GRADES 3-8
INERTIA

BACKGROUND INFORMATION:

Sir Isaac Newton, 1642-1727, was an English scientist, mathematician, and philosopher whose three laws of motion defined the modern day study of dynamics. A branch of physical science, dynamics defines the relationship between forces, energy, and motion.

Forces are pushes or pulls on an object. A force is a vector; it has both a magnitude (a number value) and a direction. The fact that forces have directions becomes very important in the study of dynamics. For example, if you push straight down on a book on a table, no matter how hard you push down, the book will not move to the side along the table. If you push hard on the side of the book parallel to the table, however, it will move across the table! The force is usually created through some type of energy. Energy can take many forms such as work, heat, or chemical. In the book example, you provided the energy to push the book. In dynamics, when forces are applied to objects, they usually move, and we can determine mathematically how they move through the use of Newton's laws.

When we examine the motion of an object, we usually do so by evaluating its velocity or acceleration. The velocity is a measure of how fast something is moving, and it, too, is a vector. The direction of the movement is equally as important as its speed! Often, when we talk about velocity, we call it the rate of change of the distance over time. You may use the saying, "speed times time equals distance" when you are trying to determine your travel plans, where you multiply your speed by the time to see if you can cover the miles you need to go.

Acceleration occurs when the velocity is changing. If the velocity is increasing, the acceleration is positive, and if it is decreasing, we have a negative acceleration or deceleration. Acceleration is also a vector. We sometimes call acceleration, the rate of change of velocity with time.

Sir Isaac Newton deduced the relationships between forces, velocities, and accelerations. In his first law of motion, he stated that an object at rest remained at rest and an object in motion remained in motion until some outside force acted to change the situation. We refer to this concept as inertia.

In the egg demonstration, a light force was applied to the eggshell, causing it to stop spinning. The insides of the raw egg, however, were still spinning, and no force had been applied to it. When you removed your finger, the eggshell began to spin again because of the motion inside it.

In the wagon example, even though the wagon stopped after hitting the barrier, the blocks inside, which were also traveling at the same velocity as the wagon, were not forced to stop, so they continued to travel right out of the wagon. Inertia is why it is so important to wear seatbelts when traveling in a moving vehicle like a car or plane. Even though the passengers feel as though they are sitting still, they are still traveling at the same speed as the vehicle. If the vehicle comes to a sudden stop, the passengers will continue to travel in the original direction unless stopped by an additional force such as a seatbelt. o

Newton's second law of motion gives us an expression to compute inertia effects. His second law is usually stated that an object acted upon by a force experiences an acceleration in the same direction as the force with a magnitude that is proportional to it. We usually give this law as an equation, F = ma, where m is the mass of the object. The boldface is used to remind us that both the force and the acceleration are vectors. If you are sitting in a car that is accelerating, for example, you don't feel the force that is pushing the car forward, even though you are experiencing it. In fact, you think you are sitting still and not moving! Although you are at equilibrium (sitting still), you actually experience a force equal to the total vehicle mass times the acceleration of the vehicle pushing you back into your seat. This is another example of an inertial force!