A car's trip has five moments: cruising straight at a steady 60 km/h, rounding a corner at that same steady 60 km/h, cruising straight again, slowing for a red light, and waiting at rest.
In which of the five moments is the car in equilibrium?
Equilibrium means the net force is zero. Newton's first law turns that into something you can see: with zero net force, the velocity is not changing.
So ask one question of any motion: is the velocity changing?
Velocity is speed with a direction. It changes if the speed changes, and it changes if the direction changes.
Moment one, cruising straight at a steady 60 km/h: the speed is steady and the direction is steady. The velocity is not changing, so the car is in equilibrium.
Moment two, rounding the corner at a steady 60 km/h: the speed holds, but the direction is turning. The velocity is changing, so the car is not in equilibrium.
The corner is the case that fools people. A steady speed is not a steady velocity, because direction counts.
Moment three, back on the straight, still at a steady 60 km/h: the turning has ended. The velocity is not changing, so the car is in equilibrium again.
Demi rides an escalator that carries her in a straight line at a steady speed. Is Demi in equilibrium?
Moment four, slowing for the red light: the speed is falling. The velocity is changing, so the car is not in equilibrium.
A pickup truck slows as it nears a stop sign. Is it in equilibrium?
Moment five, waiting at rest: the velocity is zero and staying zero. The velocity is not changing, so the car is in equilibrium.
A carousel horse moves at a steady speed around its circle. Is it in equilibrium?
A baggage belt carries a suitcase in a straight line at a steady speed. Is the suitcase in equilibrium?
Speed: steady. Direction: steady.
So the velocity is not changing.
An unchanging velocity means zero net force.
Yes: the suitcase is in equilibrium.
A runner rounds the curve of a track at a steady pace. Is she in equilibrium?
Her speed is steady, but her direction is turning.
Direction is part of velocity, so her velocity is changing.
A changing velocity means a nonzero net force.
No: on the curve she is not in equilibrium.
Which object is in equilibrium?
A cyclist rounds a corner at a steady 15 km/h. She is not in equilibrium. Which statement gives the reason?
A barbell lies at rest on the gym floor. Which statement is correct?