This is the first slide in the CELL Earth’s Forces. Investigation One introduces students to the concept of force and gravity. Students will perform experiments aimed at discovering the difference between mass and weight and explore Newton’s Laws of Motion.

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This slide pays tribute to one of the most influential and brilliant scientists who ever lived, Sir Isaac Newton. Although the cartoon sequence shown here is based on Newton’s observations at his family home in Hampthorp, England, which did have an apple tree, the story has so often been retold and dramatized it is difficult to call it pure history.

Nonetheless, as students proceed through science classes in LabLearner middle school, high school, and beyond, they will repeatedly come in contact with Newton’s many achievements. He was certainly one of the best minds of the scientific revolution of the seventeenth and eighteenth centuries. 

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This introductory slide presents Newton’s Three Laws of Motion. Review the three laws only briefly as they are each expanded upon in later slides.

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Newton’s First Law of Motion: 

An object will remain stationary, or continue at the same speed in the same direction unless acted upon by an unbalanced force.

The first part of this law is easy to imagine. An object at rest is not in motion. It doesn’t move, it is stationary. Also, the First Law states that it will remain at rest unless an unbalanced force acts on it. In the example on the left, the mass of the cart pushes down on it with exactly the same force that the Earth pushes up on it. No other forces are acting on the cart in this situation. It doesn’t move up or down or left or right.

However, once the man exerts forces on the cart, the forces are no longer balanced and the cart moves in the direction of the push.

It is the second part of Newton’s First Law of Motion that is a bit more difficult to imagine. That is, the cart will continue moving in the same direction and at the same speed unless another force is applied to stop it. In other words, the First Law of Motion suggests that the shopping cart should continue moving in the same direction and at the same speed shown in the right-hand picture… forever. 

However, our experience tells us that this would not happen. We know that if we push the cart, it will slow down and stop after some distance. However, this is only because other forces do, in fact, act upon the cart. What are these forces? Wind resistance and friction are the main forces acting on the moving cart. These forces will slow the cart down until it stops. This is completely consistent with Newton’s First Law of Motion. The cart did keep moving until a force(s) stopped it.

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Newton’s Second Law of Motion: 

An object’s acceleration is directly proportional to the unbalanced force applied and inversely proportional to the mass of the object.

A quick look at the force equation for the Second Law of Motion, tells us several important things.

∑F =ma

First, the total force is equal to the mass of the object times its acceleration (speeding up). That means that if the same amount of force is applied to two objects of different masses, the object with the lower mass will accelerate (speed up) more.

Second, if the mass of an object is increased, it will take more force to obtain the same amount of acceleration. This is fairly easy to visualize. For example, try pushing a chair and then a heavy desk with the same amount of force. Which will speed up (accelerate) more, the heavy desk or the lighter chair? The chair will accelerate more!

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Newton’s Third Law of Motion: 

For every force, there is an equal and opposite force. 

This is perhaps the easiest of Newton’s Three Laws of Motion to understand. When we push on something, it pushes right back on us. If you run, trip, and fall, not only do you hit the ground with a certain amount of force, but the ground “hits” you back with exactly the same force. Typically, the Earth will hurt you when you fall much more than you will hurt the Earth!

Also, as you stand on the Earth, the Earth pushes back on you in an equal and opposite direction. There are so many real-life examples of the Third Law that we likely take it for granted.

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This slide shows a common application of Newton’s Third Law of Motion. When the swimmer cups his hand and pushes the water backward, he moves in the water in the opposite direction, forward.

Ask Students: Can you think of other examples of Newton’s Third Law? Student answers will vary. Examples may include, a runner’s foot pushing against the track to make them accelerate (move) forward, a rocket taking off, rowing a canoe, and so on.

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Explaining to students that this CELL will explore force and motion. Use the following demonstration and discussion to help explain the role of force in changing motion:

• Hold a book so that it is stationary, a few inches above a desk.
• Refer students to the definition of motion in their Scientist’s Glossary. Ask students: How would you describe the motion of the book? Student answers may vary. The book is stationary.

Motion: how an object is moving

• Tell students that the motion can be described as zero or no speed.
• Ask students to make a prediction: What will happen if I let go of the book? Student answers may vary. The book will fall toward the ground.
• Test the prediction by letting go of the book.

Ask the class whether or not the book had motion when you let it go. Students should indicate that the book had motion – it moved towards the table or floor.

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Explain that as soon as you let go of the book, the book started to speed up (accelerate) towards the ground. Tell students that you know the book speeded up because it had zero speed to begin with. 

• Refer students to their Scientist’s Glossary. Ask students to find the term for a change in motion. That is, when an object speeds up, slows down or changes direction.  Students should locate the term, acceleration.

Acceleration: speeding up, slowing down, or changing direction.

Ask students: What causes the book to accelerate towards the ground? Student answers may vary. The book accelerates towards the ground due to gravity.

• Ask a student to read the term gravity from their Scientist’s Glossary. Reinforce that gravity is a force. Tell students that the force of gravity on an object is what gives an object its weight. 

Gravity: the force that exists between all objects and tends to pull them together.

• Refer students to the definition of weight in their Scientist’s Glossary.

Weight: the force of gravity on an object or substance. 

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Ask students: Could we draw a diagram to show a force? Student answers may vary.

• Explain that forces can be represented or drawn, by the direction and length of an arrow. Draw attention to the slide, which shows the force on the book after you let go and before it hits the desk. To ensure student comprehension of the diagram, ask the following questions:

Point to the arrow and ask students: Which direction is the arrow pointing? Students should indicate that the arrow is pointing downwards.

Ask Students: Which direction is the force of gravity acting? Students should indicate that the force of gravity is pointing downwards.

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Ask Students: If the force of gravity were less than what is shown on the book, how could we show it on the diagram? Students should indicate that the arrow would be shorter. 

Ask students to look at the diagram carefully. Ask Students: How many forces are acting on the falling books? One force, gravity, is acting on the book.

• Explain that there is an unbalanced force on the book. This means that there is no equal force present to act in the opposite direction. The unbalanced force causes the book to accelerate in the direction of the force. In this case, this means that the book will speed up downwards. Ask a student volunteer to read the definitions of unbalanced force and balanced force in their Scientist’s Glossary. 

Balanced forces: when forces are equal and act in opposite directions.

Unbalanced force: when a force does not have an equal force acting in the opposite direction on the same object.

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Tell students that they will now perform a simple activity to explore forces right at their desks.

• Ask students to hold up a book in front of them in the palm of their hand. The book should not move.

Ask Students: Is the force of gravity still acting on the book when you hold it still?  Student answers may vary. Yes, the force of gravity is still present. The force of gravity (weight) is still acting downwards.

Ask students: Why isn’t the book accelerating, or changing its motion, when you hold it in the palm of your hand? Student answers may vary. The hand applies a force upwards that is equal and opposite to the force of gravity.

Next, tell students to put the book down gently on the table.

• Refer to this slide. The figure on the left represents the equal and opposite forces exerted on the book when it is held stationary above the floor.

Explain to students that the force of the hand on the book is called the normal contact force. In this case, the forces are balanced.

• Explain that when the forces on the book are balanced, the book does not accelerate (speed up, slow down or change direction). Instead, the book remains stationary. Explain that when an object is in motion and the forces are balanced, the object remains in motion at the same speed and in the same direction.

• Tell students that normal contact forces exist between themselves and the floor when standing. That is, a normal contact force does not only apply to objects such as books. Refer students to the definition of normal contact force in their Scientist’s Glossary:

The normal contact force is the force that exists between

two objects in contact with one another.

• Tell students to once again lift up the book and hold it stationary in the air.

Ask students: What would happen if there was no force of gravity and we let go of the book? Student answers may vary. The book would remain stationary because no forces would act to accelerate the book.

Ask students: What would happen if there was no force of gravity and we threw the book up in the air?  Student answers may vary. The book would continue moving upwards with the same speed as when we let go of it because there would be no force to slow it down, speed it up or change its direction (because there is no gravity).

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PreLab Activity:

Divide students into five cooperative groups. Students should discuss the questions in Problem 1a through 1c in their Student Data Records. After several minutes, review answers to help students summarize the key ideas about force and motion from the previous activity.

Continue to work through the initial portion of the Student Data Record by working with the class to complete Problem 2. Answering these questions will provide an opportunity to informally assess whether or not students understand that balanced forces will not change motion and that unbalanced forces cause acceleration in the direction of the force.

• Remaining in cooperative groups, direct students’ attention to Problem 2a. 
• Explain that the diagram is similar to the diagram on the board, in that it shows an object and the size and direction of the force acting on it. If necessary, remind students that when one force is shown, the force is unbalanced.
• Work through Problem 2a as a class. Students should record their answers in their Student Data Records.
• Ask students to continue to collaborate as they answer Problems 2b through 2d.
• Review students’ answers. Discuss the sample answers and correct any wrong answers.

Close the pre-lab portion of the Investigation by explaining that students will investigate more about the force of gravity in the Lab. As they do so, they should keep in mind the following questions:

• Can the force of gravity be changed?

• Can the force of gravity be measured?