# Eureka inertia and mass relationship

### What is Newton's first law? (article) | Khan Academy Center of Mass no 572233.info · Centripetal A and Projectile without 572233.info · Electric Circuits. . Eureka Video Qs mass inertia 572233.info The graphs and questions from the "Determining the Relationship between Two Variables. Mass and Inertia both are different.. Inertia is property of matter. Inertia is natural tendency of an object to resist change in its state of motion, such as at rest. Also called the law of inertia, this is the most important thing to realize about Mass can be determined by measuring how difficult an object is to accelerate. In deep interstellar space, we would observe that if an object had a velocity, it would continue moving with that velocity until there was some force to cause a change in the motion. Similarly, if an object were at rest in interstellar space, it would remain at rest until there was a force to cause it to change its motion.

In the video below, we can see that objects in the international space station either remain at rest or continue with constant velocity relative to the space station until acted upon by a force.

Wait, isn't there still gravity at the space station? But since the space station is in free fall orbiting around the Earth, from inside the ISS, it's as if there were no gravitational force at all.

There is air in the ISS—since astronauts have to breathe—and so there would be some air resistance. However, since everything is in free fall, objects don't immediately plummet and make contact with the floor, so the force of friction is much less of a distracting influence on the motion of an object.

The idea that objects only change their velocity due to a force is encapsulated in Newton's first law. An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force.

## Assignments

Note the repeated use of the verb remains. We can think of this law as preserving the status quo of motion. An object sliding across a table or floor slows down due to the net force of friction acting on the object. But on an air hockey table, where air keeps the puck from touching the table, the air hockey puck continues moving with a roughly constant velocity until a force acts on it—like when it bumps into the side of the table.

What if we made an air hockey table that stretched around the Earth? Well, there would be basically zero friction since air hockey pucks aren't touching the table, but there would still be a small amount of air resistance.

Galileo and the Concept of Inertia Galileo, a premier scientist in the seventeenth century, developed the concept of inertia. Galileo reasoned that moving objects eventually stop because of a force called friction. In experiments using a pair of inclined planes facing each other, Galileo observed that a ball would roll down one plane and up the opposite plane to approximately the same height. If smoother planes were used, the ball would roll up the opposite plane even closer to the original height.

Galileo reasoned that any difference between initial and final heights was due to the presence of friction. Galileo postulated that if friction could be entirely eliminated, then the ball would reach exactly the same height. Galileo further observed that regardless of the angle at which the planes were oriented, the final height was almost always equal to the initial height.

### Force and Energy (with videos & activities)

If the slope of the opposite incline were reduced, then the ball would roll a further distance in order to reach that original height. Galileo's reasoning continued - if the opposite incline were elevated at nearly a 0-degree angle, then the ball would roll almost forever in an effort to reach the original height. And if the opposing incline was not even inclined at all that is, if it were oriented along the horizontalthen Newton's first law of motion declares that a force is not needed to keep an object in motion.

Slide a book across a table and watch it slide to a rest position. The book in motion on the table top does not come to a rest position because of the absence of a force; rather it is the presence of a force - that force being the force of friction - that brings the book to a rest position. In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever!

Or at least to the end of the table top. A force is not required to keep a moving book in motion. In actuality, it is a force that brings the book to rest.

Mass as a Measure of the Amount of Inertia All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? The tendency of an object to resist changes in its state of motion varies with mass.