Gravity near Earth

Gravity as an attractive force

Gravity is an attractive force between objects with mass. Every object with mass attracts every other object with mass, but gravity is usually noticeable only when at least one object is very massive. Earth is massive, so it pulls nearby objects downward. This pull is what we call weight.

Mass and weight are related but not the same. Mass measures how much matter and inertia an object has. Weight is the gravitational force on that mass. Near Earth's surface, weight is given by:

$W = mg$

Here $W$ is weight, $m$ is mass, and $g$ is the gravitational field strength near Earth. The value of $g$ is about $9.8 m/s^2$. For many simple estimates, people use $10 m/s^2$.

Mass versus weight

Your mass would be the same on Earth and on the Moon, because you are made of the same amount of matter. But your weight would be less on the Moon because the Moon's gravity is weaker. This is why astronauts can hop more easily on the lunar surface.

Mass is measured in kilograms. Weight, like all forces, is measured in newtons. In everyday speech, people often use weight when they mean mass, but physics keeps them separate because the distinction matters.

Free fall

An object is in free fall when gravity is the only significant force acting on it. Near Earth, ignoring air resistance, all freely falling objects accelerate downward at about $9.8 m/s^2$. This means their downward velocity changes by about $9.8 m/s$ every second.

A common misconception is that heavier objects fall faster because they have more weight. Heavier objects do have more gravitational force on them, but they also have more mass, meaning more inertia. The two effects balance so that, without air resistance, objects fall with the same acceleration.

This is why a hammer and a feather fall together on the Moon, where there is almost no air resistance. On Earth, a feather falls slowly because air resistance is significant compared with its weight.

Air resistance

Air resistance is a force from air pushing against a moving object. It acts opposite the motion. A flat sheet of paper falls slowly because it pushes through a lot of air compared with its weight. If you crumple the paper into a ball, it falls faster because air resistance is reduced.

When an object falls through air, its speed may increase until air resistance grows large enough to balance weight. At that point, the net force is zero and the object falls at constant velocity. This speed is called terminal velocity.

Gravity and projectile motion

Gravity also affects objects thrown sideways. If you throw a ball horizontally, it moves forward while gravity accelerates it downward. The forward motion and downward motion happen at the same time. This is why projectiles follow curved paths.

The horizontal motion does not need a horizontal force to continue, ignoring air resistance. Newton's first law says the object keeps moving horizontally unless a horizontal net force changes that motion. Gravity changes only the vertical part of the motion.

The big idea

Gravity near Earth gives objects weight and causes free-fall acceleration. Mass is not weight. Weight depends on gravity; mass does not. Without air resistance, all objects near Earth's surface fall with the same acceleration. Air resistance explains why everyday falling objects often behave differently from the ideal model.