Unicycling

Originally posted by sonhouse
The only thing holding back the box from being accelerated is the ground in the way. If it were a hundred meters in midair it would be accelerating so it has at least potential kinetic energy.

I think we are wondering aimlessly here. Let's, take a step back and explain to me what you aren't understanding about a box moving vertically at constant speed has zero NET force acting on it?

Originally posted by joe shmo
I think we are wondering aimlessly here. Let's, take a step back and explain to me what you aren't understanding about a box moving vertically at constant speed has zero NET force acting on it?

Say the box masses 1 Kg and is moving upwards against gravity. So there is a 1 Kg force acting on it all the time it is on the ground and then it needs a bit more to get off the ground, just like a helicopter. But in space, that same force will accelerate it a 1 G plus whatever thrust it needed to make it go constant velocity up and so in space it might accelerate at 10 M/s^2 or so, just a bit above 1 G.

Originally posted by sonhouse
The only thing holding back the box from being accelerated is the ground in the way. If it were a hundred meters in midair it would be accelerating so it has at least potential kinetic energy.

Ok., I get what you are saying now. In a Newtonian picture there is the downwards force from gravity and an upward force from the ground on your 2.2 lb piece of matter. In an Einsteinian picture there is no downward force, the ground represents the reference point for an accelerated reference frame and the only force is the upward force from the ground. An inertial observer would see the ground accelerating towards them. It's just that space-time is not flat so it appears that there is a downward force.

Originally posted by DeepThought
Ok., I get what you are saying now. In a Newtonian picture there is the downwards force from gravity and an upward force from the ground on your 2.2 lb piece of matter. In an Einsteinian picture there is no downward force, the ground represents the reference point for an accelerated reference frame and the only force is the upward force from the ground ...[text shortened]... wards them. It's just that space-time is not flat so it appears that there is a downward force.

No offense sonhouse, but I'm not quite as confident in your level of understanding in this concept as Deep Thought.

This is your original statement: "If you are lifting something straight up, you are accelerating it, against gravitational acceleration so there has to be a force action on the body to continue dragging it uphill against gravity."

"If you are lifting something straight up, you are accelerating it,..." This part of the statement is false. If you are lifting something up you can be accelerating it, but you can also be lifiting something up with a constant speed (once its in motion). In that case you are clearly not accelerating the body, by definition!

Question: I can drive 70 miles per hour for 10 miles. What was my rate of acceleration over those 10 miles? You better believe the answer is 0, my velocity did not change, it was 70 miles per hour. If I was moving 5 miles per hour upward in an elevator, for one minute, what was my acceleration for that one minute...you better have figured zero...again. Acceleration is a concept independent of "force". It is just the rate of how a body's speed changes with time, or the rate of the rate of a bodys change in position with time. It has no necessary connection to "force", it is a quantity all its own. If a body's speed is not changing (no matter which direction it is going, so long as it doesn't change direction), its acceleration is zero.

"there has to be a force action on the body to continue dragging it uphill against gravity."

This part of the statement is true. Like I said, there is a "force" necessary to keep the body in motion vertically upwards. However, in the case of a body that is moving upwards without changing its speed that force is exactly equal to and opposite the force of the bodys weight. What is the result when you add quantities that are equal and opposite? You better believe you get zero!

F_pull - F_weight = m*a; Newtons Second Law

if F_pull = F_weight then the equation becomes

0 = m*a This is true for a massless body ( m= 0, a>=0) that is accelerating, or a "real" body with mass ( m<>0, a > 0 ) that is not accelerating.

Now, if the body is accelerating in the upwards direction its speed is changing, and the force that is pulling/lifting it up must be greater than the force of the body's wieght. This inbalance of forces causes the body to accelerate (change its speed). It is the difference between the upward force and the downward force that sets the magnitude and direction of this quantity. In general, you add up ALL of the forces acting on the body ( taking their direction into account) and equate it to the quantity mass*acceleration. That is Newtons Second Law.

In the case of [i]accelerating[i/] upwards (body's speed is changing):

F_pull - F_weight = m*a

F_pull - F_weigth > 0

m*a > 0

Any body with mass will obtain some amount of acceleration in this case(the body's speed will change).

Do you have any specific questions about this explanation?

Originally posted by joe shmo
No offense sonhouse, but I'm not quite as confident in your level of understanding in this concept as Deep Thought.

This is your original statement: "If you are lifting something straight up, you are accelerating it, against gravitational acceleration so there has to be a force action on the body to continue dragging it uphill against gravity."

"If yo ...[text shortened]... ase(the body's speed will change).

Do you have any specific questions about this explanation?

Ok, I get the concept of zero net force. But there is a differance between mass and weight.

If the mass is in a gravity well on top of a platform it has weight which could be thought of as potential energy since it will accelerate if shoved off the platform and it will accelerate on Earth at 9.8 m/s^2 or 32 f/s^2 take your pick.

There would be no acceleration if the mass is between two gravity wells or if away from any gravity well, like a million light years away from a galaxy. Even there, there would be some small acceleration because space is still 'bent' there but you would be hard pressed to measure it🙂

Still, as per your post, if say a 1 Kg mass is pulled up against gravity at a constant velocity, it will not be changing velocity but it is still in a gravity well so the force on it to keep it moving at say a constant 1 m/s is also constant.

Away from a gravity well, the force required to get it from 0 to 1 M/sec only has to be applied once, when it gets to 1 m/s no more force is required and if the same force is used that was used to obtain 1 m/s of upwards movement, continuously applied, the same object outside a gravity well would be accelerating at 9.8 M/s^2 plus a tiny bit so that same force now results in a large change in velocity as long as that force is applied.

I hope we are now on the same page.

Originally posted by sonhouse
Ok, I get the concept of zero net force. But there is a differance between mass and weight.

If the mass is in a gravity well on top of a platform it has weight which could be thought of as potential energy since it will accelerate if shoved off the platform and it will accelerate on Earth at 9.8 m/s^2 or 32 f/s^2 take your pick.

There would be no ac ...[text shortened]... arge change in velocity as long as that force is applied.

I hope we are now on the same page.

It seems like we now are.