Begin the investigation by describing the focus of the CELL to students.

This CELL is centered around the concepts of force, work, and simple machines. During the Investigations, students will determine the force needed to move objects, the work done when moving objects, and the work done when using simple machines.

In order to assess students’ previous knowledge or familiarity with these concepts: 

Ask students to share their ideas about what force and work mean, what simple machines are, and where they may have heard these terms previously. Record student ideas on the board.

Tell students that as they conduct their Investigations, you will come back to the list of items to see whether students think about work, force, and simple machines in ways that are similar or different to the ideas they first described.

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The word work is one of many scientific words that have an entirely different, more specific meaning, than often used by the general public as non-scientific vocabulary. For example:

“Let’s work this out.”

“This can opener doesn’t work.”

“We may work around this.”

“I love to go to work.”

“Nice work!”

While the use of the word work in all of these examples implies some expenditure of effort or energy, the scientific meaning of work is much more specific. In scientific usage work means:

Work = Force X Distance

Work, in its scientific usage, has a specific unit associated with it, the Joule (abbreviated J) and can be measured. Therefore, in science work is a quantitative property. It can be measured and reported in numbers. The nonscientific usage of the word work, typically has no units and can not be measured (hard work, busy work, important work). Therefore, in common use, work is described in qualitative terms. We will talk about the science of work much more in Investigation 2. 

There are many other examples in which the general usage of a word has a much less precise meaning than when used in science. Can you think of others? Student examples may include words like energy, force, mass, matter, volume. “For that matter, I can’t force them to turn down the volume of the music as I simply don’t have the energy!” 

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Narrow the focus of the discussion by explaining that in Investigation One of the CELL students will investigate force. Ask students the following questions as a way to initiate a discussion:

What do you think the term force means? Students may reply with suggestions such as something they are made to do, when someone pushes you, a military group, etc.

Can you think of any types of forces? Student answers will vary.

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Explain to students that the term force can have a variety of meanings in everyday life, but in the scientific field the term has a specific meaning. Tell students the class will participate in a demonstration to help them understand what force means in scientific terms.

Ask students to get a book and place it upon their desk. Direct the students to slide the book across the top of the desk, using a constant motion.

Ask the class: What did you do to move the book? Students should reply that they pushed or pulled the book to move it.

Direct students to reference their Scientist’s Glossary. Ask students: What is the definition for the term force?

“The application of a constant effort, such as a push or pull.”

Ask students: Did you exert a force on the book? Yes, they applied a force to the book to move it across the desk.

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Help students relate the concept of force to actions that occur in everyday life:

Ask students: Have you done a chore that required you to lift an object? Did you apply a force to lift that object? Student answers may vary, but they should answer yes because a constant effort would have been applied as the object was lifted.

Ask students: Are you applying a force when you lift a pencil? When you comb your hair? Student answers may vary. Yes, a force is applied in both cases.

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Pose the following question to students: What might you use to measure the force exerted to move the book?

Explain to students that Force (F) is measured in units called newtons (N) using a scientific tool called the spring scale.Point out the term spring scale in the Scientist’s Glossary and discuss how students think a spring scale might measure force. Explain to students that they will have the opportunity to measure force with a spring scale during the first Investigation.

Continue a discussion of the terms in the Scientist’s Glossary. The following points may be useful in the discussion:

• Remind students mass is the amount of matter in an object or substance. Solids, liquids, and gases are all matter, so all have mass. In the metric system, mass is measured in units of grams, milligrams, kilograms, etc.
• When measuring force, the object moved by a force is termed the load. The load is always moved over a distance, and in the metric system distance is measured in meters, millimeters, centimeters, kilometers, etc.

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This slides shows a few examples in which scales are used in everyday life. Ask students if they can think of others.

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The triple-beam balance works by offsetting the mass of a sample placed on the platform with the mass and position of the poises. Therefore, the same Martian gravitational force that pushes down on the sample in the Mars photo below also pushes down on the poises.

As a result, the triple-beam balance will give us exactly the same mass reading for a  sample regardless if we perform the measurement on Earth, Mars, or anywhere else in the Universe. The same gravitational force pushes down on both the sample and poise side of the balance, no matter where the balance is located.

However, the situation is different for scales like a spring scale. The are no “balancing” poises on a scale. Instead, a spring scale measures the force gravity exerts on the mass of an object. That is why, when you calculate weight, you use an equation that takes into account both the mass of the object and the gravitational force acting on it. Students will learn more about mass versus weight in future LabLearner CELLs and Investigations.

Since the force of gravity is not the same everywhere in the Universe, neither will the weight of any object be the same everywhere in the Universe either.

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This slide compares the mass and weight of the same baby on Earth and Mars.

Explain to students they will further examine the concept of force as they continue with this investigation. Some questions they may want to think about as they conduct their experiments include:

• Can all states of matter exert force?
• Does the mass of a load affect the force exerted to lift it?

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This slide compares the weight of an average 10 year old (31.9 kg or 70.5 pounds) at various locations in our Solar System.

Notice that the larger and more massive the object (planet, Sun, Moon), the heavier you would be on its surface. This is because the more mass an object has, the larger its gravitational pull will exert on your body.

Notice that, on the surface of the Sun, you would be crushed by your own weight (863.5 kg or 1,903.7 pounds) in addition to being burnt to a crisp (about 5,600^oC or 10,000^oF)!

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