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

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 Thought that objects in motion must have a force keeping them moving

1564 – 1642 Italy Perhaps the first true scientist. Rolled and dropped objects to discover the true aspects of motion—that objects in motion do not need a force to keep them moving

England Developed laws for motion and gravity that explain why objects move, and worked with optics Simply put, things tend to keep on doing what they’re already doing.

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 at a constant velocity continue at a constant velocity 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. Mass and inertia are proportional. More mass, more inertia

Which has more mass, a feather pillow or a common automobile battery? Which has more volume? Which has a higher density? What has the most inertia? 3.5 Mass—A Measure of Inertia The pillow has a larger size (volume) but a smaller mass than the battery. The battery is more dense.

The stone’s inertia, or mass, is a property of the stone and not its location. The same force would be required to shake the stone with the same rhythm whether the stone was on Earth, on the moon, or in a force-free region of outer space. 3.5 Mass—A Measure of Inertia

First Law of Motion  The question is not why objects keep moving, but why they don’t keep moving.

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

REFLECT ON THE MEANING OF THIS CARTOON

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

The law of inertia states that objects in motion remain in motion and that objects at rest remain at rest if no unbalanced forces act on them. 3.6 The Moving Earth Again Constant speed and same direction!

 If the Earth is rotating at 800 mph at our latitude, why can’t we sense this?

Objects Move With Earth You can refute this argument using the idea of inertia. Earth moves at 30 km/s, but so do the tree, the worm below, and even the air in between. Objects on Earth move with Earth as Earth moves around the sun. 3.6 The Moving Earth Again

Earth does not need to be at rest for the bird to catch the worm. 3.6 The Moving Earth Again

Copernicus announced the idea of a moving Earth in the sixteenth century. One of the arguments against a moving Earth was: Consider a bird sitting at rest in the top of a tall tree. The bird sees a worm, drops down vertically, and catches it. It was argued that this would not be possible if Earth moved as Copernicus suggested. The fact that birds do catch worms from high tree branches seemed to be clear evidence that Earth must be at rest. 3.6 The Moving Earth Again

Flip a coin in an airplane, and it behaves as if the plane were at rest. The coin keeps up with you—inertia in action! 3.6 The Moving Earth Again A person flips a coin into the air while on a jet that is traveling 500 mph. Where will the coin land and why?

Objects Move With Vehicles If we flip a coin in a high-speed car, bus, or plane, we can catch the vertically moving coin as we would if the vehicle were at rest. We see evidence for the law of inertia when the horizontal motion of the coin before, during, and after the catch is the same. The vertical force of gravity affects only the vertical motion of the coin. 3.6 The Moving Earth Again

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How does the law of inertia apply to objects in motion? 3.6 The Moving Earth Again The law of inertia states that objects in motion remain in motion and that objects at rest remain at rest if no unbalanced forces act on them.

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.

The net force equals mass times acceleration. F net = ma or a = F net /m Explains the relationship between Net force, mass and acceleration.

Newton’s Second Law  F represents the vector sum of all forces acting on an object. F = F net Units for force: mass units (kg)  acceleration units (m/s 2 ) The units kgm/s 2 are also called newtons (N).

Classroom Practice Problem  Space-shuttle astronauts experience accelerations of about 35 m/s 2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude?  Answer: 2600 kgm/s 2 or 2600 N

Newton’s 2 nd law of motion  Increasing the force will increase the acceleration. Which produces a greater acceleration on a 3-kg model airplane, a force of 5 N or a force of 7 N?  Answer: the 7 N force  Increasing the mass will decrease the acceleration. A force of 5 N is exerted on two model airplanes, one with a mass of 3 kg and one with a mass of 4 kg. Which has a greater acceleration?  Answer: the 3 kg airplane

Newton’s 2 nd law of motion shows two general relationships in science  The rate of acceleration is directly related to net force  Also called directly proportional

Newton’s 2 nd law of motion shows two general relationships in science  The rate of acceleration is inversely related to the object’s mass  Also called inversely proportional

For every action force, there is an equal and opposite reaction force. Forces act in pairs!

Newton’s Third Law  Forces always exist in pairs. You push down on the chair, the chair pushes up on you Called the action force and reaction force Occur simultaneously so either force is the action force

Newton’s Third Law  For every action force there is an equal and opposite reaction force.  The forces act on different objects. Therefore, they do not balance or cancel each other. The motion of each object depends on the net force on that object.

What do you think?  Two football players, Alex and Jason, collide head-on. They have the same mass and the same speed before the collision. How does the force on Alex compare to the force on Jason? Why do you think so?

What do you think?  Suppose Alex has twice the mass of Jason. How would the forces compare? Why do you think so? Sketch as before.  Suppose Alex has twice the mass and Jason is at rest. How would the forces compare? Why do you think so? Sketch as before.

When action is A exerts force on B, the reaction is simply B exerts force on A. 7.3 Identifying Action and Reaction

When action is A exerts force on B, the reaction is simply B exerts force on A. 7.3 Identifying Action and Reaction

The dog wags the tail and the tail wags the dog. 7.2 Newton’s Third Law

The balloon recoils from the escaping air and climbs upward. 7.4 Action and Reaction on Different Masses A common misconception is that a balloon is propelled by the impact of exhaust gases against the atmosphere. Each molecule of exhaust gas acts like a tiny molecular cannonball shot downward from the balloon.

The rocket recoils from the “molecular cannonballs” it fires and climbs upward. 7.4 Action and Reaction on Different Masses Rocket pushes on gas Gas pushes on rocket

7.4 Action and Reaction on Different Masses The force the cannon exerts on the cannonball is exactly equal and opposite to the force the cannonball exerts on the cannon. cannonball cannon What can you say about the action and reaction forces experienced by the cannon and the cannonball?

F represents both the action and reaction forces; m (large), the mass of the cannon; and m (small), the mass of the cannonball. 7.4 Action and Reaction on Different Masses Which has the greater change in motion and why? The cannonball! Because it has less mass. Same force Different masses Different accelerations

Hammer Striking a Nail  What are the action/reaction pairs for a hammer striking a nail into wood? Force of hammer on nail = force of nail on hammer Force of wood on nail = force of nail on wood  Which of the action/reaction forces above act on the nail? Force of hammer on nail (downward) Force of wood on nail (upward)  Does the nail move? If so, how? F hammer-on-nail > F wood-on-nail so the nail accelerates downward

Hammer Striking a Nail  What forces act on the hammer? Force of nail on hammer (upward) Force of hand on hammer (downward)  Does the hammer move? If so, how? F nail-on-hammer > F hand-on-hammer so the hammer accelerates upward or slows down  The hammer and nail accelerate in opposite directions.

Action-Reaction: A Book on a Desk  Earth pulls down on the book (force of gravity). Reaction Force The book pushes down on the desk. Action Force  The desk pushes up on the book. The book pulls up on Earth.

Action-Reaction: A Falling Book Action  Earth pulls down on the book (force of gravity).  What is the result of the action force (if this is the only force on the book)? Unbalanced force produces an acceleration of m/s 2. Reaction The book pulls up on Earth. What is the result of the reaction force? Unbalanced force produces a very small upward acceleration (because the mass of Earth is so large).

Chapter 7, Page The cannon and cannonball have very different accelerations because equal forces on unequal masses produce unequal accelerations. The more massive cannon has more inertia and is harder to accelerate than the much less massive cannonball.

Chapter 7, Page The force that propels a rocket is the force of the exhaust gases pushing the rocket in reaction to the rocket pushing the exhaust gases. Rocket pushes on exhaust gas Exhaust gas pushes on rocket RECOIL

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

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

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