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

•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 battery has more 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

3.4 Newton’s Law of Inertia

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!

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

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.

Fnet = ma or a = Fnet/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 = Fnet

Units for force: mass units (kg) acceleration units (m/s2) The units kg•m/s2 are also called newtons (N).

Newton’s 2nd 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 2nd 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.

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

http://www.nasa.gov/audience/forstudents/brainbites/nonflash/bb_home_bolt.html

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.

cannonballcannon

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

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