3/17/2018. Interacting Objects. Interacting Objects

Transcription

1 Example 0 - Iris drags a sled containing her baby brother across the floor at a constant speed. She pulls the sled at a 20 degree above the horizontal. Draw a FBD and write out N2L for both x and y directions. Slide 6-85 Interacting Objects When a bat hits a ball, the ball exerts a force on the bat. When you pull someone with a rope in a tug-ofwar, that person pulls back on you. The bat and the ball are interacting with each other. When your chair pushes up on you (the normal force), you push down on the chair. All forces come in pairs, called action/reaction pairs. These forces occur simultaneously, and we cannot say which is the action and which is the reaction. Slide 7-22 Interacting Objects If object A exerts a force on object B, then object B exerts a force on object A. The pair of forces, as shown, is called an action/reaction pair. Slide

2 Newton s Third Law Every force occurs as one member of an action/reaction pair of forces. The two members of an action/reaction pair act on two different objects. The two members of an action/reaction pair are the same in nature, equal in magnitude, but opposite in direction: A catchy phrase, which is less precise, is: For every action there is an equal but opposite reaction. Slide 7-37 Car B is stopped for a red light. Car A, which has the same mass as car B, doesn t see the red light and runs into the back of B. Which of the following statements is true? A. B exerts a force on A but A doesn t exert a force on B. B. B exerts a larger force on A than A exerts on B. C. B exerts the same amount of force on A as A exerts on B. D. A exerts a larger force on B than B exerts on A. A small car is pushing a larger truck that has a dead battery. The mass of the truck is larger than the mass of the car. Which of the following statements is true? A. The car exerts a force on the truck but the truck doesn t exert a force on the car. B. The car exerts a larger force on the truck than the truck exerts on the car. C. The car exerts the same amount of force on the truck as the truck exerts on the car. D. The truck exerts a larger force on the car than the car exerts on the truck. 2

3 QuickCheck 7.4 A car is parked at rest on a horizontal road. The upward force of the road on the car (the normal force) is the same size as the downward pull of gravity A. Because they are an action/reaction pair. B. Because of Newton s first law. C. Both A and B. D. Neither A nor B. Some other reason. Slide 7-52 Interacting Objects Long-range forces, such as gravity, also come in pairs. If you release a ball, it falls because the earth s gravity exerts a downward force. At the same time, the ball pulls upward on the earth with a force. The ocean tides are an indication of the long-range gravitational interaction of the earth and moon. Slide 7-24 Objects, Systems, and the Environment In this chapter we extend the particle model to include two or more objects that interact. The figure shows three objects interacting via action/reaction pairs of forces. The forces can be given labels, such as. and Slide

4 Objects, Systems, and the Environment Define the system as those objects whose motion we want to analyze. Define the environment as objects external to the system. Slide 7-27 Objects, Systems, and the Environment The figure shows a new kind of diagram, an interaction diagram. The objects of the system are in a box. Interactions are represented by lines connecting the objects. Interactions with objects in the environment are called external forces. Slide 7-28 Example 1 - A man pushes a crate across a rough floor at constant speed. Draw an interaction diagram for the system, draw FBD of each object and connect N3L interacting pairs. Slide

5 Example 2 - Two strong magnets are on opposite sides of a small table. Magnetism keeps the lower magnet from falling. Draw an interaction diagram for the system, draw FBD of each object and connect N3L interacting pairs. Slide 7-43 What, if anything, is wrong with the force diagram for a bicycle that is accelerating toward the right? A. It does not draw each object separately. B. It draws interactions between one body and itself. C. It fails to identify all relevant forces. D. Nothing is wrong. QuickCheck 7.1 A mosquito runs head-on into a truck. Splat! Which is true during the collision? A. The mosquito exerts more force on the truck than the truck exerts on the mosquito. B. The truck exerts more force on the mosquito than the mosquito exerts on the truck. C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito. D. The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck. Slide

6 QuickCheck 7.2 A mosquito runs head-on into a truck. Which is true during the collision? A. The magnitude of the mosquito s acceleration is larger than that of the truck. B. The magnitude of the truck s acceleration is larger than that of the mosquito. C. The magnitude of the mosquito s acceleration is the same as that of the truck. D. The truck accelerates but the mosquito does not. Slide 7-41 Example 3 - Blocks with masses of 1 kg, 2 kg, and 3 kg are lined up in a row on a frictionless table. All three are pushed forward by a 12 N force applied to the 1 kg block. What is the acceleration of the system and the individual forces between blocks? Slide 7-43 Boxes A and B are sliding to the right across a frictionless table. The hand H is slowing them down. The mass of A is larger than the mass of B. Rank in order, from largest to smallest, the horizontal forces on A, B, and H. A. F B on H = F H on B = F A on B = F B on A B. F B on H = F H on B > F A on B = F B on A C. F B on H = F H on B < F A on B = F B on A D. F H on B = F H on A > F A on B 6