Which body is in equilibrium?
(1) a satellite orbiting Earth in a circular orbit
. No. The forces on it are unbalanced. There's only one force acting on it ... the force of gravity, pulling it toward the center of the Earth. That's a centripetal force, and the satellite is experiencing centripetal acceleration.
(2) a ball falling freely toward the surface of Earth. No. The forces on it are unbalanced. There's only one force acting on it ... the force of gravity, pulling it toward the center of the Earth. The ball is accelerating toward the ground.
<em>
(3) a car moving with a constant speed along a straight, level road. YES.</em> We don't even need to analyze the forces, just look at the car. It's moving in a straight line, and its speed is not changing. The car's acceleration is zero ! That right there tells us that the NET force ... the sum of all forces acting on the car ... is zero. THAT's called 'equilibrium'.
(4) a projectile at the highest point in its trajectory. No. The forces on it are unbalanced. There's only one force acting on it ... the force of gravity, pulling it toward the center of the Earth. The projectile is accelerating toward the ground.
