29 Body Weight
Weight as a Force
Now that we know how to find the volume of a body, we just need to measure body mass in order to find body density. We typically measure the mass of a body by first measuring the weight using a scale, and then calculating mass from the measured weight. Weight is just another name for the force of gravity on an object. In everyday experience, a force (F) is any push or pull on an object. Forces can move objects, deform objects, or both. Often W is used to abbreviate weight, but 
is also used because it reminds us that an object’s weight and the force of gravity on the object are the same thing. Throughout this book we will learn about other forces, including buoyant force, tension, normal force, friction, and air resistance. We typically represent forces with arrows that point in the direction the force pushes (or pulls). We usually try to make the length of the arrows proportional to how big the forces are, in which case the arrows can be called vectors. The SI unit for weight, and all other forces, is the Newton(N). In the U.S. we often use pounds (lbs) instead of Newtons as our unit of force. One pound is equal to 4.45 Newtons.
Reinforcement Exercises
Reinforcement Activity: Free Body Diagrams
Draw a stick figure of a person jumping on a trampoline. Then add an arrow representing gravity acting on them while in the air. The arrow should start at the center of the person and point in the direction that gravity acting. Label the force arrow.
Draw a second figure that is just standing on the trampoline and add arrows to represent the forces acting on the person. Label the forces. [Hint: There are two forces.]
Do you think the lengths of the two force vectors should be the same or different? Explain your thought process.
Free Body Diagrams
A diagram, such as you have drawn above, that represents an object in a simplified way and shows the forces acting on it using vectors is known as a free body diagram. We often make the diagrams very simple and represent the object with just a dot, so that the force vectors are easier to see.
a quantity of space, such as the volume within a box or the volume taken up by an object.
a measurement of the amount of matter in an object made by determining its resistance to changes in motion (inertial mass) or the force of gravity applied to it by another known mass from a known distance (gravitational mass). The gravitational mass and an inertial mass appear equal.
relation between the amount of a material and the space it takes up, calculated as mass divided by volume.
the force of gravity on on object, typically in reference to the force of gravity caused by Earth or another celestial body
attraction between two objects due to their mass as described by Newton's Universal Law of Gravitation
any interaction that causes objects with mass to change speed and/or direction of motion, except when balanced by other forces. We experience forces as pushes and pulls.
the upward force exerted by any fluid upon a body placed in it
the force that is provided by an object in response to being pulled tight by forces acting from opposite ends, typically in reference to a rope, cable or wire
the outward force supplied by an object in response to being compressed from opposite directions, typically in reference to solid objects.
a force that acts on surfaces in opposition to sliding motion between the surfaces
a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid
a quantity having direction as well as magnitude
a system of physical units ( SI units ) based on the meter, kilogram, second, ampere, kelvin, candela, and mole
the SI unit of force. It is equal to the force that would give a mass of one kilogram an acceleration of one meter per second per second
a unit of force equal to 4.44822 Newtons, or the the weight of a 0.4536 kg mass on Earth's surface
a graphical illustration used to visualize the forces applied to an object
