During muscle contraction calcium ions combine with tropomyosin, and this exposes active sites for cross bridge formation true or false

False

Explanation:

During muscle contraction, the sarcoplasmic reticulum releases calcium ions. These ions are released into the sarcoplasm where they bind to troponin.   Troponin serves to move the tropomyosin away from the myosin-binding sites present on the actin. This facilitates the binding of myosin to the actin and formation of cross-bridges. Cross bridge formation is followed by a power stroke to shorten the length of the sarcomere.

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom