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Old 08-24-2019, 09:35 PM   #63921 (permalink)
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Originally Posted by BassoonPlatoon View Post
So today I thought I learned some decent physics terms when I said "an object at rest has 0 velocity and 0 acceleration". Everyone just looked at me like that was so obvious, but I was trying to discuss the meaning of the terms not just their values.
An object at rest can have nonzero acceleration.
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Old 08-24-2019, 09:42 PM   #63922 (permalink)
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An object at rest can have nonzero acceleration.
explain this for the rest of us, don't just assume we understand
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Old 08-24-2019, 09:49 PM   #63923 (permalink)
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explain this for the rest of us, don't just assume we understand
You toss a ball up in the air. At the top of its motion, it's momentarily at rest (velocity zero). However, it's still accelerating downward at 9.8 m/s² (due to gravity).
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Old 08-24-2019, 09:50 PM   #63924 (permalink)
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is that like an object floating underwater that has water acting against it but has buoyancy acting to make it rise so that it's stationary but there's still a force trying to make it accelerate?
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Old 08-24-2019, 09:59 PM   #63925 (permalink)
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is that like an object floating underwater that has water acting against it but has buoyancy acting to make it rise so that it's stationary but there's still a force trying to make it accelerate?
No, in that case the forces would balance so that the acceleration would be zero. The point with the ball is that its velocity is changing, even when it is instantaneously at rest at the top of its motion.

If the ball were sitting at rest on the earth, however, then its acceleration would be zero. Even though gravity pulls down on it, there's a corresponding upward force exerted by the ground that balances it out.

Being "at rest" (i.e., zero velocity) with nonzero acceleration only happens instantaneously.
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Old 08-24-2019, 10:02 PM   #63926 (permalink)
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No, in that case the forces would balance so that the acceleration would be zero. The point with the ball is that its velocity is changing, even when it is instantaneously at rest at the top of its motion.

If the ball were sitting at rest on the earth, however, then its acceleration would be zero. Even though gravity pulls down on it, there's a corresponding upward force exerted by the ground that balances it out.

Being "at rest" (i.e., zero velocity) with nonzero acceleration only happens instantaneously.
But the air pressure on anything is still acting against anything sitting still so is still an active force, no? Like if all the air on earth suddenly disappeared everything on earth resting would "pop up"?
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Old 08-24-2019, 10:07 PM   #63927 (permalink)
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PS

Are you going to tell him the newtons equal out in that case?
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Old 08-24-2019, 10:08 PM   #63928 (permalink)
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But the air pressure on anything is still acting against anything sitting still so is still an active force, no? Like if all the air on earth suddenly disappeared everything on earth resting would "pop up"?
Huh?
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Old 08-24-2019, 10:09 PM   #63929 (permalink)
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Air pressure adds weight.
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Old 08-24-2019, 10:18 PM   #63930 (permalink)
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Huh?
I mean that the air pressure is adding permanent change to velocity that if removed would stop changing velocity. Like what's the difference between a ball hanging in midair and a ball sitting on the ground with air pushing down on it when if that air were removed would change/add velocity?
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There is only one bright spot and that is the growing habit of disgruntled men of dynamiting factories and power-stations; I hope that, encouraged now as ‘patriotism’, may remain a habit! But it won’t do any good, if it is not universal.
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