Lesson Objectives:- Newton's impact on astronomy
- Three laws of motion
How did Isaac Newton change our views on the universe? In a famous story, he observed an apple falling from a tree and realized that the same gravity pulling the apple down to the ground was also keeping the Moon orbiting the Earth. One of Aristotle's key ideas to support the idea of an Earth-centered universe had been that the heavens were separate from the Earth and that physical laws on Earth did not apply to the heavens.
Isaac Newton's discovery of gravity dismantled that. Since gravity applied to both the Earth and the heavenly bodies, they were clearly together in one universe. As a result of Newton's discoveries, people began to apply the same physical laws in effect on Earth to how objects operate in the heavens or cosmos.
In later years, this became its own field of study known as astrophysics. Astrophysics deals with the application of physical laws discovered on Earth to phenomena throughout the universe.
Isaac Newton defined three laws that apply to all motion, whether it be on Earth or the movement of a distant planet, star, or galaxy.
The first law states that an object moves at a constant velocity if there is no net force acting on it. This is why when you are traveling on a plane at cruising altitude, you can walk around and do other activities without feeling any different from being on the ground. While the plane is moving at a constant velocity, there is no net force acting on you. When the plane speeds up, slows down, or turns, however, this change in velocity results in a net force on your body. Note that this net force is experienced only as long as the plane is accelerating, or changing velocity.
Newton's second law offers a quantitative relationship. Force = mass times acceleration or (F = ma). This tells us what happens if there is a net force. A good example of this is how you can throw a baseball much farther than you can throw a shot in the shot put.
The shot has significantly greater mass than the baseball so it takes a much greater force to create the same acceleration.
The third law states that for any force, there is an equal and opposite force.
For example, when you are standing on the ground, your weight exerts a downward force. If this were the only force being applied to your body, Newton's first law would indicate that you should accelerate downward. The fact that you are not moving downward points to the fact that the ground is pushing up on you with the same force that your weight is pushing down. This is an example of Newton's third law of motion - for every force, there is an equal and opposite reaction force.
This also explains how a rocket works. A rocket engine generates a force that drives hot gas out the back, which creates an equal and opposite force that propels the rocket forward.