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# Basic Physics

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I have 2 equation for acceleration: acc. = power / (speed * mass) power = force * speed = mass * acc. * speed. Is power realy just force * speed? acc. = force / mass. Both are the same(speed cancels out), so why use power instead of force in the first place?

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P = dW/dt = F*ds/dt = F*v

Where P is power, W is work, t is time, F is force, s is displacement, and v is velocity.

The only real reason you''d use power over force is if you''re dealing with a problem that incorporates work or energy much more easier than it would use forces. As far as games go, you probably will never need this, but it''s very useful when solving physics problems.

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Thanks CmndrM that will help alot to understand physic and build the system i''m working on.

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Doesn't seem right. Moving the same direction at the same speed would imply your energy didn't change ignoring a whole bunch of other factors. Friction and gravity being two big ones. That could represent the energy lost to friction if you had a multiplier in there representing friction. The faster you go the more power it takes to keep you moving at that speed.

But then I suck at physics and could be completely wrong. And looks like I most likely am So how does force * speed represent a change in energy? The acceleration component of the force?

[edited by - LilBudyWizer on March 26, 2003 2:14:36 PM]

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If you''re applying a net force F to an object over some distance s you are doing work to it.

W = F . s

Work is a change in energy. If you take the time derivative of both sides you get:

dW/dt = d(F.s)/dt
dW/dt = F . ds/dt

Assuming the force is constant. What''s ds/dt? The rate of change of position, aka velocity. So the rate of change of work, aka power, is force times velocity.

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And to answer your question more precisely, it doesn''t represent a change it represents a rate of change.

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Oops my analysis was slightly flawed. The general equation for work is:

W = integral(F.ds)

which for a constant force F is

W = F.s

and integrating gives the equation

dW/dt = d(F.s)/dt
dW/dt = F.ds/dt
dW/dt = F.v

Edit: I totally suck today

[edited by - Dobbs on March 26, 2003 3:02:23 PM]

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quote:
Doesn''t seem right. Moving the same direction at the same speed would imply your energy didn''t change ignoring a whole bunch of other factors. Friction and gravity being two big ones. That could represent the energy lost to friction if you had a multiplier in there representing friction. The faster you go the more power it takes to keep you moving at that speed.

Kinetic energy, K, is defined as

K = (1/2)mv^2

v in this case being the speed (or magnitude of the velocity) and m being an object''s mass. It is a scalar quantity. Therefore, in an ideal world without friction, moving in a straight horizontal line would cause no change K.

However, we do have friction. So what happens is some of the kinetic energy is lost in the form of heat, sound, etc. Energy is still conserved, but you get a change in velocity (a.k.a. an acceleration).

As Dobbs stated, work is the change in K:

W = integral(F*ds) = K2 - K1

Where K2 is the final KE and K1 is the initial KE.

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