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#Laws of Inertia: Definition - Formula

Definition

Imagine a spacecraft moving through space. When the engines are turned off, the spacecraft will coast through space at the same speed and in the same direction. The book and the spacecraft have inertia. Inertia is the property of an object that resists changes in its motion. Because of inertia, an object at rest tends to stay at rest. An object in motion tends to keep moving at a constant speed in a straight line.

Newton's First Law

As shown in the picture above, a golf ball has a mass of 0.05 kilogram and the bowling ball has a mass of five kilograms. The bowling ball has 100 times more mass than the golf ball, so it has 100 times more inertia too. Now ask yourself which needs more force to start moving? If you push for same distance, the bowling ball takes MUCH more force to get it moving the same speed as the golf ball. The bowling ball needs more force because a bowling ball has more inertia than a golf ball. The greater an object's inertia, the greater the force needed to change its motion.

Effects of Inertia

You can feel the effects of inertia every day. Suppose you are riding in a car. What happens if the car comes to a sudden stop? Your body has inertia. When the car stops, you keep in moving forward. What happens when the car starts moving? Because of inertia, your body tends to stay at rest when the car moves forward. In baseball, inertia tends to keep a player running in a straight line. So base runners have to 'round' the bases instead of making sharp turns (see picture below).

A restoring force is any force that always acts to pull a system back toward equilibrium. Restoring force is related to the force of gravity or weight and the lift force (or tension) of the string of a pendulum (see the diagram above). If a pendulum is pulled forward or backward, gravity creates a restoring force that pulls it toward equilibrium. Systems with restoring forces become oscillators.

Examples

Cars and planes with more inertia take more force to accelerate (see the picture below). Since inertia is related to mass, in order to reduce inertia you must reduce mass. The mass of a car or plane is a trade-off between inertia and the strength of materials of the car or plane. You want strong materials, but you don't want them so heavy that it takes too much energy (fuel) just to get the car or plane moving!