0.12 Phy1090: force -- introduction to statics, equilibrium, and  (Page 2/5)

Impenetrable material

Fortunately, the earth is made of rock and other relatively impenetrable material, which prevents you from being pulled into the inner portions of theearth by that gravitational attraction.

In order to prevent you from being pulled into the earth, the ground that you are standing on must push up on the bottom of your feet by an amount equal to the gravitational pull that isbeing exerted on your body.

Your skeleton is critical

In addition, the bones in your inner skeleton must be capable of withstanding the pull of gravity on your body toprevent your body from turning into a puddle on the surface of the earth. For example, a raw egg doesn't have an inner skeleton; it has an exoskeleton, theshell. When you break the shell over a frying pan and dump the egg out of the shell, the attraction of gravity causes it to become a puddle in the frying pan.

The push is a force

In simplified terms, a force is a push or a pull. The push that is being exerted upward on the bottom of your feet by the surface of the earth is commonlycalled a force.

The gravitational attraction that is trying to pull you into the center of the earth is also a force (which is oftenreferred to as your weight).

Equilibrium

If you are standing in one spot without moving up or down, the entire system that constitutes your body and the earth is inequilibrium. This means that the upward force being exerted on the bottom of your feet by the surface of the earth is equal in magnitude and opposite in direction from thegravitational force that is trying to pull you into the earth.

An object is in equilibrium when it is not being accelerated. It may be at rest and remain at rest or it may be moving with a constant velocity and continue to moveat the same velocity. If it is being accelerated, which includes changing its direction of motion, it is not in equilibrium.

Force is a vector quantity

In order for a body to be in equilibrium, the vector sum of all the forces acting on that body must be zero.

The bathroom scales

Many households in the U.S. have a device that is commonly known as the bathroom scale. We use it to determine "how much we weigh."

Assume that you walk into your bathroom and step onto the bathroom scale. To understand everything that is going on, we need to recognize that the house and the scale inside the house arealso being pulled toward the center of the earth by gravitational attraction. Fortunately the surface of the earth normally exerts an upward force on thefoundation of the house to prevent that from happening. The foundation exerts an upward force on the bathroom floor, and the floor exerts an upward force on thescale.

When the strength of the earth fails...

Sometimes, under some conditions, the surface of the earth fails to exert an upward force on the foundation of a house sufficient to prevent it fromaccelerating toward the center of the earth. I recently saw a video on the news of a house that was pulled into a sinkhole in Florida. I have seen several newsvideos where houses have been pulled toward the center of the earth during landslides in California.