# Unit 3: Body Composition

Throughout Unit 1 we used the kinematic equations to model the motion of objects and make predictions about their positions, velocities, and accelerations. In unit 2 we learned about types of models and that kinematic equations are a quantitative physical model. We also learned that body fat percentage could be estimated from skinfold measurements using an empirical model that relates those two quantities, but that the uncertainty in the result was relatively large. The Unit 3 lab will investigate a more accurate quantitative empirical model that estimates body fat percentage using the body density as input. In order to use that model, we will need to understand density and how it can be determined, which in turn requires understanding of mass, volume, density, weight, apparent weight, static equilibrium, and Archimedes’ Principle. The following chapters in this unit will introduce these concepts. The learner outcomes for this unit are listed below, and below that are some related key terms to watch out for as you complete the chapter.

### Learner Objectives

- Compare and contrast mass, volume, density, weight and apparent weight and explain how each are measured.[2]
- Explain how the concept of static equilibrium plays into the measurement of weight and apparent weight. [2]
- Apply Archimedes’ Principle and density concepts to predict if objects will sink or float. [2]
- Experimentally determine an object’s mass, weight, volume, and density. [5]

### Key Terms and Concepts

Hooke's Law

Spring Constant

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.

a quantity of space, such as the volume within a box or the volume taken up by an object.

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

the reading on a scale that is used to measure the weight of an object that is submerged in a fluid

the state being in equilibrium (no unbalanced forces or torques) and also having no motion

the total amount of remaining unbalanced force on an object

the upward force exerted by any fluid upon a body placed in it

The upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid being displaced by the body

a technique for measuring the mass per unit volume of a living person's body. It is a direct application of Archimedes' principle, that an object displaces its own volume of water

the ratio of the density of a substance to the density of a standard, usually water for a liquid or solid, and air for a gas