3 Forces and Newton’s Laws 2 Newton’s Laws of Motion

MAIN IDEAWrite the Main Idea for this lesson.

Recall the definition of the Review Vocabulary term.

acceleration

Read the definitions below, then write the key term for each one in the left column.

states that when one object exerts a force on a second object, the second object exerts a force on the first that is equal in strength and opposite in direction

is the tendency of an object to resist any change in its motion

states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced force acts on it

“The acceleration of an object is in the same direction as the net force on the object, and the acceleration can be calculated from the equation a = Fnet/m.”

Use a dictionary to define the term period.

period

REVIEW VOCABULARY

acceleration

NEW VOCABULARY

ACADEMIC VOCABULARY

period

4(A), 4(C), 4(D)

Science Notebook • Forces and Newton’s Laws 38

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Summarize Newton’s first law of motion by telling how an object in motion’s inertia is changed and how an object at rest is affected.

Object in motion

Object at rest

Model a rock being thrown at a wall and a car crashing into the wall.

Predict which object will do more damage, and support your answer by using the concept of inertia.

Analyze the forces on a hockey puck sinking through water. Draw a force diagram for the puck in the water.

2 Newton’s Laws of Motion (continued)

Student Edition, pp. 80–81

Reading Essentials, pp. 48–49

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Summarize Newton’s second law of motion in your own words.

GET IT? Identify You apply a force of 2 N to a toy car and to a real car. Which car has the greater acceleration? Why?

Complete the concept map with the 3 physical properties of an object that are related by Newton’s second law of motion.

Organize the 3 variables related by Newton’s second law in the table. Show equations to find each variable if you know the values of the other two variables.

Newton’s Second Law of MotionUnknown Variable

Known Variables

Equations

Acceleration

Net force

Mass

2 Newton’s Laws of Motion (continued)

Student Edition, pp. 81–83

Reading Essentials, pp. 49–50

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2 Newton’s Laws of Motion (continued)

Summarize Newton’s third law of motion in your own words.

Predict the corresponding reaction for each action.

Action Reaction

A high-jumper lands on a mat.

A fisherman tosses an anchor away from his boat.

An airplane’s jet engine pushes air toward the back of theairplane.

GET IT? Explain why the action and reaction forces do not cancel.

Student Edition, pp. 84–85

Reading Essentials, p. 50

SUMMARIZE ITSummarize the relationship between a moving object’s mass, its inertia, and the forces acting on it.

Science Notebook • Forces and Newton’s Laws 41

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2 Newton’s Laws of Motion (continued)

REVIEW IT !

15. MAINIDEA Describe Use Newton’s laws of motion to describe what happens when you kick a soccer ball.

16. Explain why Newton’s fi rst law of motion is sometimes called the law of inertia.

17. Explain why an object with a smaller mass has a larger acceleration than an object with a larger mass if the same force acts on each.

18. Explain why an object with a smaller mass has a larger acceleration than an object with a larger mass if the same force acts on each.

19. Think Critically You push a book across a table. The book moves at a constant speed, but you do not move. Identify all of the forces on you. Then, identify all of the forces on the book.

Science Notebook • Forces and Newton’s Laws 42

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2 Newton’s Laws of Motion (continued)

20. Calculate Acceleration A student pushes on a 5-kg box with a force of 20 N forward. The force of sliding friction is 10 N backward. What is the acceleration of the box?

21. Calculate Mass You push yourself on a skateboard with a force of 30 N east and accelerate at 0.5 m/s2 east. Find the mass of the skateboard if your mass is 58 kg.

Science Notebook • Forces and Newton’s Laws 43

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