4.3 NEWTON’S THIRD LAW All forces occur in interaction pairs. tbn2.gstatic.com/images?q=tbn:ANd9GcSCEAv2yl7JcoFX_7JVm6HaIzocEfYw0HkWuI7lUSITPj SZUG7S

4.3 NEWTON’S THIRD LAW Essential Questions: What is Newton’s third law? What is the normal force?

4.3 NEWTON’S THIRD LAW A. What is Newton’s third law? 1. Interaction Pairs a. two forces that are in opposite directions b. have equal magnitudes c. act on different objects

4.3 NEWTON’S THIRD LAW d.When you exert a force on your friend to push him forward(F A on B ), he exerts an equal and opposite force on you (F B on A ), which causes you to move backward. e. The forces F A on B and F B on A are an interaction pair. d%3A140104c33ae96ea3ecd64961f50684eaea941d90ef6967d0a0faf82c%2BIMAGE_T HUMB_POSTCARD%2BIMAGE_THUMB_POSTCARD.1

4.3 NEWTON’S THIRD LAW f. When identifying an interaction pair, remember that they always occur in two different free-body diagrams and they always have the symmetry in subscripts.

4.3 NEWTON’S THIRD LAW 2. Newton’s third law states that all forces come in pairs and that the two forces are equal in strength, opposite in direction, and act on different objects. Newton’s Third Law

4.3 NEWTON’S THIRD LAW Practice Problems 28. You lift a relatively light bowling ball with your hand, accelerating it upward. What are the forces on the ball? What forces does the ball exert? What objects are these forces exerted on? Version-2.jpg

4.3 NEWTON’S THIRD LAW Practice Problems 29. A brick falls from a construction scaffold. Identify any forces acting on the brick. Also identify any forces the brick exerts and the objects on which these forces are exerted. (Air resistance may be ignored.)

4.3 NEWTON’S THIRD LAW Practice Problems 30. A suitcase sits on a stationary airport luggage cart, as in Figure 16. Draw a free-body diagram for each object and specifically indicate any interaction pairs between the two.

4.3 NEWTON’S THIRD LAW Practice Problems 31. You toss a ball up in the air. Draw a free-body diagram for the ball after it has lost contact with your hand but while it is still moving upward. Identify any forces acting on the ball. Also identify any forces that the ball exerts and the objects on which these forces are exerted. Assume that air resistance is negligible.

4.3 NEWTON’S THIRD LAW 2.What is the normal force? a. Tension is the force exerted by a string or rope. b. At any point in a rope, the tension forces are pulling equally in both directions.

4.3 NEWTON’S THIRD LAW c. Normal force is the perpendicular contact force exerted by a surface on another object. d. The normal force is not always equal to the object’s weight, as shown below. e. Finding the normal force is important when calculating the effects of friction.

4.3 NEWTON’S THIRD LAW Practice Problems 32. Diego and Mika are trying to fix a tire on Diego’s car, but they are having trouble getting the tire loose. When they pull together in the same direction, Mika with a force of 23 N and Diego with a force of 31 N, they just barely get the tire to move off the wheel. What is the magnitude of the force between the tire and the wheel?

4.3 NEWTON’S THIRD LAW Practice Problems 33. You are helping to repair a roof by loading equipment into a bucket that workers hoist to the rooftop. If the rope is guaranteed not to break as long as the tension does not exceed 450 N and you fill the bucket until it has a mass of 42 kg, what is the greatest acceleration that the workers can give the bucket as they pull it to the roof?

4.3 NEWTON’S THIRD LAW 4.3 Assignment: p. 111 #34-38 p. 115 #55-64