# Carlos Rodrigo

## A Brief History of Time

Written by Stephen Hawking.

1. Theories based on what you’ve seen in the past can help predict the future.

2. A theory, in its most basic terms, is a model that accurately explains large groups of observations.

3. Theories have two great benefits: First, they allow scientists to make definite predictions about future events. Second, theories are always disprovable, meaning they’re open to reform if new evidence that doesn’t fit the theory is found.

4. In the 1600s, Isaac Newton revolutionized the way we think about how objects move. To describe how all objects in the universe move, Newton developed three laws:

5. The first of Newton’s laws states that all objects will continue moving in a straight line if not acted on by another force. This was demonstrated in an experiment by Galileo in which he rolled balls down a slope. As gravity was the only force acting on the balls, they rolled in a straight line.

6. Newton’s second law states that an object will speed up at a rate proportional to the force acting on it. For example, a car with a more powerful engine will accelerate faster than one with a less powerful engine. This law also states that the greater the body’s mass, the less a force affects its motion. For example, if there are two cars with the same engine, the heavier car will take longer to accelerate.

7. Newton’s third law describes gravity. It states that all bodies in the universe attract other bodies with a force proportional to the mass of each object. This means that if you double the mass of one object, the force will be twice as great. If you double one object’s mass and triple the other, the force will be six times as great.