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...

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.