## 26.3 Thought Experiment: Newton's Cannonball This section explores Isaac Newton's thought experiment, which explains how an object can orbit the Earth. It connects the force of gravity that makes objects fall to the force that keeps the Moon in its orbit. ### Newton's Insight on Gravity Newton did not discover gravity, but he was the first to provide a mathematical law to explain it. He proposed that the same force of gravity that pulls an apple to the ground also keeps the Moon in its orbit around the Earth. The reason the Moon does not fall to Earth is its significant sideways motion (velocity). ### The Central Questions Newton's research was driven by two main questions: 1. Why does the Moon not fall from the sky and land on Earth, given that all objects experience gravitational acceleration towards Earth? 2. Why does the Moon move in a circle instead of a straight line, as described by Galileo's concept of inertia? Newton concluded that the Moon's orbit is a balance between its **inertia** (the tendency to move in a straight line) and the constant **gravitational acceleration** pulling it toward Earth. ### The Cannonball Thought Experiment To illustrate this concept, Newton imagined a cannon placed on a very high mountain, firing a cannonball horizontally. * The cannonball's motion is a combination of horizontal and vertical movement. * **Horizontal motion:** The distance covered is given by $X = vt$, where $v$ is the initial horizontal speed and $t$ is time. * **Vertical motion:** The distance it falls due to gravity is given by $Y = \frac{1}{2}gt^2$. Newton reasoned that if the cannonball is fired at a specific "ideal velocity," the curve of its trajectory will perfectly match the curve of the Earth. It will continuously fall toward the Earth but never get any closer, thus achieving a stable orbit. ### Centripetal Acceleration An object in a circular orbit, like the cannonball, is constantly accelerating even if its speed is constant. This is because its direction of motion (and therefore its velocity) is always changing. This is called **centripetal acceleration**. * **Formula:** The acceleration required to keep an object in a circular path is given by: $$a = \frac{v^2}{r}$$ where $v$ is the object's speed and $r$ is the radius of the orbit. ### Outcomes Based on Velocity The fate of the cannonball depends on its initial horizontal velocity: 1. **Velocity less than orbital velocity:** The cannonball's path curves more sharply than the Earth, and it falls back to the surface. 2. **Velocity equal to orbital velocity:** The cannonball's path curves at the same rate as the Earth, and it enters a stable, circular orbit. 3. **Velocity greater than orbital velocity:** The cannonball's path curves less sharply than the Earth, and it moves into an elliptical orbit or escapes Earth's gravity altogether. | Key Formula | Equation | Description | |-------------|----------|-------------| | **Centripetal Acceleration** | $a = \frac{v^2}{r}$ | Acceleration needed to maintain circular motion. |

26.3 Thought Experiment Newton's Cannonball | Nature Of Science – A Debate | Physics 12