A satellite of mass m is in a circular orbit at radius r around Earth. What force keeps the satellite in orbit, and in which direction relative to the radius?

A satellite of mass m is in a circular orbit at radius r around Earth. What force keeps the satellite in orbit, and in which direction relative to the radius?

• A. Gravitational force outward
• B. Gravitational force inward toward the center
• C. Tension along the radius outward
• D. Normal force perpendicular to radius

Answer: (B)

Gravity from Earth provides the inward force that keeps the satellite moving in a circle. This gravitational pull points along the radius toward Earth's center, supplying the centripetal acceleration needed for circular motion. In a circular orbit the required inward force is m v^2 / r, and gravity provides F_g = G M_e m / r^2 in that inward direction, so the orbit remains stable. The outward options would imply a force pulling away, which isn’t present in free space, and a tension or normal force would require a tether or contact with a surface, neither of which applies to a satellite in orbit.

Gravity from Earth provides the inward force that keeps the satellite moving in a circle. This gravitational pull points along the radius toward Earth's center, supplying the centripetal acceleration needed for circular motion. In a circular orbit the required inward force is m v^2 / r, and gravity provides F_g = G M_e m / r^2 in that inward direction, so the orbit remains stable. The outward options would imply a force pulling away, which isn’t present in free space, and a tension or normal force would require a tether or contact with a surface, neither of which applies to a satellite in orbit.