28 Body Weight

Weight as a Force

Now that we know how to find the of a body, we just need to measure body in order to find body . We typically measure the mass of a body by first measuring the using a scale, and then calculating mass from the measured weight.  Weight is just another name for the on an object. In everyday experience, a  (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 is caused by the force of gravity. Throughout this book we will learn about other forces,  including , , , and . 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 . The for weight, and all other forces, is the (N). In the U.S. we often use (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 acting on them while in the air. The arrow should start at the center of the person and point in the direction that gravity is 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.]

A diagram, such as you have drawn above, that represents an object in a simplified way and shows the forces acting on it using is known as a . We often make the diagrams very simple and represent the object with just a dot, so that the force vectors are easier to see.

Do you think the lengths of the two force should be the same or different? Explain your thought process.


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