1. In this chapter you will:  Use Newton’s laws to solve problems.  Determine the magnitude and direction of the net force that causes a change in an object’s motion.  Classify forces according to the agents that cause them.

2. CHAPTER 4 SECTIONS Section 4.1: Force and Motion Section 4.2: Using Newton's Laws Section 4.3: Interaction Forces

3. Section 4.1 Force and Motion Objectives Define force. Apply Newton’s second law to solve problems. Explain the meaning of Newton’s first law.

4. INTRO/Additional From Old Book Kinematics – study of how objects move Dynamics – study of why objects move as they do The causes of acceleration were first studied by Sir Isaac Newton. The connection between acceleration and its causes are summarized in Newton’s 3 Laws of Motion. Newton’s Laws of Motion – laws relating force and acceleration.

5. FORCE AND MOTION Force – a push or a pull. Forces can cause objects to speed up, slow down, or change direction as they move. They are Vector quantities. A force exerted on an object causes that object’s velocity to change, that is, a FORCE CAUSES AN ACCELERATION. The symbol F is a vector and represents the size and direction of a force, while F represents only the magnitude. System – the object of interest. External World – everything around the object that exerts forces on it.

6. CONTACT FORCES AND FIELD FORCES Contact Forces – forces that exist when an object from the external world touches a system and thereby exerts a force on it. Field Forces – forces that exist without being in contact. One example, is gravity. Forces result from interactions. Agent - a specific and identifiable cause of a Force. A FORCE needs both an 1) Agent and 2) System.

7. CONTACT FORCES AND FIELD FORCES For example, when you push your textbook your hand is the Agent and the textbook is the system. Free Body Diagram - A physical model which represents the forces acting on a system. Do Practice Problems p. 89 # 1-5

8. FORCE AND ACCELERATION Acceleration Force Graph – a graph that compares the Acceleration (vertical axis) to the Force (horizontal axis). This shows a linear relationship between Acceleration and Force where the greater Force is, the greater the resulting Acceleration. The slope depends on the mass. F = ma (Force = mass * acceleration) The Force causes the object to Accelerate.

9. FORCE AND ACCELERATION If you double the force you double the acceleration. If you apply the same force to several different objects, the one with the most mass will have the smallest acceleration and the one with the least mass will have the greatest acceleration. Newton – the unit we use to measure force. It is defined as the force that causes a mass of one kilogram to accelerate at a rate of one meter per second squared (m/s 2 ). It is named after Sir Isaac Newton and it is denoted by the letter capital “N”. So we have F = ma = 1 kg (1 m/s 2 ) = 1 N

10. COMBINING FORCES Net Force – the vector sum of all the forces acting on an object. Go over the examples and Figure 4-5 p. 92

11. NEWTON’S SECOND LAW Newton’s Second Law of Motion – also called Law of Acceleration. The acceleration of an object is equal to the sum of the forces on it divided by the mass of the object. It states “The acceleration of an object is directly proportional to the net force on it and inversely proportional to its mass.” This law can be summarized in the equation F = ma Force is equal to mass times acceleration

12. NEWTON’S SECOND LAW The acceleration is in the same direction as the force causing it. If the force is in the positive direction then the acceleration is positive. If the force is in the negative direction then the acceleration is negative. The larger the mass the smaller the acceleration. The smaller the mass the larger the acceleration. Do Practice Problems p. 93 # 6-8

13. NEWTON’S FIRST LAW A stationary object with no net force acting on it will stay at its position. Galileo concluded that in the Ideal Case of Zero Resistance, horizontal motion would never change. In the absence of a net force the motion (or lack of motion) of both the moving ball and the stationary object continues as it was.

14. NEWTON’S FIRST LAW Newton’s First Law of Motion – also called the Law of Inertia. The law states the following: “An object at rest will stay at rest, an object in motion will stay in motion, unless acted on by an outside force.” Or the old book version is an object with no net force acting on it moves with constant velocity. Or new book version “an object that is at rest will remain at rest and an object that is moving will continue to move in a straight line with constant speed if and only if the net force acting on the object is zero.”

15. NEWTON’S FIRST LAW Inertia – tendency of an object to resist change. It is not a force. A massive body has more inertia than a less massive body. Equilibrium – the condition in which the net force on an object is zero. An object is in equilibrium if it is at rest or if it is moving at a constant velocity.

16. NEWTON’S FIRST LAW Newton’s first law identifies a net force as something that disturbs a state of equilibrium. Thus if there is no net force acting on the object then the object does not experience a change in speed or direction and is in equilibrium. Table 4-2 p. 94 Do 4.1 Section Review p. 95 # 9-14