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 F_{g} 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, tensionnormal 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.



Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Body Physics: Motion to Metabolism Copyright © by Lawrence Davis is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book