Lesson 12.1: Gravity is a force exerted by masses

Lesson Focus

Gravity is the universal force of attraction between all objects with mass. We will explore how this fundamental force governs everything from why objects fall to the ground to how planets stay in their orbits.

Learning Objectives

• Learn how the mass of objects and the distance between them influence the strength of gravitational force.
• Discover what keeps objects in orbit, constantly falling around Earth instead of crashing into it.

Objects fall because gravity, a universal force, pulls any two masses together. This attraction formed our solar system. The mechanism is that all matter attracts other matter. Though the force exists between you and a pencil, Earth's massive pull overwhelms it. Gravity is a force that acts across any distance.

The strength of gravity changes based on two factors. The phenomenon is that its force increases with more mass but decreases with greater distance. Doubling the mass doubles the force, while doubling the distance reduces the force to one-fourth. This is why you feel Earth's gravity but not Jupiter's.

An object's mass (matter) is constant, but its weight (force of gravity) can change. The cause is the local gravitational field. We measure this force using the formula F = mg, where 'g' is acceleration due to gravity. On the Moon, your mass would be the same, but your weight less.

Objects stay in orbit because gravity acts as a centripetal force, constantly pulling them toward a central body. The Moon orbits Earth because Earth's gravity continuously bends its path. This prevents the Moon from flying off in a straight line. An orbit is essentially falling but continuously missing the ground.

Astronauts appear weightless because they are in a constant state of free fall. The spacecraft and everyone inside are falling around Earth at the same rate. This creates a microgravity environment where objects float, even though Earth's gravity is still strong. Why are they not truly weightless in orbit?