In order to estimate the amount of time students spend on homework, the principal at a high school plans to survey a sample of students. Because the principal believes that different grade levels have different amounts of homework, the plan is to take separate random samples of freshmen, sophomores, juniors, and seniors. What type of sample did the principal perform

Stratified Random sampling

Step-by-step explanation:

Stratified Random sampling simply refers to a sampling technique or strategy whereby random selection of observation of samples are made from a population or larger sample by first arranging or splitting the population of interest into smaller groups or separate catagory based on a certain criteria or criterias. Random samples are then taken from each of the groups also called strata. This technique is aimed at ensuring a more even distribution of samples across the various available groups within a population of interest. In the scenario above the stratas or groups are ; Sophomores, Junior, Senior and freshmen.

reduce the friction between its moving parts

step-by-step explanation:

in fact, by reducing the friction between the moving parts of the machine, it is possible to reduce the energy wasted due to this friction; therefore, more input energy is converted into useful work, and this will improve the efficiency of the machine.