Rotational inertia depends on the mass of the object and its distribution relative to the axis. When developing his first law, Newton drew upon the work of Galileo Galilei. In a letter to Francesco Ingoli, Galileo outlined the principle of inertia:. I tell you that if natural bodies have it from Nature to be moved by any movement, this can only be a circular motion, nor is it possible that Nature has given to any of its integral bodies a propensity to be moved by straight motion.
I have many confirmations of this proposition, but for the present one alone suffices, which is this. I suppose the parts of the universe to be in the best arrangement so that none is out of its place, which is to say that Nature and God have perfectly arranged their structure… Therefore, if the parts of the world are well ordered, the straight motion is superfluous and not natural, and they can only have it when some body is forcibly removed from its natural place, to which it would then return to a straight line.
Every body remains in its state of rest or motion in the same direction and with the same velocity, if not compelled by an external cause to forsake this state. We all experience cognitive inertia: the tendency to stick to existing ideas, beliefs, and habits even when they no longer serve us well. Few people are truly able to revise their opinions in light of disconfirmatory information. Instead, we succumb to confirmation bias and seek out verification of existing beliefs.
It takes work to overcome cognitive dissonance, just as it takes effort to stop a car or change its direction. When the environment changes, clinging to old beliefs can be harmful or even fatal. Whether we fail to perceive the changes or fail to respond to them, the result is the same.
Perspective is everything, and the perspective that matters is the one that most closely lines up with reality. Sometimes you make up your mind about something without knowing why, and your decision persists by the power of inertia.
Every year it gets harder to change. Cognitive inertia is the reason that changing our habits can be difficult. The default is always the path of least resistance, which is easy to accept and harder to question. Consider your bank, for example. Perhaps you know that there are better options at other banks. Or you have had issues with your bank that took ages to get sorted. Yet very few people actually change their banks, and many of us stick with the account we first opened.
After all, moving away from the status quo would require a lot of effort: researching alternatives, transferring balances, closing accounts, etc. And what if something goes wrong? Sounds risky. The switching costs are high, so we stick to the status quo. Sometimes inertia helps us. After all, questioning everything would be exhausting. But in many cases, it is worthwhile to overcome inertia and set something in motion, or change direction, or halt it. The important thing about inertia is that it is only the initial push that is difficult.
After that, progress tends to be smoother. Ernest Hemingway had a trick for overcoming inertia in his writing. Knowing that getting started was always the hardest part, he chose to finish work each day at a point where he had momentum rather than when he ran out of ideas. Unlike weight, mass does not vary with location. The mass of an object is the same on Earth, in orbit, or on the surface of the Moon. In practice, it is very difficult to count and identify all of the atoms and molecules in an object, so masses are not often determined in this manner.
Operationally, the masses of objects are determined by comparison with the standard kilogram. They are equal. A kilogram of one substance is equal in mass to a kilogram of another substance. The quantities that might differ between them are volume and density.
What is the relationship between weight and mass? Which is an intrinsic, unchanging property of a body? Skip to main content. Search for:. Check Your Understanding Which has more mass: a kilogram of cotton balls or a kilogram of gold? Conceptual Questions 1. The more mass, the more inertia. Momentum is another quantity in Physics which depends on both mass and speed.
Momentum will be discussed in a later unit. Supposing you were in space in a weightless environment , would it require a force to set an object in motion? Even in space objects have mass. And if they have mass, they have inertia. That is, an object in space resists changes in its state of motion. A force must be applied to set a stationary object in motion.
Newton's laws rule - everywhere! Fred spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. What affect if any does this practice have upon his inertia?
Fred will increase his mass if he makes a habit of this. And if his mass increases, then his inertia increases. Ben Tooclose is being chased through the woods by a bull moose that he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage.
Explain this in terms of inertia and Newton's first law of motion. The large mass of the bull moose means that the bull moose has a large inertia. Thus, Ben can more easily change his own state of motion make quick changes in direction while the moose has extreme difficulty changing its state of motion. Physics for better living! Two bricks are resting on edge of the lab table.
Shirley Sheshort stands on her toes and spots the two bricks. She acquires an intense desire to know which of the two bricks are most massive. Since Shirley is vertically challenged, she is unable to reach high enough and lift the bricks; she can however reach high enough to give the bricks a push.
Discuss how the process of pushing the bricks will allow Shirley to determine which of the two bricks is most massive. What difference will Shirley observe and how can this observation lead to the necessary conclusion? The bricks, like any object, possess inertia. That is, the bricks will resist changes in their state of motion. If Shirley gives them a push, then the bricks will offer resistance to this push. The one with the most mass will be the one with the most inertia.
This will be the brick which offers the most resistance. This very method of detecting the mass of an object can be used on Earth as well as in locations where gravitational forces are negligible for bricks. Physics Tutorial. My Cart Subscription Selection.
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