Which of the following is in equilibrium?

a. an object that was thrown straight upward at the moment it has zero velocity (my answer)
b. an object moving in a circle at constant speed
c. an object that moves at constant velocity
d. an object that moves at constant acceleration

If I had to pick one it would be c.

Constant velocity means no acceleration which means no net force.

To determine which of the options is in equilibrium, we need to understand what equilibrium means in the context of physics. In physics, equilibrium refers to a state in which there is no net force or net torque acting on an object.

Let's break down each option and analyze if it satisfies the condition for equilibrium:

a. an object that was thrown straight upward at the moment it has zero velocity:
In this case, the object is momentarily at rest (zero velocity) at the highest point of its trajectory. Since the net force acting on the object is zero at this moment, it is in equilibrium. So, your answer is correct.

b. an object moving in a circle at constant speed:
An object moving in a circle at a constant speed is not in equilibrium because it is constantly changing direction. To maintain circular motion, there must be a centripetal force acting towards the center of the circle. This force is required to continuously change the object's direction towards the center, which means there is a net force acting on the object, preventing it from being in equilibrium.

c. an object that moves at constant velocity:
An object that moves at constant velocity has no acceleration. In order for the object to have no acceleration, the net force acting on it must be zero, which satisfies the condition for equilibrium. So, an object moving at constant velocity is in equilibrium.

d. an object that moves at constant acceleration:
An object that moves at a constant acceleration does not satisfy the condition for equilibrium. If there is a constant acceleration, there must be a net force acting on the object in the direction of the acceleration. Therefore, an object moving at constant acceleration is not in equilibrium.

In conclusion, the objects that are in equilibrium are:
a. an object that was thrown straight upward at the moment it has zero velocity, and
c. an object that moves at constant velocity.