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?