Gravity is the pull that brings things down to the ground. It is why a ball drops, rain falls, and your feet stay on the floor. Kids notice gravity every day when they jump, toss toys, or slide at the playground.
Learning about gravity helps explain how objects move around us.
Gravity pulls on everything, from feathers to books to people. When you let go of something, gravity starts pulling it downward right away. Some things seem to fall differently because of air pushing on them, not because gravity forgot them.
Gravity also helps keep the Moon near Earth and the planets moving around the Sun.
Understanding Gravity
Earth has mass, and mass creates a gravitational field in the space around it. A field is a way to describe an effect that can act without touching. Near the surface, this field points toward Earth's center.
That is why a dropped object moves down even if it begins high on a shelf or low near the floor. The direction changes slightly at different places on Earth because down always means toward the center. Gravity becomes weaker as distance from Earth increases, though the change is too small to notice during ordinary classroom activities.
Mass and weight are related but they are not the same thing. Mass tells how much matter an object contains. It stays the same whether the object is on Earth, on the Moon, or floating in space.
Weight is the pull of gravity on that mass, so it changes when the strength of gravity changes. A bathroom scale measures your weight by sensing the support force from the floor.
On the Moon, a person would have the same mass but weigh much less because lunar gravity is weaker. This difference matters in science because using the two words as if they mean the same thing causes mistakes.
When an object is released, gravity gives it a downward acceleration. Acceleration means that its speed changes each second. Near Earth's surface, the speed of a falling object increases by about ten metres per second every second when air effects are small.
A heavier object has more gravitational pull on it, but it also has more resistance to changes in motion. These effects balance so objects fall with the same acceleration in a vacuum.
This was tested on the Moon when a hammer and a feather were dropped together. They reached the ground at the same time because there was almost no air.
Air changes what people see in everyday falls. As an object moves through air, the air pushes against its motion. A flat sheet of paper catches much more air than the same paper scrunched into a ball.
The flat sheet falls slowly because the upward air force grows as it speeds up. Eventually, that air force can equal the downward gravitational force. The object then falls at a steady speed called terminal speed.
Parachutes work by creating a large air force. When learning about falling objects, pay attention to the object's shape, its surface area, and whether the air is important. A fair test can compare two objects dropped from the same height at the same time.
Key Facts
- Gravity is a force that pulls objects toward Earth.
- Near Earth, all objects are pulled downward by gravity.
- Weight is the force of gravity on an object.
- shows how force changes motion.
- gives weight near Earth's surface.
- Air resistance can make light objects fall more slowly.
Vocabulary
- gravity
- Gravity is the pull that brings objects toward Earth.
- force
- A force is a push or pull that can change motion.
- fall
- To fall means to move downward because gravity is pulling.
- weight
- Weight is how strongly gravity pulls on an object.
- air resistance
- Air resistance is the air pushing against a moving object.
Common Mistakes to Avoid
- Thinking gravity only works when something is falling, but gravity is always pulling even when an object is resting on the ground or a table.
- Believing heavier things always fall faster, which is wrong because without much air resistance objects fall at nearly the same rate.
- Confusing mass and weight, because mass is the amount of matter while weight depends on gravity pulling on that mass.
- Ignoring air resistance, which leads to wrong predictions for feathers, paper, and other light objects that are strongly affected by air.
Practice Questions
- 1 A child drops a ball from a small height and a book from the same height at the same time. If air effects are small, which one hits the ground first?
- 2 On Earth, an object has mass . Using , what is its weight?
- 3 A feather and a coin are dropped in a place with no air. Explain why they fall the way they do.