Why dont rain drops accelerate as they get nearer to earth?

Answer 1

Because they have already reached the terminal velocity of 120mph which is the fastest any object can fall to the earth. gravity would have to increase its pull inoder for it to make objects fall faster.

Answer 2

Terminal velocity is simply the velocity at which the falling body will no longer accelerate. In a very true sense, TV should really be called terminal acceleration, because that's the point where acceleration ceases, but velocity might continue on.

If you are a math nerd, f = mg - 1/2 Cd A rho v^2; where f is the net force on the raindrop, m is the mass of the raindrop, g is the acceleration under gravity (32 ft/sec sec), Cd is a coefficient of drag unique to each raindrop, A is the cross sectional area of each raindrop, rho is the density of the surrounding air, and v is the current velocity of the raindrop.

Falling bodies that have Cd and/or A too small to totally offset mg (the weight of the body) at the velocities it is falling will terminate its velocity acceleration only upon impact with the Earth. That is, it will continue to accelerate until it smacks into the ground; at which time, other forces in opposition cause it to deccelerate and stop.

As some of your answerers also state, other forces might also come into play (wind for example). There are sea stories about people jumping out of aircraft and being held in the air for long minutes by upward drafts in storm cells for example. But the equation I provided will give you a good ballpark answer for TV in most cases.

To find the terminal velocity, set f = 0 because that's where the force of gravity is cancelled out by the drag force. So v = sqrt[mg/Cd A rho] = TV. From this you can readily see that TV will vary from drop to drop according to each drop's own m, Cd, and A, and according to current atmospheric conditions as measured by density, rho.

By the way, the 120 mph some people provided in their answers...that came from an average person falling through the air. That's the TV for a person under average conditions of Cd, A, rho, and m. Sky divers reach that velocity (or thereabouts) when doing their thing.

Answer 3

Most of the answers talk of the terminal velocity. But why does the terminal velocity comes into picture is not explained in simple term. Neglecting the effect of wind three forces act on a falling drop. One is by earth. Second is due to buoyancy and the third is due to the moving air passed. The cause of the force due to earth is force exerted by the earth and is dependent on the mass of the object concerned and it acts downwards . Here the object is rain drop. Second is the buoyant force, the reason for which is the random motion of the molecules of air which forces the air to get into the space which has been now occupied by the drop. This is proportional to the mass of the air displaced and acts upwards. The third force is due to viscosity of air which is due to the molecules of the air moving past the molecules of the raindrop. This force is proportional to the relative velocity of raindrop and air and this force always acts in the direction opposite to the direction of relative motion.As long as the net downward force due to algebraic sum of gravitational force and buoyant force is more than the viscous force there is a net unbalanced downward force and hence the rain drop accelerates initially. As its velocity increases the viscous force also increases. therefore one can easily see that at some velocity the net force on rain drop would be zero hence the rain drop can no longer accelerate. This is the stage when it acquires terminal velocity.

Answer 4

Excerpt from http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId=146 :

Rain Drop Terminal Velocity
Any thing moving through the atmosphere experiences the force of gravity and a drag force due to the friction of the air as it moves through it. When the object is falling, the force of gravity is directed towards the ground and the drag force directed upwards. As the body moves faster, the drag force becomes larger and larger until it equals the force of gravity and the object then falls at a constant velocity. This velocity is called the terminal velocity. For a person jumping out of an airplane, in skydiving, it is roughly 120-150 miles per hour. The terminal velocity of a water drop ranges from 1.6MPH for drizzle to 20 MPH for large drops during heavy rain.

See the website for an interactive demo!!!

Answer 5

cabjr is mostly right. gravity is pulling down on the raindrops, but the raindrops have to push the air out of the way. Eventually the friction from the air balances out gravity, and the raindrop doesn't fall any faster. This is called terminal velocity.

cabjr is wrong in that 120 mph isn't a universal terminal velocity. Each object has it's own terminal velocity, depending on how much air it has to move out of the way (relative to their weight). So a feather falls very slowly, while a rock falls fast, all based on how easily they can move the air out of the way

Answer 6

While cabjr1961 was close in that he did mention terminal velocity, he is still a bit off.

Terminal velocity is not 120 MPH. It varys according to air resistance (mainly) and other factors (initial velocity, air pressure, density, outside forces, etc.).

Rain drops do reach their terminal velocity a few seconds after condsening from clouds, and unless the velocity is changed by wind or other forces, it will not fall faster until it lands.

Answer 7

Everything falls toward the earth at the same rate of acceleration due to gravity, 32 feet/sec/sec, until they reach terminal velocity. Probably raindrops dont fall long enough to reach terminal velocity, so they are accelerating.

Answer 8

They reach terminal velocity and cannot go any faster. BUT the raindrops start out as much larger drops and fragment into smaller droplets as they reach the Earth

Answer 9

Terminal velocity

Answer 10

I'm no physics expert, but the term "terminal velocity" comes to mind