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**Newton’s Laws of Motion**

Read the lesson title aloud to students.

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**Learning Objectives State Newton’s first law of motion.**

State Newton’s second law of motion. State Newton’s third law of motion. Click to reveal each learning objective. Tell students: Newton’s three laws of motion explain much about the motion around you, but they also help explain motion in the universe. Ask: What is a scientific law? Answer: a statement of what will happen in a given set of circumstances Ask: What other law did Newton develop? Answer: law of universal gravitation Make sure that students understand that at the end of the presentation, they should be able to state Newton’s three laws of motion.

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Warm up 10/30 Define these words inertia

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Isaac Newton “If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone....” —Sir Isaac Newton Tell students that Isaac Newton was a British mathematician and physicist in the 1680s. He published his three laws of motion in his Mathematical Principles of Natural Philosophy book. This quote is from that publication. Lead a discussion about the meaning of the quote. Encourage students to imagine a horse connected to a stone by a rope and the effects each would have on the other. Discuss how the mass of the stone might affect the horse’s motion and the role forces might play in this scenario. Some students may relate the quote to the definition of horsepower. Tell students that this example is not a definition of horsepower. It relates to Newton’s third law of motion, which they will study in this lesson.

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**Newton’s First Law of Motion**

An object at rest will remain at rest unless acted upon by a nonzero net force AND an object moving at a constant velocity will continue moving at a constant velocity unless acted upon by a nonzero net force. Inertia: resistance to change in motion Distribute the worksheets to the students and tell them to use them to take notes on Newton’s laws. They can create a column for each law and add information about the laws in the rows. Place a pen on a desk students can see. Ask: How would you describe the pen’s motion? Answer: It is at rest. Ask: How can its motion change? Answer: A nonzero, or unbalanced, net force acts upon it. Ask: What would happen if a nonzero net force never acts upon this pen? Answer: It will stay at rest. Click to reveal the first part of Newton’s first law. Read the first part of the law aloud. Explain to students that the pen scenario is an example of Newton’s first law. Roll the pen across the desk or across the floor until it stops moving due to friction. Ask: Why did the pen stop rolling? Answer: The force of friction acted in the opposite direction of motion and slowed the pen until it stopped. Ask: What would happen if there were no frictional force or any other force acting on the rolling pen? Answer: The pen would keep rolling until an unbalanced force changed its motion. Click to reveal the second part of Newton’s first law. Read the second part of the law aloud. Tell students that all objects, moving or at rest, resist changes in motion. Resistance to change in motion is called inertia. Newton’s first law of motion is also called the law of inertia. Click to reveal the definition of inertia and the objects. Explain that the blue box will stay at rest and the red box will stay in constant motion in a straight line until external forces act upon them because they have inertia. Tell students to picture a ball rolling down a hill. Ask: What keeps the ball moving downhill? Answer: gravity and inertia Ask: Why does the ball not continuing rolling forever if it has inertia? Answer: Friction between the ball and the ground resists motion and slows the ball to a stop. Constant motion in a straight line At rest

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**Inertia Depends on Mass**

The greater the mass of an object, the greater its inertia and the greater the force required to change its motion. Tell students that mass and inertia are related. Ask a volunteer to read the sentence on the screen. Explain that this idea is like moving an empty backpack and a full backpack. Ask: Which backpack is more difficult to move from rest? Answer: The full backpack is more difficult to move than the empty one because it has more mass and therefore more inertia. Direct students’ attention to the three images. Invite students to the board to rank the skydiver, the soccer ball, and the roller coaster by how much inertia each has and why. A rank of one means it has the most inertia. Click to reveal the correct answers. 2 3 1

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**Newton’s Second Law of Motion**

An object’s acceleration depends on its mass and on the net force acting on it. Si Unit: Newtons (N) Tell students that now they know that an object’s mass affects its motion, but the force acting on an object also affects its motion. Ask: If you increase the force on a moving cart without changing its mass, what will happen to the cart’s motion? Answer: It will speed up, accelerate. Tell students that this is an example of Newton’s second law of motion. Click to reveal the text. Read the text aloud. Explain that this law is usually written as a formula. Click to reveal the first formula. Ask: How can you rearrange this equation to calculate how much force must be applied to an object to accelerate the object at a certain rate? Answer: Net Force = Mass × Acceleration Click to reveal the second formula. Explain that this is the most common form of Newton’s second law.

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**Newton’s Second Law of Motion**

The sled’s acceleration will increase if: more dogs are added to increase the force. passengers are removed to decrease the mass. OR Tell students that the formula is usually abbreviated to F = ma, which they may have seen before. Direct students’ attention to the picture. Ask: Which part of the formula would you change if you added dogs to the team? Assume that the dogs apply force to the sled to accelerate it. Answer: net force Ask: How does changing the “top” number, or numerator, of a fraction affect the value? Answer: It increases the value. Click to reveal the statement. Invite a student to fill in a correct answer on the write-on lines and explain his or her reasoning. Invite a second student to the board to write in a second correct answer. Click to reveal the correct answers. Adding dogs will increase the net force and mass, resulting in an increase in acceleration; removing passengers will decrease the mass, resulting in an increase in acceleration.

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**Newton’s Second Law of Motion**

Practice: What is the net force on a 1,200-kg race car accelerating at a rate of 3 m/s2? Remind students that acceleration is measured in meters per second (m/s2). Mass is measured in kilograms (kg). Newton’s second law shows that force is measured in kilograms times meters per second (kg × m/s2). This unit is also called the newton (N), which is the SI unit of force. One newton is the force required to give a 1-kg mass an acceleration of 1 m/s2. Tell all students to work out the problem in their notes. Invite a volunteer to the board to write the answer and show his or her work. Click to reveal the correct answer. 3,600 N

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**Newton’s Third Law of Motion**

If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. Action Reaction For every action, there is an equal and opposite reaction. Read Newton’s third law of motion aloud. Tell students that this law is often simplified as: “For every action, there is an equal and opposite reaction.” Click to reveal this statement. Ask: What does equal and opposite mean? Answer: The forces have equal strength and they act in opposite directions. Ask: Suppose you hit a nail with a hammer. What is the action force? Answer: The action force is you exerting a downward force on the nail. Ask: What is the reaction force? Answer: The reaction force is the nail exerting an opposite and equal force on the hammer, causing the motion of the hammer to stop. Ask: What are the action and reaction forces involved in jumping on a trampoline? Answer: action force: person jumping on the trampoline; reaction force: trampoline pushing up on the person Direct students’ attention to the photo of the girl on the balance beam. Invite a student to the board to draw arrows and labels to show the action and reaction forces between the gymnast and the balance beam. Click to reveal the correct arrows and labels. Ask: What are some other examples of action and reaction forces? Sample answer: You push on the ground with your foot when you walk; the ground pushes back on your foot, so you move forward.

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**Action-Reaction Forces**

Force on hands Force on ball Force on ball Remind students that two equal forces acting in opposite directions on an object cancel each other out and produce no change in motion. Explain that action and reaction forces do not cancel out because they act on different objects. The gymnast on the previous slide exerts a downward action force on the beam. The beam exerts an equal but opposite upward reaction force on her hands. The action and reaction forces act on different objects. Direct students’ attention to the volleyball players. Explain that both players exert a force on the same object—the volleyball. Each player exerts a force on the ball equal in strength but opposite in direction. The forces on the volleyball are balanced. Ask: What is the motion of the volleyball? Answer: The ball does not move toward one player or the other. Ask: What force that acts on the ball is not labeled in this image? Answer: gravity Ask: Is there a force that cancels out gravity? Answer: no Ask: What would happen to the volleyball if the players were to take their hands away from it? Answer: It would fall to the floor.

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**Laws of Motion in Action**

1. Use one of Newton’s laws to explain why the driver was thrown forward when she braked. Newton’s first law says that objects in motion stay in motion. The car stopped, but the driver’s inertia kept her moving forward. Read the following scenario to students: The driver hates killing bugs. When she saw one coming toward the windshield, she braked suddenly and hoped it would get out of the way. (Sadly, it did not.) When she hit the brakes, she felt that she was thrown forward. Click to reveal the question. Invite a student to the board to write an answer on the write-on lines. Read the student’s answer aloud and discuss any comments and questions the class may have about the student’s response. Click to reveal the correct answer.

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**Laws of Motion in Action**

2. Use one of Newton’s laws to explain how the seat belt stopped the driver’s body from moving too far forward. Newton’s third law says that for every action force, there is an equal and opposite reaction force. The driver’s body exerted an action force on the seat belt. The seat belt exerted a reaction force on the driver that stopped her motion. Read the following scenario to students: The driver always follows all the safety rules of the road. So, of course, she was wearing a seat belt. Fortunately, the seat belt stopped her from hitting the windshield when she braked. Click to reveal the question. Invite a student to the board to write an answer on the write-on lines. Read the student’s answer aloud and discuss any comments and questions the class may have about the student’s response. Click to reveal the correct answer.

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**Laws of Motion in Action**

3. Use one of Newton’s laws to determine how much force the driver will need to exert to make it to her appointment on time. Newton’s second law is represented by the formula F = ma. F = (1,100 kg)(2 m/s2) F = 2,200 N Read the following scenario to students: After the delay from braking for the bug, the driver realized she was late for her appointment. To make it to the appointment on time, she would have to accelerate the 1,100-kg car 2 ms2. Click to reveal the question. Invite a student to the board to write an answer on the write-on lines. Read the student’s answer aloud and discuss any comments and questions the class may have about the student’s response. Click to reveal the correct answer.

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**Patterns in Newtons Laws**

Q: What pattern of motion identifies with the law of inertia? A: objects resist a change in their motion Q: What pattern exists in the law of force and acceleration? A: as force increases, so does acceleration if the mass is constant A: as acceleration increases so does force if the mass is constant A: As mass increases, acceleration decreases if the force is constant A: As mass decreases, acceleration increase if the force is constant

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**Patterns in Newtons Laws**

Q: What pattern exists in the law of action-reaction? A: Every action force has a corresponding reaction force Q: What are the action- reaction forces? A: Whenever one object exerts a force on a second object, the second object exerts an EQUAL and OPPOSITE force on the first object.

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**Student Worksheet Answers**

Remind students that they should have been using their worksheets throughout the lesson to take notes on Newton’s laws of motion. Divide students into groups of three. Assign each group member one law on which to become an expert. Then, have have the groups break up and form new groups based on their law. For example, all of of the first law of motion students will gather in a group. Allow the “law groups” to compare information in their tables. Encourage students to add rows to their tables if others have additional features or characteristics to describe their law. After a while, have the original groups reconvene. Each person will take turns sharing what he or she learned in the expert groups. Every student will update his or her table with any new information. Worksheet Answers: Students’ tables should include information about each of Newton’s laws of motion, including a definition and an example.

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