UUNIT 2 Motion and Force in One Dimension CCHAPTER 5 Chapter 5 Newton’s Laws: Force and Motion Objectives: By the end of this chapter you should be able to: 4 Describe how the law of inertia affects the motion of an object. 4 Give an example of a system or invention designed to overcome inertia. 4 Measure and describe force in newtons (N) and pounds (lb). 4 Calculate the net force for two or more forces acting along the same line. 4 Calculate the acceleration of an object from the net force acting on it. 4 Determine whether an object is in equilibrium by analyzing the forces acting on it. 4 Draw a diagram showing an action-reaction pair of forces. 4 Determine the reaction force when given an action force. Key Questions: When do you encounter Newton’s laws of motion in daily life? How are force, mass, and acceleration related? What are some common action-reaction force pairs? Vocabulary action force net force Newton’s second law reaction dynamics law of inertia newton (N) Newton’s third law statics equilibrium locomotion Newton’s first law 99 Chapter 5 NEWTON’S LAWS: FORCE AND MOTION 5.1 The First Law: Force and Inertia Sir Isaac Newton (1642–1727), an English physicist and mathematician, is one of the most famous scientists who have ever lived. Before the age of 30, he made several important discoveries in physics and invented a whole new kind of mathematics—calculus. The three laws of motion discovered by Newton are probably the most widely-used natural laws in all of science. Together, Newton’s laws are the model which connects the forces acting on an object, its mass, and its resulting motion. This chapter is about Newton’s laws, and the first section is about the first law, the law of inertia. Force Changing an Suppose you want to move a box from one side of the room to the other. What object’s motion would you do? Would you yell at it until it moved? “Hey, box, get going! Move to the other side of the room!” Of course not! You would push or pull it across the room. In physics terms, you would apply a force to the box. Force is an A force is what we call a push or a pull, or any action that has the ability to action that can change an object’s motion. Forces can be used to increase the speed of an object, change motion decrease the speed of an object, or change the direction in which an object is moving. For something to be considered a force, it does not necessarily have to change the motion, but it must have the ability to do so. For example, if you push down on a table, it will probably not move. But if the legs were to break, the table could move. Therefore, your push qualifies as a force. Figure 5.1: Force is the action which has the ability to change Creating force Forces can be created by many different processes. For example, gravity creates motion. Without force, the motion force. Muscles can create force. The movement of air, water, sand, or other matter of an object cannot be started or changed. can create force. Electricity and magnetism can create force. Even light can create force. No matter how force is created, its effect on motion is always described by Newton’s three laws. Changes in Forces create changes in motion, and there can be no change in motion without motion only also having a force (Figure 5.1). Anytime there is a change in motion, a force must occur through exist, even if you cannot immediately recognize the force. For example, when a force rolling ball stops by hitting a wall, its motion changes rapidly. That change in motion is caused by the wall exerting a force that stops the ball. 100 UNIT 2 MOTION AND FORCE IN ONE DIMENSION NEWTON’S LAWS: FORCE AND MOTION Chapter 5 Inertia Which systems in a car Objects tend to Consider that box you wish to move across the room. What if the box had been overcome the law of inertia? keep doing what moving and you wanted to stop it? Again, yelling a command will not make it The engine supplies force that they are doing stop. The only way to stop the box is to apply enough force in a direction opposite allows you to change motion by to its motion. In general, objects tend to continue doing what they are already pressing the gas pedal. doing. If they are moving, they tend to keep moving, in the same direction, at the same speed. If they are at rest, they tend to stay at rest. This idea is known as Newton’s first law of motion. Newton’s Newton’s first law states that an object will continue indefinitely in its current first law state of motion, speed, and direction, unless acted upon by a net force. Intuitively, The brake system is designed to you know that the motion of a massive object is harder to change than the motion help you change your motion by of a lighter object. Inertia is a term used to measure the ability of an object to slowing down. resist a change in its state of motion. An object with a lot of inertia takes a lot of force to start or stop; an object with a small amount of inertia requires a small amount of force to start or stop. Because inertia is a key idea in Newton’s first law, the first law is sometimes referred to as the law of inertia. The steering wheel and steering system is designed to help you change your motion by changing your direction. Inertia is a The amount of inertia an object has depends on its mass. More massive objects property of have more inertia than less massive objects. Recall that mass is a measure of the mass amount of matter in an object. Big trucks are made of more matter than small cars; thus, they have greater mass and a greater amount of inertia. It takes more force to stop a moving truck because it has more inertia than a small car. This is a Can you think of three parts of a common-sense application of the first law. bicycle that are designed to overcome the law of inertia? Origin of the The word inertia comes from the Latin word inertus, which can be translated to word inertia mean “lazy.” It can be helpful to think of things that have a lot of inertia as being very lazy when it comes to change. In other words, they want to maintain the status quo and keep doing whatever they are currently doing. 5.1 THE FIRST LAW: FORCE AND INERTIA 101 Chapter 5 NEWTON’S LAWS: FORCE AND MOTION Applications of Newton’s first law Seat belts and Two very important safety features of automobiles are designed with Newton’s air bags first law in mind: seat belts and air bags. Suppose you are driving down the highway in your car at 55 miles per hour when the driver in front of you slams on the brakes. You also slam on your brakes to avoid an accident. Your car slows down but the inertia of your body resists the change in motion. Your body tries to continue doing what it was doing—traveling at 55 miles per hour. Luckily, your seat belt or air bag or both supplies a restraining force to counteract your inertia and slow your body down with the car (Figure 5.2). Cup holders A cup holder does almost the same thing for a cup. Consider what happens if you have a can of soda on the dashboard. What happens to the soda can when you turn sharply to the left? Remember, the soda can was not at rest to begin with. It was moving at the same speed as the car. When your car goes left, the soda can’s inertia causes it to keep moving forward (Figure 5.3). The result is quite a mess. Figure 5.2: Because of its inertia, Automobile cup holders are designed to keep the first law from making messes. your body tends to keep moving The tablecloth Have you ever wondered how a magician is able to pull a tablecloth out from when your car stops suddenly. This trick can cause serious injury if you are underneath dishes set on a table? It’s not a trick of magic at all, but just physics. not wearing a seat belt. The dishes have inertia and therefore tend to resist changes in motion. Before the magician pulls on the cloth, the dishes are at rest. So when the tablecloth is whisked away, the inertia of the dishes keeps them at rest. This trick works best when the tablecloth is pulled very rapidly and the table is small. It would be quite difficult to perform this trick with the long table in the diagram. Can you think why the long table would make the trick hard to do? Figure 5.3: Because of its inertia, a soda can on the dashboard will tend to keep moving forward when the car turns left. 102 UNIT 2 MOTION AND FORCE IN ONE DIMENSION NEWTON’S LAWS: FORCE AND MOTION Chapter 5 5.2 The Second Law: Force, Mass, and Acceleration Newton’s discovery of the connection among force, mass, and acceleration was a milestone in our understanding of science.