Primarygun Posted November 6, 2004 Share Posted November 6, 2004 What's the relation between the speed of a falling object and the air resistance? Why does the resistance increase with speed? Is that similar to liquid? When a heavy thing is thrown into water, it sinks with decreasing speed, right? But if an object falls down from a mountain, it falls with increasing velocity but decreasing acceleration. Why there is a difference between liquid and gases? Aren't both of them fluid? Could anyone help me? Link to comment Share on other sites More sharing options...
Mokele Posted November 6, 2004 Share Posted November 6, 2004 Drag = 0.5 * (density of the fluid) * Velocity^2 * surface area * coefficient of drag. The equations applies to both gasses and liquids. The last term, coefficient of drag, is a measured quantity that, without a supercomputer, can't be determined any way except experimentally. So.... What's the relation between the speed of a falling object and the air resistance? Resistance increases to the square as speed does. So, if you double your speed, your resistance rises by a factor of 4 Why does the resistance increase with speed? Most, if not all fluids (gas or liquid) have a viscosity. This makes them stick to surfaces and their own molecules (vaguely). Thus, the fluid in contact with a moving objecty doesn't move. The fluid molecules right on top of those move slowly, and those still higher move a bit faster. Eventually you get up to free-stream speed. The thickness of this layer of slowed-fluid is called the boundary layer thickness. The boundary layer basically means that an object moving through air is slowing some of the air down. This takes energy, and thus reduces the speed of the object (or requires more power output for powered flight). As speed increases, so does the boundary layer thickness and the amount of energy it sucks up. The energy in the boundary layer is effectively thermal energy, but this only comes into effective play at *very* high speeds, like orbital re-entry by spacecrafts. Is that similar to liquid? Yep, just modify the drag coefficient and plug in the new density, and it's the same. When a heavy thing is thrown into water, it sinks with decreasing speed, right?But if an object falls down from a mountain, it falls with increasing velocity but decreasing acceleration. Why there is a difference between liquid and gases? Aren't both of them fluid? Ok, let's say I drop two balls off a mountain. Both will accelerate, but the acceleration will decrese until it's zero, when drag equals gravity, a speed called terminal velocity. One of the balls hits can kills a pedestrian bellow. Yay! The other falls into a lake. Now it's moved from a low-drag fluid into a high-drag fluid. This alters the forces it experiences. It'll decellerate, due to the high drag, but eventually reach a new terminal velocity. The reason it decellerates is that the drag force applied to it suddenly increases. If you dropped a ball underwater (the ball never being above water), it would behave just like in air, accelerating but the acceleration decreasing over time until it hits a constant terminal velocity. Of course, because water is high-drag, this means the speeds involved will be much slower, but it'll still be the same forces. Mokele Link to comment Share on other sites More sharing options...
Primarygun Posted November 7, 2004 Author Share Posted November 7, 2004 Thank you very much. The information helps me a lot. I wonder when it will be taught. It seems to be very useful. Oh. By the way, could air resistance slow down a falling object? And what happens if the atmosphere is made of homogenous air layer( every area of the atmosphere has the same number of gas molecules)? Link to comment Share on other sites More sharing options...
swansont Posted November 7, 2004 Share Posted November 7, 2004 Oh. By the way' date=' could air resistance [b']slow down[/b] a falling object? You'd need a change in surface area (which may change the drag coefficient) or viscosity. Parachutes are an example of the former, the object hitting water is an (extreme) example of the latter. A less extreme example would be freefall from a very high altitude - terminal velocity 20 miles up should be faster than terminal velocity 1 mile up. Link to comment Share on other sites More sharing options...
Primarygun Posted November 8, 2004 Author Share Posted November 8, 2004 I'm quite confused about the force and motion. Force used to determine acceleration. Is the acceleration per second? How to find out how much time the acceleration last for? Come to my case, Air molecules exert force back to the falling object to decelerates it. Why velocity is required to know? Isn't the acceleration constant and the reacting force from air molecules is deceleration? When the sum of them are 0 , the object stops accelerate, but in the equation, why do we need to find out the velocity? I don't know how to account for it but I know it is indeed a factor in reality. Please help me/ Link to comment Share on other sites More sharing options...
mak10 Posted November 8, 2004 Share Posted November 8, 2004 when acceleration = 0, velocity = constant but non-zero. -mak10 Link to comment Share on other sites More sharing options...
Primarygun Posted November 9, 2004 Author Share Posted November 9, 2004 How does the velocity of an object affect the forces? Link to comment Share on other sites More sharing options...
mak10 Posted November 9, 2004 Share Posted November 9, 2004 velocity steadily increasing = constant acceleration = constant force velocity constant = zero acceleration = zero force -mak10 Link to comment Share on other sites More sharing options...
Primarygun Posted November 9, 2004 Author Share Posted November 9, 2004 Yes. The case above shows falling with constant acceleration, but the velocity determine the drag, I don't know why it is determined by the velocity with constant acceleration. Link to comment Share on other sites More sharing options...
cyeokpeng Posted November 11, 2004 Share Posted November 11, 2004 What is the difference betwen air resistance and uptrust? Link to comment Share on other sites More sharing options...
mak10 Posted November 11, 2004 Share Posted November 11, 2004 What is the difference betwen air resistance and uptrust? air resistance is the friction force exerted on the motion of an object in air and increases with speed. upthrust is the upward force exerted on the motion of an object in a liquid, arises due to pressure differences and increases with depth. -mak10 Link to comment Share on other sites More sharing options...
Primarygun Posted November 12, 2004 Author Share Posted November 12, 2004 Anyone would like to help my previous question? I'm quite confused. Link to comment Share on other sites More sharing options...
mak10 Posted November 12, 2004 Share Posted November 12, 2004 Yes. The case above shows falling with constant acceleration, but the velocity determine the drag, I don't know why it is determined by the velocity with constant acceleration. more the velocity... more the drag force, thats how I think they're related. and thus, at some point, the drag force upwards will equalize the weight downwards and the resultant force becomes zero... and the acceleration of the object is now effectively zero = object falling at constant velocity. Link to comment Share on other sites More sharing options...
gene Posted November 13, 2004 Share Posted November 13, 2004 Correct me if i'm wrong. Anything that falls from an a certain height travels at 10m/s^2. (g) So, the object increases it's speed of 10m/s for every second. This i provided that air resistance in neglect. So, If were to relase 2 objects with differnt mass, would their speed still remain constant, given that there is air resistance? I'm confused about drag. Link to comment Share on other sites More sharing options...
Mokele Posted November 13, 2004 Share Posted November 13, 2004 So, If were to relase 2 objects with differnt mass, would their speed still remain constant, given that there is air resistance? Yes, though their final speeds would be different if both had the same shape, since the force of gravity depends on mass, but the force of drag depends on surface area and velocity. Oh, and drag = air resistance Link to comment Share on other sites More sharing options...
Primarygun Posted November 13, 2004 Author Share Posted November 13, 2004 but the force of drag depends on surface area and velocity. Why do drags depend on velocity? Doesn't air resistance depend on the action-reaction force by the object to the air molecules? Link to comment Share on other sites More sharing options...
Mokele Posted November 13, 2004 Share Posted November 13, 2004 There's no really simple was to explain the dependency on velocity. Ok, air, water, any fluid is viscous. Some fluids, like gasses, have very low viscosity, others, like syrup, have very high viscosity. Basically, in crude terms, viscosity is a measure of how "sticky" the particles/molecules are, both to each other and to other surfaces. When any object is moving in a fluid the molecules that touch it's surface are "stuck", and don't move. Those molecules slow down other fluid molecules, almost but not quite to a stop. The next layer is slowed a little less, and so on until you get to no slowing at all. This slowed-down layer is called the boundary layer. Now, if you think about it, the faster the object is moving, the farther away the fluid will be that's not slowed down (in other words, the faster you go, the thicker the boundary layer gets). This means there's a greater mass of air that's "tagging along" as speed increases. A ball that's moving through fluid has a certain amount of kinetic energy. If there were no viscous forces at all, there would be almost no drag (not entirely true, but the non-viscous drag forces can be ungodly complex, so I'll ignore them here). But, with a viscous fluid, you have a boundary layer of slowed fluid. In order to slow something down, you have to exert a force on it, and do work on it. This work comes from the kinetic energy of the ball, slowing it down, and goes into the boundary layer. If you think about it from the perspective of the still air, this thing comes zooming through, and both forces molecules out of the way, but also grabs some and imparts velocity to them, thereby decreasing the object's velocity. Mokele Link to comment Share on other sites More sharing options...
gene Posted November 13, 2004 Share Posted November 13, 2004 " If you think about it from the perspective of the still air, this thing comes zooming through, and both forces molecules out of the way, but also grabs some and imparts velocity to them, thereby decreasing the object's velocity. " If you see in this context, if the speed of the wind which is opposing the applied force on the object, it would faster slow down the object right? but how does the speed of the air molecules allow it to fast absob the object's velocity? Link to comment Share on other sites More sharing options...
Primarygun Posted November 13, 2004 Author Share Posted November 13, 2004 Thank you Mokele. I neglected the intermolecular forces. I just thought of the action-reaction force. It is relative. When you travel in a bus, the people before you seems to be coming while the people behind seems to be leaving. In the case, this is the same. Link to comment Share on other sites More sharing options...
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