Lesson 10/5

Now that you know how much you weigh, we need to figure out what exactly that number is telling you?

When you step on the scale, the only force acting upon you is the gravitational force due to the Earth's mass. So, what exactly what is weight? It is the gravitational force experienced by an object.

Remember that gravitational force is a field force whose magnitude is directly proportional to the mass of the object experiencing the force? In equation form, the gravitional force, which equals weight, can be written as F_g=mg. The mass of the object is m and g, called the gravitational field, is a vector quantity that relates the mass of an object to the gravitational force it experiences at a given location.

Near Earth's surface, g is 9.8 N/kg toward Earth's center. In other words, objects near Earth's surface experience 9.8 N of force for every kilogram of mass.

When you stand on a scale, the scale exerts a force upward on you. Because you are not accelerating, the net force acting on you must be zero. Therefore, the magnitude of force exerted by the scale (F_{scale on you}) pushing up must equal the magnitude of F_g pulling down on you. Inside the scale, springs provide the upward force necessary to make the net force equal to zero.

The scale is calibrated to convert the stretch of the springs to a weight. The measurement on the scale is affected by the gravitational field on Earth's surface.