One type of **force** that everyone is familiar with is weight. This is the amount of force that the Earth exerts on you. There are two interesting things about this force:

- It pulls you down, or, more exactly, toward the center of the Earth.
- It is proportional to your mass. If you have more mass, the Earth exerts a greater force on you.

When you step on a bathroom scale, you exert a force on the scale. The force you apply to the scale compresses a spring, which moves the needle. When you throw a baseball, you apply a force to the ball, which makes it speed up. An airplane engine creates a force, which pushes the plane through the air. A car's tires exert a force on the ground, which pushes the car along.

Force causes **acceleration**. If you apply a force to a toy car (for example, by pushing on it with your hand), it will start to move. This may sound simple, but it is a very important fact. The movement of the car is governed by **Isaac Newton's Second Law**, which forms the foundation for classical mechanics. Newton's Second Law states that the *acceleration (a) of an object is directly proportional to the force (F) applied, and inversely proportional to the object's mass (m)*. That is, the more force you apply to an object, the greater the rate of acceleration; and the more mass the object has, the lower the rate of acceleration. Newton's Second Law is usually summarized in equation form:

**a = F/m, or F = ma**

To honor Newton's achievement, the standard unit of force in the SI system was named the **newton**. One newton (N) of force is enough to accelerate 1 kilogram (kg) of mass at a rate of 1 meter per second squared (m/s^{2}). In fact, this is really how force and mass are defined. A **kilogram** is the amount of weight at which 1 N of force will accelerate at a rate of 1 m/s^{2}. In English units, a **slug** is the amount of mass that 1 pound of force will accelerate at 1 ft/s^{2}, and a **pound** mass is the amount of mass that 1 lb of force will accelerate at 32 feet/s^{2}.

The Earth exerts enough force to accelerate objects that are dropped at a rate of 9.8 m/s^{2}, or 32 feet/s^{2}. This gravitational force is often referred to as **g** in equations. If you drop something off a cliff, for each second it falls it will speed up by 9.8 m/s. So, if it falls for five seconds, it will reach a speed of 49 m/s. This is a pretty fast rate of acceleration. If a car accelerated this quickly, it would reach 60 miles per hour (97 kph) in less than three seconds!

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