Summary
Highlights
Newton's third law states that for every action, there is an equal and opposite reaction. When two objects interact, they exert equal and opposite forces on each other. This is illustrated with an example of two kids on skateboards pushing each other and a person pushing a wall or chair, explaining how differences in mass affect the resulting motion.
Newton's third law applies to universal gravity. The Earth pulls on the Moon with the same force that the Moon pulls on the Earth. They don't collide due to the Moon's orbit. The force of gravity is calculated using a constant G, the masses of the two objects, and the distance between them squared.
Newton was unable to measure the gravitational constant (G), which was later determined by Henry Cavendish in 1798. This constant is a very small number (6.67 x 10^-11). Knowing G allows for the calculation of the mass of celestial objects, revealing that the Moon is 1/80th the mass of Earth, and the Sun is over 300,000 times the mass of Earth.
Surface gravity describes the strength of gravity on a planet's surface. On the Moon, surface gravity is one-sixth that of Earth, meaning a person would weigh one-sixth of their Earth weight. Surface gravity affects an object's shape, pulling large enough objects into spherical forms. It also determines whether an object can retain an atmosphere.
Earth's strong surface gravity allows it to hold onto an atmosphere, preventing particles from escaping. The Moon, with lower surface gravity, cannot retain an atmosphere because solar energy blows away any gas. However, objects like Titan, a moon of Saturn, can have an atmosphere despite a similar size to Earth's moon because its greater distance from the Sun reduces the energy imparted to gas particles.
Surface gravity depends on the object's mass divided by its radius squared. Earth's surface gravity is about 9.8 m/s². On the Moon, one weighs about one-sixth of their Earth weight, while on Jupiter, it's about three times more. This explains why large objects have strong surface gravity and small objects have weak surface gravity.
Escape velocity is the speed an object needs to attain to permanently escape a celestial body's gravitational pull. For Earth, this is approximately 11 kilometers per second (7.5 miles per second). Larger objects have higher escape velocities. Black holes have an escape velocity greater than the speed of light, making escape impossible.
To get into space or to other planets, objects must reach a certain velocity, which is why giant rocket ships are necessary. For instance, escaping Earth's gravity to reach Mars requires achieving escape velocity. The next lectures will cover modern astronomy, including light, physics, and telescopes.