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Pet_Sounds · 08-24-2019, 08:35 PM

Quote:

Originally Posted by BassoonPlatoon

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.

~~jwb~~ · 08-24-2019, 08:42 PM

Quote:

Originally Posted by Pet_Sounds

An object at rest can have nonzero acceleration.

explain this for the rest of us, don't just assume we understand

Pet_Sounds · 08-24-2019, 08:49 PM

Quote:

Originally Posted by jwb

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).

The Batlord · 08-24-2019, 08:50 PM

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?

Pet_Sounds · 08-24-2019, 08:59 PM

Quote:

Originally Posted by Doug McClasky

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.

The Batlord · 08-24-2019, 09:02 PM

Quote:

Originally Posted by Pet_Sounds

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"?

OccultHawk · 08-24-2019, 09:07 PM

PS

Are you going to tell him the newtons equal out in that case?

Pet_Sounds · 08-24-2019, 09:08 PM

Quote:

Originally Posted by Doug McClasky

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?

OccultHawk · 08-24-2019, 09:09 PM

Air pressure adds weight.

The Batlord · 08-24-2019, 09:18 PM

Quote:

Originally Posted by Pet_Sounds

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?

08-24-2019, 09:07 PM	#63927 (permalink)
OccultHawk one-balled nipple jockey Join Date: Dec 2010 Location: Dirty Souf Biatch Posts: 22,033	PS Are you going to tell him the newtons equal out in that case? __________________ 2016 2017 2018 2019 2020 Member of the Year & Journal of the Year Champion Behold the Writing of THE LEGEND: https://www.musicbanter.com/members-...p-lighter.html

08-24-2019, 09:09 PM	#63929 (permalink)
OccultHawk one-balled nipple jockey Join Date: Dec 2010 Location: Dirty Souf Biatch Posts: 22,033	Air pressure adds weight. __________________ 2016 2017 2018 2019 2020 Member of the Year & Journal of the Year Champion Behold the Writing of THE LEGEND: https://www.musicbanter.com/members-...p-lighter.html