UNIT I: CONCEPT ENHANCER – INERTIA

Inertia is generally defined as a resistance to change. The Aristotelian explanation for motion held for 2000 years. Objects had natural motions. The motion of these objects was based on the amount of the basic elements (earth, water, wind, and fire) they contained. Objects composed of primarily “earth” would fall to the earth when released or had no motion. Remaining at rest was considered a natural state. Moving objects in the absence of a force would slow to a rest. Common experiences supported the conclusion that objects have a tendency not to move. Galileo developed the concept of inertia - the natural tendency of objects to resist changes in their state of motion (or non-motion) that Newton formulated into what we call Newton’s First Law of Motion or the Law of Inertia.

Aristotle required a push or a pull to keep an object moving. Galileo denied this by stating that if there were no resistance by a moving object it would keep moving forever in a straight line. His famous inclined plane experiments lead him to this assertion. In his Principia, Newton stated that every object continues in its state of rest or of uniform motion in a straight line, unless it is compelled to change its state by forces impressed upon it. In the abbreviated form of Newton’s First Law of Inertia we say, objects at rest remain at rest and objects in motion will remain in motion at constant speed along a straight line unless acted on by an unbalanced or net force.

Inertia is defined as an object’s resistance to a change in its motion (or non-motion). In other words, the inertia of an object at rest provides the resistance in the attempt to move it. The inertia of an object moving at constant speed along a straight line provides the resistance in the attempt to slow it down to a rest or to speed it up. The relative measure of an object’s inertia is the amount of mass an object has. The more mass an object has, the greater its inertia or resistance to a change in its state of motion (or non-motion). Also keep in mind that the mass of an object can be used as a relative measure of its weight since the force due to an object’s weight is equal to the product of the object’s mass (in kg) and the acceleration of gravity (9.8 m/s2). It is also important not to confuse mass and volume. Massive objects are not necessarily large in volume. Although they have the same mass, a kilogram of iron has much less volume than a kilogram of Styrofoam packing peanuts.

In Pool Ball - Silver Dollar Trick, the inertia of the coin is evident. If the force is rapidly applied, there is not enough time for the force to be transferred to the coin. It tends to remain at rest. The coin drops as the ball moves from under it, the inertia of the coin on the pool ball prevents the applied force from being transferred to it.

Most reporters have no concept of the physics involved in a car crash. In fact, often a statement such as “The driver was thrown from the car.” accompanies a report of a person found outside their vehicle at the scene of the accident. Unless the person was driving James Bond’s 007 car or Batman’s Batmobile, inertia, not ejection, was the cause of the body leaving the vehicle. The relationship that exists between Newton’s 1st Law and automobile collisions was discussed in some detail in the Barbie, Ken, and Sir Isaac activity. So buckle up your seat belts and prepare yourself for the upcoming ride. In the meantime to wrap up this discussion, the key to inertia is the word “continues.” An object will continue in its state of motion (or non-motion) unless forced to do otherwise. The difference between the everyday usage of the word inertia and its use in physics is the inclusion of the idea that the state of uniform motion in a straight line and non-motion are equivalent with respect to inertia.

The establishment of reference frames for describing motion is another important property of inertia. An inertial frame of reference is one in which the reference frame is not accelerating. All of Newton’s laws apply to an inertial frame of reference, or to a reference frame that is not accelerating.

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UNIT I: CONCEPTUAL PRACTICE – INERTIA

  1. Explain the physics of a neck injury called “whiplash” and the purpose of a headrest.
  1. You are pushing a car across a parking lot. When you stop pushing, the car coasts to rest. Contrast an Aristotelian description of the change in motion with that of Galileo or Newton.
  1. In terms of the physics involved what happens to an automobile that attempts to turn a corner too fast on an icy road (missing the curve)?
  1. Prior to going to bed, reach up and mark the highest spot on a wall that you can reach. In the morning re-measure your reach. Explain the difference using the concept of inertia and weight and the discs between your vertebrae.
  1. How could you determine if two objects have the same inertia without lifting them?
  1. In an automobile collision, there are three types of collisions that take place. Describe each. Why is the third often the most harmful even though physical appearances may not support the fact?
  1. If the earth is traveling around the sun at roughly 30 km/sec, why are you not left behind when you jump on a trampoline and go into the air?

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