<< Back to Lessons Index

6th Grade Science / Lesson 11 - Things That Float

Things That Float

What will we be learning today?

  • In this lesson, we are going to learn all about why do things float.

  • why do things float.

    Have you ever dived deep under the water while swimming? If so you know that you feel more pressure on your ears as you go deeper. Pressure is the weight or force on a given area. The weight, in this case, comes from the air and water above you.

    When any object is submerged in a fluid, the fluid "pushes" in on the object. The push is greater at the bottom than at the top, so the fluid actually pushes the object toward the surface. This push is called the buoyant force.

    Any submerged object displaces fluid (moves fluid out of the way). The photograph on the next slide shows how the water level rises when a lead mass is submerged in water. If you look at the weight readings on the spring scale, you will see that the reading is less when the lead mass is submerged-the buoyant force has made it seem lighter. It turns out that the amount of buoyant force equals the weight of the fluid displaced by the lead mass.

    An object seems to lose weight when it is placed underwater. This is because the water pushes the object upward. The push is called the buoyant force. The unit of force is the newton (N).

    When an object first begins sinking into a fluid, little fluid is displaced. There is little buoyant force, and the object keeps sinking. However, some objects may displace a weight of fluid equal to their own weight before they sink all the way in. These objects float on the fluid. Other objects, though, cannot displace enough water to float. Even when fully submerged, these objects have more weight than buoyant force, and they sink to the bottom.

    Let's Review What We Learned Today!

  • Now complete your PDA!