This task is a refinement of ``Carbon 14 dating'' which focuses on accuracy. Because radioactive decay is an atomic process modeled by the laws of quantum mechanics, it is not possible to know with certainty when half of a given quantity of Carbon 14 atoms will decay. This type of question is very important in science and it also provides an opportunity to study the very subtle question of how errors behave when applying a function: in some cases the errors can be magnified while in others they are lessened.

This problem involves solving a system of algebraic equations from a context: depending how the problem is interpreted, there may be one equation or two.

This task is a somewhat more complicated version of "Accurately weighing pennies I'' as a third equation is needed in order to solve part (a) explicitly. Instead, students have to combine the algebraic techniques with some additional problem-solving (numerical reasoning, informed guess-and-check, etc.)

In this activity, students explore the effect of chemical erosion on statues and monuments. They use chalk to see what happens when limestone is placed in liquids with different pH values. They also learn several things that engineers are doing to reduce the effects of acid rain.

Students conduct a simple experiment to model and explore the harmful effects of acid rain (vinegar) on living (green leaf and eggshell) and non-living (paper clip) objects.

Students are introduced to the differences between acids and bases and how to use indicators, such as pH paper and red cabbage juice, to distinguish between them.

Students play and record the “Mary Had a Little Lamb” song using musical instruments and analyze the intensity of the sound using free audio editing and recording software. Then they use hollow Styrofoam half-spheres as acoustic mirrors (devices that reflect and focus sound), determine the radius of curvature of the mirror and calculate its focal length. Students place a microphone at the acoustic mirror focal point, re-record their songs, and compare the sound intensity on plot spectrums generated from their recordings both with and without the acoustic mirrors. A worksheet and KWL chart are provided.

Students learn to use manipulatives and calculators to explore what happens when they change one number at a time in an addition number sentence. They record and develop the pattern that develops.

Explains the properties of acute, right, and obtuse angles. [5:33]

Khan Academy learning modules include a Community space where users can ask questions and seek help from community members. Educators should consult with their Technology administrators to determine the use of Khan Academy learning modules in their classroom. Please review materials from external sites before sharing with students.

Practice adding two-digit numbers by making tens on Khan Academy. If you are stuck, watch a refresher video or choose to get a hint. Test yourself to see how many you can get correct!

Khan Academy learning modules include a Community space where users can ask questions and seek help from community members. Educators should consult with their Technology administrators to determine the use of Khan Academy learning modules in their classroom. Please review materials from external sites before sharing with students.

This video and accompanying interactive assessment practice was designed to teach overview of addition sums to first graders.

This short video and interactive assessment activity is designed to give fifth graders an overview of addition and subtraction of weights (metric units).

This short video and interactive assessment activity is designed to teach fifth graders about subtracting decimals - word problems.

This short video and interactive assessment activity is designed to teach fourth graders about adding and subtracting money using words.

This is a video overview of addition word problems (to sums of 100) for first graders.

This short video and interactive assessment activity is designed to teach third graders an overview of mixed operations.

Demonstrates how to add and subtract pieces of fruit using the numbers 5 and 3. [2:17]

Khan Academy learning modules include a Community space where users can ask questions and seek help from community members. Educators should consult with their Technology administrators to determine the use of Khan Academy learning modules in their classroom. Please review materials from external sites before sharing with students.

This short video and interactive assessment activity is designed to teach second graders about adding and subtracting (numbers up to 20) - word problems.

This short video and interactive assessment activity is designed to teach fifth graders about adding and subtracting fractions with different denominators.

This site provides students practice with the concept of adding and subtracting decimals. Students can learn about the topic by completing an interactive tutorial. Students can then take a ten question timed test to practice the skill.