Tension

No.

In the above situations, it is true that T does not change.
However, under some criteria, T DOSE CHANGE.

(Wish you do not mind me using English. =P)

When the system undergoes a change in ACCELERATION, at least, in the following example, T changes.

Suppose a fish of 1 kg is "connected" to a fishing line, and if the fisherman pulls it upwards with

(1) a constant velocity,
the tension of the fishing line,
F = ma
T - W = ma
T = (1)(0) + 1(10)
T = 10 N

(2) an acceleration of 1 m/s^2,
the tension becomes,
F = ma
T - W = ma
T = (1)(1) + (1)(10)
T = 11 N

(3) a deceleration of 2 m/s^2,
the tension is then,
F = ma
T - W = ma
T = (1)(-2) + (1)(10)
T = 8 N

If the above 3 cases happen successively, the tension of the fishing line indeed CHANGES from time to time.

Please note that this situation is very exceptional.


Hope it helps. =]]

2008-06-08 22:15:37 補充：
Actually, in your examples, T changes as well.

Say, pulling different masses.

(1) pulling a mass of 2 kg with acceleration of 2 m/s^2
T = ma
T = (2)(2)
T = 4 N

(2) pulling a mass of 10 kg with same acceleration
T = ma
T = 10(2)
T = 20 N

BEWARE, mate. =]]

2008-06-08 22:17:05 補充：
In the above cases, please neglect friction.