- #1
sawer
- 65
- 2
According to work formula
$$
W = \mathbf F\cdot\Delta\mathbf x
$$
and
$$
W = F\Delta x\cos\theta
$$
If an object falls:
F = Gravitational Force = negative
Delta x = Final Position - Initial Position = negative ==> (Like 2 - 5 = -3 ; because it is falling)
cos 0 = 1 ==>(because x and F have same direction)
And work is = negative.negative.positive = POSITIVE
The work is done by gravitational force for falling body is positive. Right?
But if an object rises
F = Gravitational Force = negative
Delta x = Final Position - Initial Position = positive==> (Like 5 - 2 = 3 ; because it is rising)
cos 180 = -1 ==>(because x points upward and F points downward direction)
And work is = negative.positive.negative = POSITIVE
The work is done by gravitational force for lifting body is still positive.
But it must be negative. Right?
So my question is: What is wrong?
$$
W = \mathbf F\cdot\Delta\mathbf x
$$
and
$$
W = F\Delta x\cos\theta
$$
If an object falls:
F = Gravitational Force = negative
Delta x = Final Position - Initial Position = negative ==> (Like 2 - 5 = -3 ; because it is falling)
cos 0 = 1 ==>(because x and F have same direction)
And work is = negative.negative.positive = POSITIVE
The work is done by gravitational force for falling body is positive. Right?
But if an object rises
F = Gravitational Force = negative
Delta x = Final Position - Initial Position = positive==> (Like 5 - 2 = 3 ; because it is rising)
cos 180 = -1 ==>(because x points upward and F points downward direction)
And work is = negative.positive.negative = POSITIVE
The work is done by gravitational force for lifting body is still positive.
But it must be negative. Right?
So my question is: What is wrong?