0.12 Phy1090: force -- introduction to statics, equilibrium, and  (Page 4/5)

• pound force
• newtons
• kg*m/s^2

The following conversion factor can be used to convert from English units to SI units:

1 pound force = 4.44822162 newtons

To convert your weight from pounds to newtons, multiply your weight in pounds by 4.448.

To convert your weight from newtons to pounds, divide your weight in newtons by 4.448.

If your bathroom scale says that your weight is 100 pounds, a bathroom scale calibrated in newtons would say that your weight is 444.8 newtons.

The basis for your weight

You may be wondering why you weigh as much as you do (other than as a result of yourappetite for sweets). As mentioned earlier, the basis for your weight is the gravitational attraction between your body massand the mass of the earth.

The acceleration of gravity

In an earlier module, you learned that two bodies in free fall above the surface of the earth, in the absence of an atmosphere, willaccelerate toward the center of the earth at the same rate, which is about 32.2 ft/sec^2 or 9.8 m/s^2.

Your weight, or the gravitational force exerted on your body, is the force necessary to cause your body to accelerate toward the center of the earthat 32.2 ft/sec^2.

Acceleration = force/mass

You also learned in an earlier module that the acceleration of an object due to an applied force is directly proportional to the force and inversely proportional to the mass of the object.

Therefore, a greater force must be applied to an object with greater mass to achieve the same acceleration. The greater your body mass, the greater must be the forcethat will cause your body to accelerate at 32.2 ft/sec^2 toward the center of the earth.

Your weight

That force is what we commonly refer to as your weight. The greater your body mass, the greater will be your weight.

Your body in equilibrium

If you climb out and sit on the limb of a tree, that limb must be capable of exerting an amount of upward force sufficient to cancel the gravitational force acting on yourbody. Otherwise, you will fall to the surface of the earth or to something between the limb and the surface of the earth that is capable of exerting thatupward force..

If the limb doesn't break and you are able to sit there in comfort, the entire system including thetree, the limb, your body, and the earth will be in equilibrium.

Your body out of equilibrium

If the limb is incapable of sustaining that amount of upward force, it will break and allow you to accelerate towards the center of the earth. That means that the system is not in equilibrium.

Other examples of force

So far, I have limited the discussion to forces that are directly attributable to gravitational attraction. I began this explanation with that limitation becausethe effects of gravitational force are something that most of us understand pretty well.

However, there are many other examples of force, most which involve the conversion of energy into force.

A human who lifts weights

For example, a human who lifts weight eats a diet that contains energy, commonly measured in calories. The human body has the abilityto convert that energy into the contraction of muscle tissue.