To answer question 1, Jim should control the initial temperature to which he heats the solid object. This is because in his experiment on the rate of cooling of solid objects in air, the initial temperature can affect the rate of cooling and introduce variability if not controlled.
To answer question 2, William Herschel's independent variable was the amount of sunlight. This is because in his experiment on the discovery of infrared light, Herschel was specifically investigating the relationship between the amount of sunlight and the heating effect it had.
To answer question 3, a potential source of error in Sara's projectile motion experiment is if she uses different stopwatches to measure the time it takes for the ball to hit the ground. This is because using different stopwatches can introduce variability in the measurement of time, leading to inaccurate results.
To answer question 5, the purpose of Herschel's extra thermometers in his infrared light experiments was to control for changes in ambient temperature. By using multiple thermometers, Herschel could ensure that any changes in temperature readings were not due to external factors but rather the effect of the different colors of light being tested.