When a hypothesis is tested and shown to be incorrect is the experiment a failure?

No. Testing hypothesis is part of scientific method, and proving or rejecting then are both valid results of the experiments.

If the hypothesis is shown to be incorrect the experiment is a succesful result. The role of science is to try discover facts and form theory. The rejection of hypothesis also produces knowledges and provide insight leading to the development of science.

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water