In Investigation Three, students will study the important force of friction. In doing so, they will return to Newton’s first Law of Motion, which states that a stationary object will remain stationary and a moving object will continue moving at the same speed and in the same direction unless another force acts on them.

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This slide reminds students of Newton’s First Law of Motion. In the picture, the woman is skating toward the edge of a cliff! Unless another force acts on her, she will continue right off the edge!

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Fortunately, someone arrives at the last moment and applies a force in the opposite direction of her movement.

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This slide is included simply to remind students that acceleration is a change in velocity. Also, acceleration can be either positive (+) or negative (-). Acceleration is positive if an object is increasing its velocity (speed in a certain direction). Acceleration is negative if the object is slowing down with decreasing velocity. 

Ask students: What can you say about the acceleration at each of the three positions shown on this slide? How do you know? Students should answer that the acceleration is zero (0) in the first position, positive (+) as the bicycle speeds up going downhill, and negative in the third position as the rider coasts to an eventual stop.

Ask students: What if you ride your bicycle really, really fast and keep up that speed for 2 minutes What is the acceleration over the two minutes if you ride in a straight line on a trail? Students should answer that it doesn’t matter how fast you are moving, if you do not slow down, speed up, or change directions your acceleration will be zero.

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Begin the Investigation with a brief review of the concepts encountered in Investigations One and Two. The following questions are intended to guide the discussion.

Ask the class: What have we learned about forces in the last two Investigations? Student answers may vary. Examples include that when forces are balanced, an object either remains stationary or remains in motion at a constant speed; that unbalanced forces produce a change in motion, otherwise termed acceleration; that gravity is a force; that the force of gravity acting on an object is equal to its weight; and that the force of gravity acting on an object (its weight) can be measured in Newtons.

• Record students’ statements on the board, underneath the word force.

Ask students: Other than gravity, can you think of another force that causes our motion to change? Student answers may vary. Guide students to think of the word friction by giving examples of motion such as sliding on ice, or a car skidding to a halt on the road.

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Explain that this Investigation is all about the force of friction. 

Ask students: What should we do when getting ready to learn more about a topic? Students should recognize the need to Recall what they already know about that topic.

The following questions serve as a guide in eliciting prior knowledge of the concept of friction:

Ask students: What is friction? Student answers may vary. Students may know that friction is a force and hence can change motion (cause acceleration).

Ask students: Can you give an example of a situation in which friction plays a role? Student answers may vary. Friction may play an important role in slowing down or stopping an object. For example, the braking system in a car uses the force of friction of brakes against the wheel, to slow and stop the car. In other cases, friction may cause injuries. Students who have had experienced falling from a bicycle or skateboard may have experienced cuts and scrapes to their skin because of the force of friction between their body and the ground.

Ask students: How do our examples show that friction is a force? Students should realize that in each example, a motion was changed. An object either sped up, slowed down, or changed direction. Since forces create this effect, friction is a force.

Quick Activity:

Have students rub their hands together fast for about 10 seconds. Ask: What do you feel? Students should indicate that they feel frictional force between their palms as they rub them together. They may indicate that their hands feel warm.

Tell students that the heat energy they feel is a product of friction. Remind students that with the force of friction, often objects feel hot to the touch. However, in their experiments, the objects will not become warm to the touch. Therefore, frictional force can, but does not always yield, a noticeable increase in temperature.

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Use the following in-class activity to help students connect the concept of friction to other concepts previously learned, such as balanced and unbalanced forces, and acceleration.

1. Place a book on a table, and request that students do the same. Ask students: What do we know about the forces acting on this book? Is the book accelerating? The forces acting on the book are balanced. The book is not accelerating. 

2. Ask students: What forces are acting on this book, and what direction do they act? Students should indicate the force of gravity acting downwards and the normal force acting upwards.

3. If necessary, refer students to the appropriate definitions in their Scientist’s Glossary (included in the slide).

4. Remind students that, in the diagram, the forces are balanced.

5. Ask students: Is the force of friction present in this diagram? There is no frictional force yet because no force is acting against the side of the book.

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(Continued from the previous slide)

6. Direct students’ attention back to the demonstration books on the tables.

7. Apply a small force to the side of the book so that it still remains stationary. Tell students to do the same. 

Ask students: What force do you feel that opposes the force you applied? Student answers may vary.

8. Explain to students that the force that opposed changing motion was a frictional force between the table and the book. Show students the Forces applied to a stationary book in this slide. Explain to students that the push force and friction force must have been balanced (equal and opposite) because the book did not accelerate (i.e. it remained stationary).

9. Ask students: Was the push force greater than the frictional force? No. If it had been greater than the frictional force, the book would have begun to move.

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(Continued from the previous slide)

10. Direct students’ attention back to the books on their desks, and to the instructor’s desk in particular. Push the book so that it begins to slide and stops before the end of the table. Tell the students to do the same, and watch the movement of their books, and feel the force they produced with their hands.

11. Ask students: In order to make the book move, what can we say about the difference between the push force and the friction force? The push force must have been greater than the friction force.

12. Ask students: Were the forces balanced or unbalanced? How do you know? The forces were unbalanced because the book began to accelerate (change its motion).

13. Show students the Push forces to start to move the book in the illustration on this slide. The diagram represents the forces on the book as it was (accelerated) speeded up by the push force.

14. Ask students: Describe the motion of the book after you pushed it (you stopped touching it). Student answers may vary. The book accelerated, it slowed down.

15. Ask students: What force acted on the book to slow it down? Frictional force.

16. Direct students’ attention to the slide. The slide uses arrows to denote both the direction and relative magnitude of the forces that are acting on the book. Hence the slide depicts the forces present as the book is slowed down by friction. Note that the direction of friction is the opposite of the direction of motion.

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Follow up the in-class activity by completing Problem 1 in their Student Data Record.

Ask students to predict: What would happen to acceleration if the frictional force on an object is increased? Student answers may vary. Students may predict that if the frictional force on a book were increased, it may come to a stop sooner.

• Direct students to complete Problem 1 in their Student Data Record.

Ask students: How can frictional force be increased? Student answers may vary. Students may indicate that a different surface type for the object, the desk, or both, may increase frictional force. Students may also include other variables, such as speed or surface area. Accept all answers at this point.

• Direct students to write their list of ways to increase frictional force in Problem 2 of their Student Data Record.

Close this portion of the Investigation by informing students that they will be conducting experiments to learn more about frictional force in the Lab. As they do so, they should keep in mind the following question:

• How does surface type affect friction?

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