force acts

Now let us move from forces' in solids to forces in liquids. If a solid is placed into a liquid the solid will push the liquid out of its path with the result that the liquid will push back on the solid. This is an example of Newton's Third law of motion which states 'action and reaction are equal and opposite'. The upward force exerted by liquids on objects placed in the liquid is known as the upthrust. Upthrust acts on a body in a liquid even if the body is resting on some support such as the bottom of a vessel, provided the space between the body and the support is not evacuated. A marble dropped into water in a beaker will sink to the bottom of the beaker as in Fig. 3.13(a). The forces acting on the marble are its weight W, the upthrust U of the water and the reaction R of the base of the beaker; W = R + U. A marble resting in a hole in the container will not have an upthrust acting on it. This time the downward forces on the marble are its weight W and the weight w of the liquid above it pressing the marble into the hole. The total reaction on the edges of the hole R2 = W + w, which is much greater than the reaction R in the beaker where R = W — U.