Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Floating Needle or Razor Blade

Objective

To see surface tension supporting a razor blade on the water surface.

Introduction

You can float a metallic needle or razor blade on the water surface without giving them any special shape. The floating of a razor blade is described here. The buoyancy force do not have practically no role in floating of blade. The water surface gets bent at the edges of the blade and the direction of the surface tension force becomes inclined to the horizontal. As a result, there is net vertical force due to surface tension which balances the weight of the blade.

Apparatus

a petri dish or glass, needle or razor blade, paper clip

Procedure

Bend the paper clip into an L shape such that the needle/blade can be laid across it. Lay the needle or blade on it, and carefully lower onto the surface of water. Gently take away the paper clip. The needle or blade float on the water surface.

You can do this in another way too. Float a piece of newspaper and place a blade on it. Once the paper gets wet, push it down with a matchstick. The paper sink leaving the needle or blade to float. The stability is also very good and small disturbances do not make it sink.

Discussion

You can carefully look at the water surface near the edge of the blade. The surface has taken inclined shape. The inclined surface in contact with the edge pull it along the incline and hence generates vertically upward force. The force from all sides finally balances the blade.

Now prepare a soap solution and gently put a drop of soap solution on the water surface on one side of the blade. The blade will immediately shift in opposite direction on the water surface. The soap solution reduces the surface tension on one side of the blade. So the blade is pulled with lesser force from this side. From the other side, where no soap solution has reached, the force is high and hence the blade moves.

Rub the needle on a candle wax, this help it to float better.

The surface tension helps many insects to comfortably stride on the water.