Gravity is the force that pulls objects with mass toward one another, and near Earth it makes things fall toward the ground. It shapes everyday events like dropping a book, jumping, and rain falling from clouds. Gravity also controls the motion of planets, moons, and satellites, so it is one of the most important forces in physics.
Understanding gravity helps explain both simple motion on Earth and large scale motion in space.
Near Earth's surface, gravity gives most objects the same downward acceleration, about , if air resistance is small. The gravitational force on an object is its weight, given by , so heavier objects feel a larger force. However, a larger mass also means more inertia, which is why many objects fall with the same acceleration in a vacuum.
Air resistance can change what we observe, making light or spread out objects like feathers fall more slowly in air.
Understanding Gravity: Why Things Fall
Weight is not quite the same thing as the feeling of being supported. A bathroom scale does not directly measure gravity. It measures the upward push from the floor on your feet.
This push is called a normal force. When you stand still, the floor pushes upward with the same strength that gravity pulls downward, so the scale gives your usual reading. In a falling lift, both you and the scale accelerate downward together.
Your feet press much less on the scale, so the reading drops. During true free fall, there is no supporting push at all. That is why astronauts in orbit feel weightless even though Earth’s gravity is still acting strongly on them.
The equal fall rate comes from two linked effects. Gravity pulls harder on an object with more mass. Yet a larger mass is harder to accelerate by exactly the same proportion.
These effects cancel when air has little influence. This is a useful place to separate force from acceleration. A truck experiences a larger gravitational pull than a tennis ball, but neither has a built in advantage in free fall.
The famous hammer and feather demonstration on the Moon worked because the Moon has almost no atmosphere. On Earth, a feather meets a lot of air for its small weight, while a compact hammer does not.
Air resistance acts opposite to motion through air. Its size depends on speed, shape, surface area, and air density. As a dropped object speeds up, drag usually increases.
Eventually the upward drag can equal the downward gravitational pull. The object then continues at a steady speed called terminal velocity. A skydiver uses a parachute to create far more drag, which greatly lowers terminal velocity.
This explains why a flat sheet of paper falls slowly, while the same paper crumpled into a ball reaches the ground sooner. It is not because gravity changed. The air changed the net force.
Gravity becomes weaker with distance from the center of Earth. The distance matters greatly because spreading the separation to twice its value makes the gravitational force one quarter as strong. This pattern helps explain why high satellites need careful planning.
An orbit is not a place with no gravity. It is continuous free fall around a curved planet. A satellite has enough sideways speed that, while it falls, Earth’s surface curves away beneath it.
Learning gravity well means drawing force arrows, stating which forces act, and separating mass, weight, speed, and acceleration. These quantities are often confused, especially in questions about lifts, parachutes, planets, and astronauts.
Key Facts
- Gravity is an attractive force between any two masses.
- Weight is the gravitational force on an object: .
- Near Earth, gravitational acceleration is downward.
- Universal gravitation is given by .
- In the absence of air resistance, all objects fall with the same acceleration.
- Mass measures amount of matter, but weight depends on local gravity.
Vocabulary
- gravity
- Gravity is the attractive force that pulls masses toward each other.
- mass
- Mass is the amount of matter in an object and also measures its resistance to changes in motion.
- weight
- Weight is the force of gravity acting on an object.
- acceleration
- Acceleration is the rate at which velocity changes with time.
- air resistance
- Air resistance is a force from the air that opposes the motion of an object moving through it.
Common Mistakes to Avoid
- Thinking heavier objects always fall faster, which is wrong because in a vacuum all objects near Earth accelerate at the same rate regardless of mass.
- Confusing mass with weight, which is wrong because mass stays the same while weight changes if the gravitational field changes.
- Using as a force instead of an acceleration, which is wrong because has units of and must be multiplied by mass to find weight.
- Ignoring air resistance in real situations like feathers or paper, which is wrong because drag can strongly reduce the falling speed and change the motion you observe.
Practice Questions
- 1 A kg backpack is dropped near Earth's surface. What is its weight in newtons? Use .
- 2 Two objects, one with mass 2.0 kg and one with mass 8.0 kg, are dropped in a vacuum. What is the acceleration of each object as they fall near Earth?
- 3 A hammer and a feather are dropped at the same time. In air they land at different times, but on the Moon they can land together. Explain why this happens.