Isaac Newton first described gravity mathematically! According to legend, Newton saw an apple fall from a tree and wondered why the moon does not fall to Earth. He realized that the same force that pulls the apple down also pulls the moon toward Earth, but the moon's forward motion keeps it in orbit. Newton's law of universal gravitation states that every particle attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. This law explained both falling apples and planetary motion. Newton is considered one of the greatest scientists in history. The story of the apple is likely true, but the details (apple hitting his head) were probably embellished by his biographer.

Your weight would decrease! Weight = mass × gravity (W = mg). The Moon's gravity is about 1.62 m/s² (compared to Earth's 9.8 m/s²). So your weight on the Moon would be about 1/6 of your Earth weight. Your mass (the amount of matter in your body) does not change. That is why astronauts can jump higher on the Moon. On Jupiter, your weight would be much higher because Jupiter's gravity is stronger. The International Space Station (ISS) orbits Earth; astronauts are in free fall, so they feel weightless (not because gravity is absent, but because they are falling at the same rate as their spacecraft). Weightlessness does not mean no gravity – gravity is still 90% as strong at the ISS altitude.

They would hit at the same time! This was a revolutionary discovery. Galileo's experiments (and thought experiments) contradicted Aristotle, who had been accepted for nearly 2,000 years. The moon landing astronauts demonstrated this by dropping a feather and a hammer on the Moon (no air) – they hit the lunar surface at the same time. Air resistance can affect falling objects: a feather falls slower because air pushes up against it. In a vacuum (no air), all objects fall together. The acceleration due to gravity on Earth is about 9.8 meters per second squared (32 ft/s²). This means that every second, the speed of a falling object increases by about 9.8 m/s (if air resistance is ignored).

Inertia keeps an object moving! Inertia is the tendency of an object to resist changes in its motion. If there is no net force, the object will continue doing what it is doing (either staying still or moving at constant velocity). On Earth, friction and air resistance are forces that eventually stop moving objects. In space (near vacuum), there is almost no friction, so objects can travel huge distances without slowing down. The Voyager spacecraft (launched in 1977) are still coasting through interstellar space. The first law is often called the law of inertia. The word "inertia" comes from Latin for "laziness" or "inactivity." Seat belts protect you because of inertia – when a car stops suddenly, your body wants to keep moving forward. Newton's first law was a major breakthrough because it contradicted Aristotle (who thought objects needed a constant force to keep moving).

F = ma is the equation! Force (F) is measured in newtons (N). Mass (m) is measured in kilograms (kg). Acceleration (a) is measured in meters per second squared (m/s²). One newton is the force required to accelerate a 1 kg mass at 1 m/s². The second law explains why heavier objects are harder to push: they have more mass, so they require more force to accelerate. If you push a shopping cart (low mass), it accelerates quickly; if you push a loaded cart (high mass), it accelerates slowly. The second law also explains why you feel pressure during takeoff in an airplane – the plane is accelerating, and your body (mass) feels the force. The second law is one of the most important equations in physics. It allows us to calculate the motion of everything from a baseball to a rocket ship.

The rocket pushes gas down, and the gas pushes the rocket up! This is Newton's third law. The action and reaction forces are equal in size but opposite in direction. They act on different objects: the action acts on the gas, the reaction acts on the rocket. The same principle explains how you swim – you push water backward (action), and the water pushes you forward (reaction). When you walk, your foot pushes the ground backward, and the ground pushes you forward. Birds fly by pushing air down and backward. The third law applies to all forces, not just contact forces. Even gravity follows the third law: Earth pulls on you (action), and you pull on Earth with an equal force (reaction). You don't notice Earth moving because it has much more mass (F=ma, so a = F/m, so Earth's acceleration is tiny).

Static friction prevents the car from rolling! Static friction acts when objects are not moving relative to each other. It must be overcome to start moving. Kinetic (or sliding) friction acts when objects are sliding past each other. Static friction is usually greater than kinetic friction – that is why it is harder to start pushing a heavy box than to keep it moving. Friction can be reduced by using lubricants (oil, grease), ball bearings, or wheels. Friction can be increased by using rougher surfaces (sand on icy roads) or by pressing surfaces together (brake pads on a bicycle wheel). Friction is also responsible for heat: when you rub your hands together, friction converts kinetic energy into thermal energy (heat). Without friction, we could not walk, drive, or hold objects. But too much friction can cause wear and inefficiency.

Gravity and tangential velocity keep the Moon in orbit! The Moon is constantly falling toward Earth, but its sideways speed (about 2,288 mph / 3,683 km/h) means it keeps missing. Isaac Newton explained this with his "thought experiment" – if you fire a cannonball from a high mountain with enough speed, it will go into orbit around Earth. The International Space Station (ISS) orbits Earth at about 17,500 mph (28,000 km/h). At that speed, it falls around Earth rather than toward it. Astronauts feel weightless not because gravity is absent (gravity is about 90% as strong at the ISS altitude as on Earth's surface) but because they are in free fall – the ISS and everything inside is falling together. The same principle applies to all orbiting objects: planets around the Sun, satellites around Earth, and stars around the center of galaxies.

The acceleration due to gravity (g) is about 9.8 m/s²! On the Moon, g is about 1.6 m/s² (about 1/6 of Earth). On Jupiter, g is about 24.8 m/s² (about 2.5 times Earth). The value varies slightly on Earth: 9.78 m/s² at the equator (due to Earth's rotation and bulge) and 9.83 m/s² at the poles. At the top of Mount Everest (8,848 m), g is slightly lower (about 9.76 m/s²). The symbol "g" is used for acceleration due to gravity. The term "g-force" refers to acceleration relative to Earth's gravity: a fighter pilot experiencing 5 g feels five times their weight. Astronauts experience high g-forces during launch (up to 3-4 g) and re-entry (up to 3 g). The formula for distance fallen (starting from rest) is d = ½gt². After 1 second, an object falls about 4.9 m (16 ft); after 2 seconds, about 19.6 m (64 ft).

General relativity says gravity is curved spacetime! Imagine a heavy ball on a rubber sheet – it creates a dip. If you roll a marble near the ball, it will follow the curve. In the same way, the Sun curves spacetime, and planets follow the curves (orbits). This theory has been confirmed by many experiments: the bending of starlight by the Sun (Eddington, 1919), gravitational redshift, the precession of Mercury's orbit, and the detection of gravitational waves (LIGO, 2015). GPS satellites must account for both special and general relativity to give accurate positions – if they didn't, they would drift by several miles per day. Einstein's theory is one of the pillars of modern physics. Newton's theory of gravity works well for most everyday situations (it got us to the Moon), but Einstein's theory is more accurate, especially for very strong gravity or very fast motion.