Which body is in equilibrium? (1) a satellite orbiting earth in a circular orbit (2) a ball falling freely toward the surface of earth (3) a car moving with a constant speed along a straight, level road (4) a projectile at the highest point in its trajectory

A car moving with a constant speed along a straight line road is the body in equilibrium.  

•When the forces are balanced, then the object is considered to be in equilibrium.  

•In equilibrium, the net force is zero and the acceleration of the body is also zero.  

•The acceleration of 0 m/s does not signify that the body is at rest. An object is considered to be in equilibrium when it is either at rest and staying or rest, or is in motion, however, having same speed and direction.  

•Therefore, when the car is moving with a similar speed along a straight line, it is considered to be having zero acceleration due to constant velocity and is moving in a straight line showing thus the car is in equilibrium.  

Thus, a car moving with a constant speed along a straight level road is in equilibrium.  

To know more about:

link

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.  

(3) a car moving with a constant speed along a  straight, level road.  YES. 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.

answer( sicar );