1. Aristotle, Galileo and Newton and Newton’s Laws of Motion
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2. ARISTOTLE 384 BC – 322 BC Ancient Greece
One of the first to try to explain the natural world
Geocentric view of the universe
Ideas based on observations that seemed to be true

3. GALILEO 1564 – 1642 Italy Perhaps the first true scientist.
Rolled and dropped objects to discover the true aspects of motion

4. NEWTON
England
Developed laws for motion and gravity that explain why objects move, and worked with optics

5. 3.1 Aristotle on Motion Objects do not move without a force.
Objects in motion always require a force to keep them moving.
Objects seek their natural state, which is at rest.
Mechanical equilibrium can only be static.

6. 3.1 Galileo and Newton on Motion
Objects do not change motion without unbalanced force.
Objects in motion do not always require a force to keep them moving.
Objects have two “natural” states of motion, at rest (static equilibrium) and moving at a constant speed and direction (dynamic equilibrium).
Simply put, things tend to keep on doing what they’re already doing.

7. 3.4 Newton’s Law of Inertia
Is a force required to keep an object moving?
Newton’s first law, usually called the law of inertia, is a restatement of Galileo’s idea that a force is not needed to keep an object moving.
Galileo argued that only when friction is present is a force needed to keep an object moving.
Galileo stated that if friction were entirely absent, a ball moving horizontally would move forever at the same speed and in the same direction (at a constant velocity).

8. 3.4 Newton’s Law of Inertia
The law of inertia provides a completely different way of viewing motion from the ancients.
Objects continue to move by themselves.
Forces are needed to overcome any friction that may be present and to set objects in motion initially.
Once the object is moving in a force-free environment, it will move in a straight line indefinitely.

9. The First Law of Motion
Objects at rest stay at rest and objects in motion stay in motion with the same speed and in the same direction (or at a constant velocity) unless acted upon by an unbalanced force.
Inertia: the tendency of an object to resist acceleration
Inertia is not a force, it’s a property of matter.
More mass, more inertia
(also called the law of inertia).

10. The Second Law of Motion
The net force equals mass times acceleration.
Fnet = ma or a = Fnet/m
This equation explains the relationship between net force (accelerating force), mass and acceleration.

11. For every action force, there is an equal and opposite reaction force.
The Third Law of Motion
For every action force, there is an equal and opposite reaction force.
Forces act in pairs!

12. 6.1 Force Causes Acceleration
What causes an object to accelerate?
Unbalanced forces acting on an object cause the object to accelerate.
Net Force > 0

13. 3.3 Galileo on Motion think!
A ball is rolled across a counter top and rolls slowly to a stop. How would Aristotle interpret this behavior? How would Galileo interpret it?

14. 3.3 Galileo on Motion think!
A ball is rolled across a counter top and rolls slowly to a stop. How would Aristotle interpret this behavior? How would Galileo and Newton interpret it?
Answer:
Aristotle would say that the ball stops because it seeks its natural state of rest.
Galileo and Newton would say that the friction between the ball and the table overcomes the ball’s natural tendency to continue rolling—overcomes the ball’s inertia—and brings it to a stop.

15. 3.3 Galileo on Motion
According to Galileo and Newton, when is a force needed to keep an object moving?
Only when friction (or some other oppositional force) is present is a force needed to keep an object moving.

16. 3.4 Newton’s Law of Inertia
Objects at Rest
Objects in a state of rest tend to remain at rest.
Only a force will change that state.

17. 3.4 Newton’s Law of Inertia
Objects in Motion
In the absence of forces, a moving object tends to move in a straight line indefinitely.
Toss an object from a space station located in the vacuum of outer space, and the object will move forever due to inertia.

18. 3.4 Newton’s Law of Inertia
Objects at rest tend to remain at rest.

19. 3.4 Newton’s Law of Inertia

20. 3.4 Newton’s Law of Inertia

21. 3.4 Newton’s Law of Inertia
think!
A force of gravity between the sun and its planets holds the planets in orbit around the sun. If that force of gravity suddenly disappeared, in what kind of path would the planets move?

22. 3.4 Newton’s Law of Inertia
think!
A force of gravity between the sun and its planets holds the planets in orbit around the sun. If that force of gravity suddenly disappeared, in what kind of path would the planets move?
Answer: Each planet would move in a straight line at constant speed.