All objects will continue moving along geodesics (a straight line in curved-space but sometimes a curved line in straight-space) if there are no external forces acting on them. Unless, by different velocity you mean the direction is not entirely radial to us. In that case, the expansion will cause the object's path to appear to "bend" away from us, but this is still a geodesic for the object. Show As for how far it will get, this depends on your distance measurement. In proper distance (the distance you would measure with a ruler), the object can get infinitely far away if you wait an infinite amount of time. There is no limit in proper distance. There is also something called comoving distance, which factors out the expansion of the universe. Two points that are only getting farther apart because the universe is expanding remain the same distance apart in comoving coordinates. In this distance measure, there is a fixed distance that the object can get to. The image below shows the maximum comoving distance that we can reach at different velocities.  The blue, purple, green, and black lines are 0.5c, 0.8c, 0.99c, and c respectively. Today, comoving distance equals proper distance. So the distances you see on that graph are essentially the maximum today-distances you could reach (even though the actual distance travelled would be substantially larger). Anything beyond those distances are unreachable from where we are now. Anything at those distances would take 70 billion years to reach at the given velocities. So you see, an object would travel along a geodesic path and a ruler would measure it going an unlimited distance, but there is a limit on how far something can travel on a comoving scale. And that is the more useful thing to know.
SfC Home > Physics > Mechanics > by Ron Kurtus Inertia is a property of space and matter such that an object has a tendency to remain in its state of motion unless acted upon by a force. This is stated in Newton's First Law of Motion which is often called the Law of Inertia. What the law states is that if an object is motionless, it will stay motionless unless acted upon by some force. Likewise, if an object is moving at a constant velocity, it will continue at that speed unless acted upon by some force along the line of motion. And finally, if an object is moving, it will move in a straight line unless acted upon by some force at an angle . This law is important in defining how things around us behave. Questions you may have include:
This lesson will answer those questions. Useful tool: Units Conversion The Law of Inertia states that objects that are not moving will remain motionless, unless you apply a force on them. This law of nature makes sure things will stay where you put them. It requires that you provide a push or pull to get something moving. A trick using inertiaThere is the parlor trick that you may have seen where a person quickly pulls a tablecloth from under a setting of heavy dishes or some other objects. If it is does correctly, the objects remain in place on the table after the tablecloth is pulled from under them. This trick works because the inertia of the heavy objects tends to keep them in place. By quickly pulling the tablecloth, the force of friction is easily overcome. If the tablecloth was pulled slowly, the friction would be greater than the inertia, and the dishes would follow along.
Object will continue motionOnce you start an object moving, it will keep moving unless you apply a force in the opposite direction to slow it down. Typically, the force of friction will slow things down. But in outer space, where friction is almost zero, an object will move at its given velocity forever unless acted upon by some external force. If you push a moving object in the direction of motion, it will accelerate to a new velocity. Once you stop pushing, the object will continue at the velocity it had once you stopped pushing. Things move in straight linesThe Law of Inertia states that moving objects go in a straight line. You must apply a force on an object to make it go in a circular motion. For example, when you spin an object around on a string, you are applying a force on that object from the string to make it go around. Once you let the string (or the force) go, the object will fly off in a straight line. Likewise, the Moon is attracted to the Earth by the gravitational force. That force is just enough to make the Moon spin around the Earth. If gravitation would stop, the Moon would fly off in a straight line into outer space. SummaryThe Law of Inertia states that matter remains in its state of motion and direction unless acted upon by a force. Motionless objects will stay motionless unless acted upon by some force. Moving objects continue at a constant speed unless acted upon by some force. Also, moving objects will move in a straight line unless acted upon at an angle by some force. Don't let your personal inertia hold you back Resources and referencesRon Kurtus' Credentials WebsitesNewton's First Law of Motion - Physics Classroom Newton's laws of motion - Wikipedia Physics Resources Books(Notice: The School for Champions may earn commissions from book purchases) Top-rated books on the Physics of Motion Share this pageClick on a button to bookmark or share this page through Twitter, Facebook, email, or other services: Students and researchersThe Web address of this page is: Please include it as a link on your website or as a reference in your report, document, or thesis. Copyright © Restrictions Where are you now?School for Champions
The First Law is concerned with changes in velocity caused by non-zero net forces. Isaac Newton stated three laws of motion; the first law deals with forces and changes in velocity. For just a moment, let us imagine that you can apply only one force to an object. That is, you could choose push the object to the right or you could choose to push it to the left, but not to the left and right at the same time, and also not up and to the right at the same time, and so on. Under these conditions the first law says that if an object is not pushed or pulled upon, its velocity will naturally remain constant. This means that if an object is moving along, untouched by a force of any kind, it will continue to move along in a perfectly straight line at a constant speed. In
the animation above we see an object (blue box) that is already moving
along when we encounter it. As long as we do not push or pull it, as
long as it does not run into something else or rub against something
else, it will continue to move in a straight line at constant speed. It
will continue to do this on its own without the need for any influence
from other agents. The first law also means that if an object is standing still and is not contacted by any forces, it will continue to remain motionless. Actually, a motionless object is just a special case of an object that is maintaining constant velocity. Its velocity is constantly 0 m/s. In the scene above we see an object (blue box) that is standing still. On its own it will continue to stand still as long as we do not influence it with a force, that is, with an unbalanced, non-zero force. Several forces can act at once on the object. Now, what about if there is more than one force on the object? You really can push an object, say, to the left and down at the same time, so, what happens then? Under these conditions we must realize that a group of forces on an object adds up so that all the forces appear to the object as one force. This one force that is the sum of all the forces is called the net force. (You might want to read some information on net force. It is here.) The word net in this context means total. It is this net force that may change the velocity of the object. Let us look at some examples. Imagine that two forces act at the
same time on an object. One is a very strong force to the left, and the
other is a weaker force to the right. These two forces add up to one
net force. Since the force to the left is stronger, the net force is to
the left. This net force to the left will cause the velocity of the object to change. The object experiences this one net force as if this was the only force pushing it, although, actually, there are two separate forces present. Next let us see what happens when two forces act, but they are equal in strength. Imagine that two forces, one to the right and
one to the left, push on an object, and imagine that the two forces are the
same size. These two forces add up as before, but this time one of them
does not overpower the other. They cancel each other out. So, in this
example the net force is zero. It is as though no forces were really
acting on the object. Under these conditions the velocity of the object would not change. If it was moving in a straight line at constant speed before the two forces were applied, then it would continue to move in a straight line at constant speed after these two equal and opposite forces were applied. If it was standing still before the application of these forces, it would continue to stand still afterwards. The net force is the total force. It could be the sum of two forces or more than two forces. If only one force acts upon an object, then this one force would be the net force. If the net force on an object is zero, then the object experiences no velocity change. If the net force on an object is not zero, then the object will show a change in velocity. Lastly, this net force must be external to the object. The net force can not come from the object itself. You could not, for example, put on ice skates, stand on a frozen pond, push on your back by reaching around with your arms, and expect to get going. Although if someone else came up from behind and gave a you a shove, then your velocity would change. But skaters do get going all by themselves, so, how does that happen? Well, that answer is in Newton's third law of motion. They push on the ice, and it is the the ice pushing back on them that changes their velocity and gets them going. Newton's first law of motion contains the same information as Galileo's explanation of inertia. Other ways to say Newton's First Law Click here and link to a collection of other ways to say Newton's First Law. An explanation for each way is included. Hopefully, the explanation will show that each way of saying this law actually describes the same physics. 3-D VRML First Law Demonstration To see this law in action go to the following VRML 2.0world. Newton's First Law Demonstration You will find an object there upon which you can apply a force. You can apply only one force on the object at a time. The object's velocity will only change while this one force is being applied. The VRML world looks like this picture:
The object to be moved floats in the center of the world. There are six dark arrows touching it. Each arrow represents a force that can be activated upon the object. One arrow points in the positive x direction (to the right), another points in the positive y direction (upward), and so on. Click and hold down the mouse on one of the boxes in the foreground to activate the force that is described by the label on the box. The corresponding arrow on the object will turn white, and a force in its direction will be applied. Notice that with no net force on the object, its velocity does not change. The only way to change the velocity is to apply a non-zero net force. If the object drifts away, either use your browser viewpoint options to get a better look or hit the Reset button. That button will bring the object back to its original position. |
What happens when you move in a straight line?
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