Which equation expresses the gravitational force on an object near Earth's surface?

Which equation expresses the gravitational force on an object near Earth's surface?

• A. Fg = mg
• B. Fg = ma
• C. Fg = G m1 m2 / r^2
• D. Fe = qE

Answer: (A)

Gravitational force near Earth’s surface is the weight of the object, which is the mass times the acceleration due to gravity. Because gravity pulls downward with a nearly constant acceleration g at the surface, the force on an object of mass m is Fg = m g. This is just Newton’s second law F = ma applied to gravity, with a set equal to g. The other expressions describe gravity in different ways or describe other forces. The general law F = G M m / r^2 is the universal gravitation formula that applies for any two masses at distance r, not the simplified near-surface weight. The statement Fg = ma is true in general but doesn’t specify gravity unless you plug in a = g; and Fe = qE is the electric force, not gravitational.

Gravitational force near Earth’s surface is the weight of the object, which is the mass times the acceleration due to gravity. Because gravity pulls downward with a nearly constant acceleration g at the surface, the force on an object of mass m is Fg = m g. This is just Newton’s second law F = ma applied to gravity, with a set equal to g.

The other expressions describe gravity in different ways or describe other forces. The general law F = G M m / r^2 is the universal gravitation formula that applies for any two masses at distance r, not the simplified near-surface weight. The statement Fg = ma is true in general but doesn’t specify gravity unless you plug in a = g; and Fe = qE is the electric force, not gravitational.