A force is always part of a mutual action that involves another ..., Lecture notes of Law

Download A force is always part of a mutual action that involves another ... and more Lecture notes Law in PDF only on Docsity! 1 7 Newton’s Third Law of Motion–Action and Reaction For every force, there is an equal and opposite force. 7 Newton’s Third Law of Motion–Action and Reaction A force is always part of a mutual action that involves another force. 7.1 Forces and Interactions 7 Newton’s Third Law of Motion–Action and Reaction In the simplest sense, a force is a push or a pull. A mutual action is an interaction between one thing and another. 7.1 Forces and Interactions 2 7 Newton’s Third Law of Motion–Action and Reaction When you push on the wall, the wall pushes on you. 7.1 Forces and Interactions 7 Newton’s Third Law of Motion–Action and Reaction The interaction that drives the nail is the same as the one that halts the hammer. 7.1 Forces and Interactions 7 Newton’s Third Law of Motion–Action and Reaction A hammer exerts a force on the nail and drives it into a board. • There must also be a force exerted on the hammer to halt it in the process. • Newton reasoned that while the hammer exerts a force on the nail, the nail exerts a force on the hammer. • In the interaction, there are a pair of forces, one acting on the nail and the other acting on the hammer. 7.1 Forces and Interactions 5 7 Newton’s Third Law of Motion–Action and Reaction In every interaction, the forces always occur in pairs. • You push against the floor, and the floor simultaneously pushes against you. • The tires of a car interact with the road to produce the car’s motion. The tires push against the road, and the road simultaneously pushes back on the tires. • When swimming, you push the water backward, and the water pushes you forward. 7.2 Newton’s Third Law 7 Newton’s Third Law of Motion–Action and Reaction The interactions in these examples depend on friction. A person trying to walk on ice, where friction is minimal, may not be able to exert an action force against the ice. Without the action force there cannot be a reaction force, and thus there is no resulting forward motion. 7.2 Newton’s Third Law 7 Newton’s Third Law of Motion–Action and Reaction When the girl jumps to shore, the boat moves backward. 7.2 Newton’s Third Law 6 7 Newton’s Third Law of Motion–Action and Reaction The dog wags the tail and the tail wags the dog. 7.2 Newton’s Third Law 7 Newton’s Third Law of Motion–Action and Reaction What happens when an object exerts a force on another object? 7.2 Newton’s Third Law 7 Newton’s Third Law of Motion–Action and Reaction To identify a pair of action-reaction forces, first identify the interacting objects A and B, and if the action is A on B, the reaction is B on A. 7.3 Identifying Action and Reaction 7 7 Newton’s Third Law of Motion–Action and Reaction There is a simple recipe for treating action and reaction forces: • First identify the interaction. Let’s say one object, A, interacts with another object, B. • The action and reaction forces are stated in the form: Action: Object A exerts a force on object B. Reaction: Object B exerts a force on object A. 7.3 Identifying Action and Reaction 7 Newton’s Third Law of Motion–Action and Reaction Sometimes the identity of the pair of action and reaction forces in an interaction is not immediately obvious. • For example, what are the action and reaction forces in the case of a falling boulder? • If we call the action Earth exerting a force on the boulder, then the reaction is the boulder simultaneously exerting a force on Earth. 7.3 Identifying Action and Reaction 7 Newton’s Third Law of Motion–Action and Reaction When action is A exerts force on B, the reaction is simply B exerts force on A. 7.3 Identifying Action and Reaction 10 7 Newton’s Third Law of Motion–Action and Reaction In the interaction between the boulder and Earth, the boulder pulls up on Earth with as much force as Earth pulls down on the boulder. • The forces are equal in strength and opposite in direction. • The boulder falls to Earth and Earth falls to the boulder, but the distance Earth falls is much less. 7.4 Action and Reaction on Different Masses 7 Newton’s Third Law of Motion–Action and Reaction Although the pair of forces between the boulder and Earth is the same, the masses are quite unequal. Acceleration is not only proportional to the net force, but it is also inversely proportional to the mass. Because Earth has a huge mass, we don’t sense its infinitesimally small acceleration. 7.4 Action and Reaction on Different Masses 7 Newton’s Third Law of Motion–Action and Reaction Force and Mass When a cannon is fired, there is an interaction between the cannon and the cannonball. • The force the cannon exerts on the cannonball is exactly equal and opposite to the force the cannonball exerts on the cannon. • You might expect the cannon to kick more than it does. • The cannonball moves so fast compared with the cannon. • According to Newton’s second law, we must also consider the masses. 7.4 Action and Reaction on Different Masses 11 7 Newton’s Third Law of Motion–Action and Reaction The cannonball undergoes more acceleration than the cannon because its mass is much smaller. 7.4 Action and Reaction on Different Masses 7 Newton’s Third Law of Motion–Action and Reaction F represents both the action and reaction forces; m (large), the mass of the cannon; and m (small), the mass of the cannonball. Do you see why the change in the velocity of the cannonball is greater compared with the change in velocity of the cannon? 7.4 Action and Reaction on Different Masses 7 Newton’s Third Law of Motion–Action and Reaction We can extend the basic idea of a cannon recoiling from the cannonball it launches to understand rocket propulsion. 7.4 Action and Reaction on Different Masses 12 7 Newton’s Third Law of Motion–Action and Reaction The balloon recoils from the escaping air and climbs upward. 7.4 Action and Reaction on Different Masses 7 Newton’s Third Law of Motion–Action and Reaction If a balloon is released and allowed to move, it accelerates as the air comes out. A rocket accelerates in much the same way—it continually recoils from the exhaust gases ejected from its engine. Each molecule of exhaust gas acts like a tiny molecular cannonball shot downward from the rocket. 7.4 Action and Reaction on Different Masses 7 Newton’s Third Law of Motion–Action and Reaction A common misconception is that a rocket is propelled by the impact of exhaust gases against the atmosphere. Both the rocket and recoiling cannon accelerate because of the reaction forces created by the “cannonballs” they fire— air or no air. In fact, rockets work better above the atmosphere where there is no air resistance. 7.4 Action and Reaction on Different Masses 15 7 Newton’s Third Law of Motion–Action and Reaction Action and reaction forces do not cancel each other when either of the forces is external to the system being considered. 7.5 Defining Systems 7 Newton’s Third Law of Motion–Action and Reaction Since action and reaction forces are equal and opposite, why don’t they cancel to zero? To answer this question, we must consider the system involved. 7.5 Defining Systems 7 Newton’s Third Law of Motion–Action and Reaction A force acts on the orange, and the orange accelerates to the right. The dashed line surrounding the orange encloses and defines the system. 7.5 Defining Systems 16 7 Newton’s Third Law of Motion–Action and Reaction The vector that pokes outside the dashed line represents an external force on the system. The system (that is, the orange) accelerates in accord with Newton’s second law. 7.5 Defining Systems 7 Newton’s Third Law of Motion–Action and Reaction The force on the orange, provided by the apple, is not cancelled by the reaction force on the apple. The orange still accelerates. 7.5 Defining Systems 7 Newton’s Third Law of Motion–Action and Reaction The force is provided by an apple, which doesn’t change our analysis. The apple is outside the system. The fact that the orange simultaneously exerts a force on the apple, which is external to the system, may affect the apple (another system), but not the orange. You can’t cancel a force on the orange with a force on the apple. So in this case the action and reaction forces don’t cancel. 7.5 Defining Systems 17 7 Newton’s Third Law of Motion–Action and Reaction a. Action and reaction forces cancel. 7.5 Defining Systems 7 Newton’s Third Law of Motion–Action and Reaction a. Action and reaction forces cancel. b. When the floor pushes on the apple (reaction to the apple’s push on the floor), the orange-apple system accelerates. 7.5 Defining Systems 7 Newton’s Third Law of Motion–Action and Reaction When the force pair is internal to the orange-apple system, the forces do cancel each other. They play no role in accelerating the system. A force external to the system is needed for acceleration. • When the apple pushes against the floor, the floor simultaneously pushes on the apple—an external force on the system. • The system accelerates to the right. 7.5 Defining Systems 20 7 Newton’s Third Law of Motion–Action and Reaction Why don’t action-reaction forces cancel each other? 7.5 Defining Systems 7 Newton’s Third Law of Motion–Action and Reaction If the horse in the horse-cart system pushes the ground with a greater force than it pulls on the cart, there is a net force on the horse, and the horse-cart system accelerates. 7.6 The Horse-Cart Problem 7 Newton’s Third Law of Motion–Action and Reaction All the pairs of forces that act on the horse and cart are shown. The acceleration of the horse-cart system is due to the net force F – f. 7.6 The Horse-Cart Problem 21 7 Newton’s Third Law of Motion–Action and Reaction Will the horse’s pull on the cart be canceled by the opposite and equal pull by the cart on the horse, thus making acceleration impossible? From the farmer’s point of view, the only concern is with the force that is exerted on the cart system. • The net force on the cart, divided by the mass of the cart, is the acceleration. • The farmer doesn’t care about the reaction on the horse. 7.6 The Horse-Cart Problem 7 Newton’s Third Law of Motion–Action and Reaction Now look at the horse system. • The opposite reaction force by the cart on the horse restrains the horse. • Without this force, the horse could freely gallop to the market. • The horse moves forward by interacting with the ground. • When the horse pushes backward on the ground, the ground simultaneously pushes forward on the horse. 7.6 The Horse-Cart Problem 7 Newton’s Third Law of Motion–Action and Reaction Look at the horse-cart system as a whole. • The pull of the horse on the cart and the reaction of the cart on the horse are internal forces within the system. • They contribute nothing to the acceleration of the horse-cart system. They cancel and can be neglected. 7.6 The Horse-Cart Problem 22 7 Newton’s Third Law of Motion–Action and Reaction • To move across the ground, there must be an interaction between the horse-cart system and the ground. • It is the outside reaction by the ground that pushes the system. 7.6 The Horse-Cart Problem 7 Newton’s Third Law of Motion–Action and Reaction 7.6 The Horse-Cart Problem 7 Newton’s Third Law of Motion–Action and Reaction think! What is the net force that acts on the cart? On the horse? On the ground? 7.6 The Horse-Cart Problem 25 7 Newton’s Third Law of Motion–Action and Reaction You cannot touch without being touched—Newton’s third law. 7.7 Action Equals Reaction 7 Newton’s Third Law of Motion–Action and Reaction What must occur in every interaction between things? 7.7 Action Equals Reaction 7 Newton’s Third Law of Motion–Action and Reaction 1. A force interaction requires at least a(n) a. single force. b. pair of forces. c. action force. d. reaction force. Assessment Questions 26 7 Newton’s Third Law of Motion–Action and Reaction 1. A force interaction requires at least a(n) a. single force. b. pair of forces. c. action force. d. reaction force. Answer: B Assessment Questions 7 Newton’s Third Law of Motion–Action and Reaction 2. Whenever one object exerts a force on a second object, the second object exerts a force on the first that is a. opposite in direction and equal in magnitude at the same time. b. in the same direction and equal in magnitude a moment later. c. opposite in direction and greater in magnitude at the same time. d. in the same direction and weaker in magnitude a moment later. Assessment Questions 7 Newton’s Third Law of Motion–Action and Reaction 2. Whenever one object exerts a force on a second object, the second object exerts a force on the first that is a. opposite in direction and equal in magnitude at the same time. b. in the same direction and equal in magnitude a moment later. c. opposite in direction and greater in magnitude at the same time. d. in the same direction and weaker in magnitude a moment later. Answer: A Assessment Questions 27 7 Newton’s Third Law of Motion–Action and Reaction 3. The force that directly propels a motor scooter along a highway is that provided by the a. engine. b. fuel. c. tires. d. road. Assessment Questions 7 Newton’s Third Law of Motion–Action and Reaction 3. The force that directly propels a motor scooter along a highway is that provided by the a. engine. b. fuel. c. tires. d. road. Answer: D Assessment Questions 7 Newton’s Third Law of Motion–Action and Reaction 4. When you jump vertically upward, strictly speaking, you cause Earth to a. move downward. b. also move upward with you. c. remain stationary. d. move sideways a bit. Assessment Questions