Archimedes' Principle

Essential Question
What happens to an object's weight when it is put in water? What happens to the water's weight?

Objectives

 * Associate one demonstration of force with another opposite force.

Introduction & Student Background
When a weight is dangled in water, does its weight increase or decrease? Does the weight of the cup of water increase or decrease? What weight of water is displaced by the dangled object?

Archimedes' principle indicates that 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 that the body displaces. Students will demonstrate this using weights in water.

Materials

 * Digital scale
 * Spring scale
 * Metal mass on a loop of thread
 * Cup of water
 * Overflow cup
 * Graduated cylinder

Lesson Plan

 * 1) Weigh the object with a spring scale. Record its weight.
 * 2) Place the cup of water on the digital scale. Record its weight.
 * 3) Dangle the weight in the cup of water. The object's weight on the spring scale should decrease as the weight of the cup of water increases at exactly the same amount at the same rate.
 * 4) Fill the overflow cup. Measure the volume of water displaced by the object. The volume (and weight) of the water displaced by the object is exactly equal to the decrease in the object's weight and the increase in the cup of water's weight.

How it fits to Standard

 * Forces and motions can be measured.
 * Carry out investigations by using instruments, observing, recording, and drawing evidence-based conclusions.

Follow Up
Archimedes' Principle demonstrates the concept of equal and opposite forces. What other examples can you think of that demonstrates this?