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Gravity: Why Things Fall
Weight, Free Fall, and Universal Gravitation
Related Labs
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 9.8 m/s^2, if air resistance is small. The gravitational force on an object is its weight, given by F = mg, 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.
Key Facts
- Gravity is an attractive force between any two masses.
- Weight is the gravitational force on an object: F = mg.
- Near Earth, gravitational acceleration is g = 9.8 m/s^2 downward.
- Universal gravitation is given by F = Gm1m2/r^2.
- 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 g as a force instead of an acceleration, which is wrong because g has units of m/s^2 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 3.0 kg backpack is dropped near Earth's surface. What is its weight in newtons? Use g = 9.8 m/s^2.
- 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.