Which of the following is not an example of work?
a. raising a barbell over your head
b. picking up a box off the floor
c. holding a tray in the cafeteria line
d. pushing a box across the floor

When a vertical beam of light passes through a transparent medium, the rate at which its intensity i decreases is proportional to i(t), where t represents the thickness of the medium (in feet). in clear seawater, the intensity 3 feet below the surface is 25% of the initial intensity i0 of the incident beam. what is the intensity of the beam "10" feet below the surface? (give your answer in terms of i0. round any constants or coefficients to five decimal places.)