Buoyancy and surface tension

Outcomes

Define and give examples of buoyancy and surface tension

Surface Tension

Molecules within a liquid are pulled equally in all directions by intermolecular forces. However, molecules at the surface are pulled downwards and sideways by other liquid molecules, but not upwards away from the surface. The overall effect is that the surface molecules are pulled into the liquid, creating a surface that is tightened like a film (see A in Figure below). The surface tension of a liquid is a measure of the elastic force in the liquid’s surface. Liquids that have strong intermolecular forces, like the hydrogen bonding in water, exhibit the greatest surface tension. Surface tension allows objects that are denser than water, such as the paper clip shown in B in Figure below, to nonetheless float on its surface. It is also responsible for the beading up of water droplets on a freshly waxed car because there are no attractions between the polar water molecules and the nonpolar wax.

Buoyancy

Archimedes’ Principle states that the upward buoyant force on an object in the water is equal to the weight of the displaced volume of water. The reason for this upward force is that the bottom of the object is at lower depth, and therefore higher pressure, than the top. If an object has a higher density than the density of water, the weight of the displaced volume will be less than the object’s weight, and the object will sink. Otherwise, the object will float. The ratio of an object or substance's density to the density of water is called it's specific gravity.

Weight and Buoyant Force

Weight is a measure of the force of gravity pulling down on an object, whereas buoyant force pushes up on an object. Which force is greater determines whether an object sinks or floats. Look at the Figure below. On the left, the object’s weight is the same as the buoyant force acting on it, so the object floats. On the right, the object’s weight is greater than the buoyant force acting on it, so the object sinks.

Density and Buoyant Force

Density, or the amount of mass in a given volume, is also related to the ability of an object to float. That’s because density affects weight. A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice is less dense than liquidwater. This explains why the giant ice berg in the Figure below is floating in the ocean.

Activities:

Define surface tension.
What is responsible for the strong surface tension in water?
What is buoyant force?
Why does buoyant force occur?
Why is a heavier object more likely than a lighter object to sink instead of float in water?
Assume you have an ice cube and also a small rock that is the same size and shape as the ice cube. Predict what would happen if you placed the ice cube and rock in a glass of water. Explain your prediction.

Practical: Coin Drop Experiment

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