## 9 (c) SI

Scientists all over the world use the same system of measurement so that they can communicate clearly. This system of measurement is called the International System of Units or, in French, Système International (SI). When you measure distance, you measure length. The SI unit of length is the meter (m). A meter is a little longer than a yard. A football field is about 91 meters long. The length of an object smaller than a meter often is measured in a unit called the centimeter (cm). The prefix centi- means “one hundredth.” A centimeter is one hundredth of a meter, so there are 100 centimeters in a meter. For lengths smaller than a centimeter, the millimeter (mm) is used. The prefix milli- means “one thousandth,” so there are 1,000 millimeters in a meter. Distances too long to be measured in meters often are measured in kilometers (km). The prefix kilo- means “one thousand.” There are 1,000 meters in a kilometer.

Mass is a measure of the amount of matter in an object. The SI unit of mass is the kilogram. The strength of a force is measured in the SI unit called the newton (N).

What Is Acceleration?Suppose you are a passenger in a car stopped at a red light. When the light changes to green, the driver steps on the accelerator. As a result, the car speeds up, or accelerates. In everyday language, acceleration means “the process of speeding up.”

Acceleration has a more precise definition in science. Scientists define acceleration as the rate at which velocity changes. Recall that velocity describes both the speed and direction of an object. A change in velocity can involve a change in either speed or direction—or both. In science, acceleration refers to increasing speed, decreasing speed, or changing direction.

Acceleration has a more precise definition in science. Scientists define acceleration as the rate at which velocity changes. Recall that velocity describes both the speed and direction of an object. A change in velocity can involve a change in either speed or direction—or both. In science, acceleration refers to increasing speed, decreasing speed, or changing direction.

What is a force. A force is a push or a pull. The strength of a force is measured in the SI unit called the newton (N). This unit is named after the English scientist and mathematician Isaac Newton. You exert about one newton of force when you lift an apple. It is a force that makes an apple fall to the ground.

Inertia (in ur shuh) is the tendency of an object to resist a change in motion. Newton’s first law of motion is also called the law of inertia.

Inertia explains many common events, such as why you move forward in your seat when a car stops suddenly. When the car stops, inertia keeps you moving forward.

Inertia explains many common events, such as why you move forward in your seat when a car stops suddenly. When the car stops, inertia keeps you moving forward.

Inertia Depends on Mass: The greater the mass of an object is, the greater its inertia, and the greater the force required to change its motion. How is mass related to inertia?