Newton’s Second and Third Law October 29, 2015

Bell Work Hannah just finished building a house of cards that stands four-stories high. She is worried that it will fall down. Which of the following statements is definitely true? A house of cards is too fragile and must fall down eventually. If Hannah adds another card to the house of cards, then it will fall down. If no unbalanced force acts upon the house of cards, then it will remain standing forever. As long as nobody touches the house of cards, it will remain standing. Explain your reasoning.

Learning Target Students will be able to identify the acceleration and velocity from a graphical representation. Students will be able to recognize that forces act in pairs and Students will apply Newton’s third law to examples to demonstrate comprehension.

Newton’s Third Law

Review Newton's First Law tells you what happens to an object if the net force on it is zero.  Newton's Second Law tells you what happens to an object if the net force on it is not zero. What is left for Newton's Third Law to tell you? Newton's First and Second Laws tell you what forces do. Newton's Third Law tells you what forces are.

Newton’s Third Law States For every action there is an equal and opposite reaction.

"action...reaction" means that forces always occur in pairs. A force is never a single occurrence Either force can be labeled the action or reaction-it depends on perspective.

Don’t Be Confused by Cause and Effect Not Newton’s Third Law People associate "action" and "reaction" with "first an action, then a reaction" as in, first Suzie smacks Johnnie (action) then Johnny says "Mommy! Suzie hit me!" (reaction). That is NOT what is going on here! The action and reaction forces exist at the same time.

Examples of Newton’s Third Law When you push on a wall the wall pushes back on you.  When you walk on the ground, you push the ground and the ground pushes you. When the bat hits the ball the ball hits the bat.  When your finger touches your nose your nose touches your finger. When you push on a table the table pushes on you

What is meant by equal and opposite? "equal" means two things: Both forces are exactly the same size. They are equal in magnitude. Both forces exist at exactly the same time. They both start at exactly the same instant, and they both stop at exactly the same instant. They are equal in time. "opposite" means that the two forces always act in opposite directions - exactly 180o apart.