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.
Draw a picture of yourself jumping on a trampoline (a stick figure will work). Then add an arrow representing gravity acting on you while you are in the air. The arrows should start at your center and point in the direction that the force is pushing or pulling you. Label the forces arrows.
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.) Do you think the lengths of the two arrows should be the same or different? Explain your thought process.