As a river flows, it is flowing downstream and gravity will increase the river's speed. The trouble am having is explaining why this happens on paper; would the pit be able to help me out?

Thanks a lot guys,

Michal

EDITed for mistakes
Last edited by michal23 at Apr 16, 2008,
Draw a diagram instead. If you're not allowed just write a little bit then use a diagram to help get your point across.
Quote by michal23
As a river flows downstream, it is flowing downstream and gravity will increase the river's speed. The trouble am having is explaining why this happens on paper; would the pit be able to help me out?

Thanks a lot guys,

Michal

doesnt make much sense, that bit

but gravity is pulling it down, acting as a force to push it downhill

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Like a ball rolling down a hill...same thing.
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Quote by michal23
As a river flows downstream, it is flowing downstream and gravity will increase the river's speed. The trouble am having is explaining why this happens on paper; would the pit be able to help me out?

Thanks a lot guys,

Michal

but seriously, what exactly do you mean? are you asking why increase in the steepness of the fall increases the speed?
I think it's all about if the river is on a level plane or not. The Colorado river is not on a level plane which is why it flows very fast in some areas and has carved the Grand Canyon. The Amazon river on the other hand is on a pretty flat plane and that causes it to be very slow moving.
The river water will accelerate downstream because of the gravitational component acting in the direction of the river's motion making it accelerate until it reaches a terminal velocity.
Quote by tomathy
doesnt make much sense, that bit

but gravity is pulling it down, acting as a force to push it downhill

>_< I didn't proof read my post, sorry.

But see, the problem is I'm having trouble explaining why the river accelerates, because if for example I drop my guitar out of the window (Not that I am planning on doing so) it will drop at a constant speed, rather than accelerate.
Draw a vector triangle.

As gravity (9.8ms^-2) acts upon the water, it's speed (V) increases since it's initial speed of 0ms^-1 (U). The vertical velocity (j) increases due to gravity, but the horizontal velocity (i), unless acted upon by a force, remains constant at whatever velocity it is.

For example, lets say the initial horizontal velocity is 5ms^-1. The equation is

River speed = 5ms^-1(i) + (9.8ms^-2(j) x T)

T = Time since the river first set off.
U(initial velocity)j = 0
Ui = 5
V (final velocity)i = 9.8 x total time (terminal velocity being reached is negligible, of course )
Vj = 5
Last edited by TheBF at Apr 16, 2008,
the sediments in the water can make a difference, for example larger sediments and debris will sink to the bottom of the river which causes the river to slow down, while lighter sediments will ride with the water which can increase the rivers speed
Quote by michal23
if for example I drop my guitar out of the window (Not that I am planning on doing so) it will drop at a constant speed, rather than accelerate.

No it won't, it'll accelerate due to gravity, at a rate of 9.8m/s, until it hits it's terminal velocity. It's the same story for the water.
Quote by gen0doom
No it won't, it'll accelerate due to gravity, at a rate of 9.8m/s, until it hits it's terminal velocity. It's the same story for the water.

Ah okay; the terminal velocity, I assume, is the maximum speed an object can have as a result of gravity, without any other forces acting on it... poorly phrased, but is it correct? I looked on both dictionary.com and wikipedia but didn't quite understand it =/
Yeah, terminal velocity is when the forces acting vertically on the object are balanced, so it can't accelerate anymore.
Quote by Mutant Corn
Like a ball rolling down a hill...same thing.

Right. a ball rolls faster down a steeper slope, a river runs slower over a steeper slope. There's a physics equation involving a = g * sin(theta) or something but it's been a while so i forget the specifics.
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Quote by michal23
Ah okay; the terminal velocity, I assume, is the maximum speed an object can have as a result of gravity, without any other forces acting on it... poorly phrased, but is it correct? I looked on both dictionary.com and wikipedia but didn't quite understand it =/

Terminal velocity is the maximum velocity something can fall, when the acceleration of gravity = acceleration of air resistance, so that net acceleration is 0 and velocity remains the same.
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^ wow i actually almost missed that hahaha iforgot your a genious

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fuck that police!
Quote by michal23
Ah okay; the terminal velocity, I assume, is the maximum speed an object can have as a result of gravity, without any other forces acting on it... poorly phrased, but is it correct? I looked on both dictionary.com and wikipedia but didn't quite understand it =/

No. Terminal velocity takes all forces into account.

As in, Gravity (force down) - Resistance (force up).

Gravity is constant, but the resistance is what give the terminal velocity. A small, heavy object will have a higher terminal velocity than a large light one.

This is for objects in freefall, in air.
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Okay, well thanks for your help guys, I've managed to phrase it by explaining the terminal velocity, which I was unaware of before (i.e. unaware of the terminology, I knew that such an effect existed =P)
Terminal velocity: The point at which the forces acting upon an object are balanced, and it ceases to accelerate. For example, a ball would continue to accelerate downwards until the air resistance acting upon it is at 9.8N. That is why a parachute has a lower terminal velocity than a ball, because there is higher air resistance than the ball due to greater area for the air to act upon, as well as the effect of very poor streamlining.

In space, the ball would continue to accelerate indefinitely, the light-speed-limit and gravity from other spatial objects being negligible.
Gravity will only increase the flow velocity if the river bed slope angle is increasing, otherwise it remains constant, ignoring friction, temperature and the like.
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