This is the final regular Investigation in the LabLearner CELL Earth’s Forces. In Investigation Five, students will study the effect of applied force and mass on the acceleration of objects.

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A. Begin the Investigation with a review of concepts examined thus far.

Note to Teacher: Should the instructor wish to use the following questions as an informal quiz, divide the class into cooperative groups and offer a question to each group in turn.

Tell students that they have learned a great deal about forces and motion. Encourage students to recognize what they have learned by posing the following questions to cooperative groups of students.

1. Stand up straight. Ask students: What force is acting on me? The force of gravity is acting on the instructor.

2. Ask students: How is my weight related to the force of gravity that acts on me? Students should recall that weight in Newtons is equal to the force of gravity that acts on a body or an object.

3. Hold an object such as a book in the air. Ask students: Are the forces on this object balanced or unbalanced? Students should indicate that the forces are balanced since the object is stationary.

4. Ask students: What would happen if the forces become unbalanced? Students should indicate that the object will accelerate if the forces become unbalanced. 

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5. Ask students: What force acts when an object moves along a surface? Students should recall that friction acts when an object moves along a surface.

6. Push an object along a desk surface. Ask students: In which direction is friction acting? Students should indicate that friction is acting in the opposite direction to the direction of motion of the object.

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B. Continue the investigation with a review of the effect of balanced and unbalanced forces on the motion of an object.

1. Push a book along a table at a constant speed. 

2. Invite students to do the same with an object at their desks. Then, Ask students: If an object is moving at a constant speed of 10 m/s, what will happen to the object if the forces remain balanced? Students should indicate the object will continue to move in the same direction at a constant speed of 10 m/s.

3. Invite students to focus on the book on your demonstration table. Remind students that the forces on this stationary object are balanced. Ask students: What forces are acting on this book? What forces are acting on the table? Students should recognize that gravity is acting on the book and that the book is exerting a force on the table (which is also affected by gravity).

4. Ask students: Why isn’t the book falling? Students should indicate that the table is exerting a force on the book.

5. Inform students that, to begin with, the forces on the stationary object are balanced. Ask students: If the upward force is increased, what will happen to the object? Students should indicate the object will start to move (accelerate) upwards.

6. Place the book on the table for students to view. Push the book gently to the right, so that it does not move. Invite students to do the same with their objects. Ask students: Is there a force acting to the left? Students should indicate that the force of friction is acting to the left. That is, the force of friction is acting in the direction opposite to the direction of motion.

7. To begin with, the forces on a stationary object are balanced. Ask students: If the force to the left is decreased, what will happen to the object? Students should indicate the object will start to move (accelerate) towards the left.

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C. Conduct an in-class activity in which students move objects vertically in order to demonstrate the following diagrams. 

1. Refer to the figure on the slide, focusing first on drawing a. Ask students: If the object is stationary, what changes could be made to the upward force on the object to make it start moving upwards?  What changes could be made to the downward force on the object to make it start moving upwards? Students should indicate to increase the upward force or decrease the downward force.

2. Illustrate the answers to the problem above by explaining drawings b and c on this slide. Point out that it is not necessary to remove the downward force, only to make the forces unbalanced in favor of the upward force. Emphasize the unbalanced nature of the forces, as shown by the differing length of arrows.

3. Guide students to hold their book or other object stationary. Ask students to accelerate the book upwards, and as they do so, match the motion to the diagram by pointing to the differences between the upward and downward forces. 

Ask students to describe what they do in order to move the book upwards. Students should indicate that they push the book upwards. 

Emphasize that the push they exert is what is represented on the diagram as the larger arrow, or force. 

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4. Refer to drawing a on this slide. Ask students: If the object is stationary, what changes could be made to the forces on the object to make it start moving downwards? Students should indicate to increase the downward force or decrease the upward force.

5. Illustrate the answers to the problem above by explaining the drawings b and c on the slide. Point out that it is not necessary to remove the upward force, only to make the forces unbalanced in favor of the downward force.

6. Guide students to hold their book or other object stationary. Ask students to accelerate the book downwards, and as they do so, match the motion to the diagram by pointing to the differences between the upward and downward forces. 

Ask students to describe what they do in order to move the book upwards. Students should indicate that they stop holding the book with the same upward force. 

Emphasize that the decrease in upward force they exert is what is represented on the diagram as the larger arrow, or force downwards.

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7. Refer to the figure on the slide, focusing first on drawing a. Ask students: If the object is moving downwards at a constant speed of 2 m/s, what changes could be made to the forces on the object to make it slow down? Students should say increase the upward force or decrease the downward force.

8. Illustrate the answers to the problem above by explaining drawings b and c on this slide. Point out that it is not necessary to remove the downward force, only to make the forces unbalanced in favor of the upward force.

9. Guide students to hold their book or other object and to begin to lower it at a constant speed. Ask students to accelerate the book and to begin to slow it down. Match the motion to the diagram by pointing to the differences between the upward and downward forces. Emphasize that to slow the book or object down, they must increase the upward force. 

Ask students to describe what they do in order to move the book upwards. Students should indicate that they begin to exert a force on the book to slow it down. 

Encourage students to repeat the exercise at slightly different speeds, as with higher speeds students are more likely to notice the effect. 

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10. Refer to the figure on the slide, focusing first on drawing a. Ask students: If the object in drawing a is moving upwards at a constant speed of 10 m/s, what changes could be made to the forces on the object to make it slow down? Students should say increase the downward force or decrease the upward force.

11. Illustrate the answers to the problem above by explaining drawings b and c on this slide. Point out that it is not necessary to remove the upward force, only to make the forces unbalanced in favor of the downward force.

12. Guide students to hold their book or other object and to begin to raise it at a constant speed by accelerating the book then beginning to slow it down to a stop. Match the motion to the diagram by pointing to the differences between the upward and downward forces. Emphasize that to slow the book or object down, they must increase the downward force, which can be achieved by decreasing the upward force. 

Ask students to describe what they do in order to move the book. Students should indicate that they decrease the push force on the book in order to slow it down when it is moving upwards.  

Encourage students to repeat the exercise at slightly different speeds, as with higher speeds students are more likely to notice the effect. 

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D. Conduct an in-class activity in which students move objects horizontally in order to demonstrate the following diagrams on this slide. 

1. Focusing first on drawing a, Ask students: If the object in drawing a is stationary, what changes could be made to the forces on the object to make it starts moving to the right? Students should say increase the force to the right or decrease the force to the left.

2. Illustrate the answers to the problem above by explaining drawings b and c on this slide. Point out that it is not necessary to remove the force to the left, only to make the forces unbalanced in favor of the force to the right.

3. Guide students to hold their book or other object and to begin to move it at a constant speed to the right. Match the motion to the diagram by pointing to the differences between the force acting to the left and the force acting to the right. 

Ask students to describe what they do in order to move the book. Students should indicate that they increase the push force on the book to the right in order to accelerate the book. Students may indicate that once the book is moving, a lesser force is required to move the book to the right at a constant speed.  

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4. Focusing first on drawing a, Ask students: If the object is stationary, what changes could be made to the forces on the object to make it start moving to the left? Students should say increase the force to the left or decrease the force to the right.

5. Illustrate the answers to the problem above by explaining drawings b and c on this slide. Point out that it is not necessary to remove the force to the left, only to make the forces unbalanced in favor of the force to the left.

6. Guide students to hold their book or other object and to begin to move it at a constant speed to the left. Match the motion to the diagram by pointing to the differences between the force acting to the left and the force acting to the right. 

Ask students to describe what they do in order to move the book. Students should indicate that they increase the push force on the book to the left in order to accelerate the book. Students may indicate that once the book is moving, a lesser force is required to move the book to the left at a constant speed.  

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7. Focusing first on drawing a, Ask students: If the object is moving at 5 m/s to the right, what changes could be made to the forces on the object to make it slow down? Students should say increase the force to the left or decrease the force to the right.

8. Illustrate the answers to the problem above by explaining drawings b and c on this slide. Point out that it is not necessary to remove the force to the left, only to make the forces unbalanced in favor of the force to the right.

9. Guide students to place their book or another object on a table and to begin to move it at a constant speed to the right. Direct students to try to slow the object down. Match the motion to the diagram by pointing to the differences between the force acting to the left and the force acting to the right.

Ask students to describe what they do in order to move the book. Students should indicate that they decrease the push force on the book to the right in order to accelerate the book (slow it down).  

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10. Focusing first on drawing a, Ask students: If the object is moving at 3 m/s to the left, what changes could be made to the forces on the object to make it slow down? Students should say increase the force to the right or decrease the force to the left.

11. Illustrate the answers to the problem above by explaining drawings b and c on this slide. Point out that it is not necessary to remove the force to the left, only to make the forces unbalanced in favor of the force to the right.

12. Guide students to place their book or another object on a table and to begin to move it at a constant speed to the left. Direct students to try to slow the object down. Match the motion to the diagram by pointing to the differences between the force acting to the left and the force acting to the right. 

Ask students to describe what they do in order to move the book. Students should indicate that they decrease the push force on the book to the left in order to accelerate the book (slow it down).  

• • • Close the lesson by explaining that in the Lab, students will conduct experiments that will allow them to explore the forces required to accelerate objects of different masses.  

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