Aristotle, Galileo and Newton and Newton’s Laws of Motion

Chapter 3.1-3.6Chapter 6.1-6.3Chapter 7.1-7.4

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

1564 – 1642

Italy

Perhaps the first true scientist.

Rolled and dropped objects to discover the true aspects of motion

1642-1727

England

Developed laws for motion and gravity that explain why objects move, and worked with optics

1. Objects do not move without a force.2. Objects in motion always require a force to keep

them moving.3. Objects seek their natural state, which is at rest.4. Mechanical equilibrium can only be static.

3.1 Aristotle on Motion

1. Objects do not change motion without unbalanced force.

2. Objects in motion do not always require a force to keep them moving.

3. Objects have two “natural” states of motion, at rest (static equilibrium) and moving at a constant speed and direction (dynamic equilibrium).

3.1 Galileo and Newton on Motion

Simply put, things tend to keep on doing what they’re already doing.

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).

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.

3.4 Newton’s Law of Inertia

•Objects at rest stay at rest and objects in motion stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. (also called the law of inertia).

• Inertia: the tendency of an object to resist acceleration

• Inertia is not a force, it’s a property of matter.

• More mass, more inertia

The net force equals mass times acceleration.

Fnet = ma or a = Fnet/m

Explains the relationship between Net force, mass and acceleration (direct vs. inverse relationships)

In the presence of a net force (greater than zero) an object will experience acceleration

For every action force, there is an equal and opposite reaction force.

Forces act in pairs!

What causes an object to accelerate?

6.1 Force Causes Acceleration

Unbalanced forces acting on an object cause the object to accelerate.

Net Force > 0

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?

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.

3.3 Galileo on Motion

According to Galileo and Newton, when is a force needed to keep an object moving?

3.3 Galileo on Motion

Only when friction (or some other oppositional force) is present is a force needed to keep an object moving.

Objects at Rest

• Objects in a state of rest tend to remain at rest.

• Only a force will change that state.

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.

3.4 Newton’s Law of Inertia

Objects at rest tend to remain at rest.

3.4 Newton’s Law of Inertia

3.4 Newton’s Law of Inertia

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?

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.

3.4 Newton’s Law of Inertia