Friday, January 14, 2011

A body is in equilibrium when a) acceleration is uniformb) speed is uniformc) acceleration and speed is uniformd) acceleration is non-uniform ...

Hello!


I see such an option (a), but it is not a correct one.


By the Newton's First law, a body is in equilibrium (when no forces acting on it or all forces are balanced) moves with a constant (uniform) velocity in any inertial frame of reference.


(For this problem we can use the word "speed" instead of "velocity", although usually "velocity" means vector quantity (including a direction), while "speed" isn't. Sometimes speed is uniform but velocity is not.)


It is temptingly to say "such a body is in rest", but movement must be considered with respect to a frame of reference. All inertial frames of reference move with uniform velocities with respect to each other, and physical processes are indistinguishable in them. Therefore we must say "moves with a constant velocity".


In other words, if a body has a non-zero acceleration, it means that its velocity is changing. The cause of a velocity change is an unbalanced force. Thus if a body has non-zero acceleration, it is not in equilibrium.


And the correct answer is b) speed (well, velocity) is uniform.

No comments:

Post a Comment

How does the choice of details set the tone of the sermon?

Edwards is remembered for his choice of details, particularly in this classic sermon. His goal was not to tell people about his beliefs; he ...