f4 phy

1.Q: What does the balance measure?Weight or normal force?

A: The balance measures the force that exerts on it. Th normal force R is the reaction force given by the balance. Because action and reaction forces are equal in magnitude, the reading on the balance gives the magnitude of the normal force R.

Q: But the normal force remains unchanged although the weight increases.
Why does the reading decrease?

A; No, the weight doesn't change. The weight is the pull of gravity on the man, which would not change due to motion/acceleraton. The reading on the balance changes because of a change of normal force R.

2. Both diagrams in fact show forces in action-reaction pairs.

Q:Why it doesn't apply to one object by itself?
A: An object could only acts as a "force provider" or a "force receiver". It cannot be both, as a force could only act on another object.



2012-01-28 23:38:02 補充：
Your suppl question:
When the lift moves downward, you are now falling slower relative to the lift than when the lift is at rest. Hence, the force exerted onto the balance decreases (just imagine if the lift moves downward at your falling speed, there would not be any force exerts on the balance).

2012-01-28 23:40:49 補充：
(cont'd)... Thus, the normal reaction force becomes reduced. On the other hand, if the lift moves upward, it has to push your body up. The force exerted on the balance increases, this causes the normal force to increase.