PreClass Notes: Chapter 4, Sections

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PreClass Notes: Chapter 4, Sections 4.1-4.4 From Essential University Physics 3 rd Edition by Richard Wolfson, Middlebury College 2016 by Pearson Education, Inc.

1 PreClass Notes: Chapter 4, Sections From Essential University Physics 3 rd Edition by Richard Wolfson, Middlebury College 2016 by Pearson Education, Inc. Narration and extra little notes by Jason Harlow, University of Toronto This video is meant for University of Toronto students taking PHY Pearson Education, Inc. Slide 1-1 Outline 4.1 Newton s First Law 4.2 Newton s Second Law 4.3 The Fundamental Forces What keeps things moving? is the wrong question. Galileo declared that the question needs no answer. R.Wolfson 4.4 The Force of Gravity 2012 Pearson Education, Inc. Slide 1-2 1

Galileo Galileo Galilei (1564-1642), was an Italian astronomer, physicist, engineer, philosopher, and mathematician who played a major role in the scientific revolution during the Renaissance.

2 Galileo Galileo Galilei ( ), was an Italian astronomer, physicist, engineer, philosopher, and mathematician who played a major role in the scientific revolution during the Renaissance. He is widely regarded as one of the greatest scientists of all time. Galileo has been called the the father of modern science Pearson Education, Inc. Slide 1-3 Galileo s Concept of Inertia Balls rolling on downwardsloping planes pick up speed. Balls rolling on upwardsloping planes lose speed. So a ball on a horizontal plane must maintain speed forever. If the ball comes to rest, it is not due to its nature, but due to friction Pearson Education, Inc. This nice explanation is from Conceptual Physics 11 th Edition by Paul G. Hewitt Slide 1-4 2

pulling. If you are both pulling in opposite directions with the same force, the net force is zero. 2012 Pearson Education, Inc.

3 Net Force Net force is the combination of all forces that change an object s state of motion. Example: If you pull on a box with 5 N and a friend pulls in the same direction with 5 N, the net force is 10 N in the direction you are both pulling. If you are both pulling in opposite directions with the same force, the net force is zero Pearson Education, Inc. Slide 1-5 Newton s First Law of Motion A body in uniform motion remains in uniform motion, and a body at rest remains at rest, unless acted on by a nonzero net force Pearson Education, Inc. Slide 1-6 3

Image from http://www.jnmjournal.com/oakville-go-train-fatality/ 2015-06-29 Got it?

4 Image from Got it? On a horizontal tabletop is a curved barrier that exerts a force on a ball, guiding its motion in a circular path as shown. After the ball leaves the barrier, which of the dashed paths shown does it follow? 2012 Pearson Education, Inc. Slide 1-7 Momentum Momentum is a property of moving things. Momentum is defined as mass of an object multiplied by its velocity: Momentum = mass velocity p = m v 2012 Pearson Education, Inc. Slide 1-8 4

5 Newton s Second Law of Motion The rate at which a body s momentum changes is equal to the net force acting on the body: F net = d p dt (Newton s 2 nd Law) 2012 Pearson Education, Inc. Slide 1-9 Newton s Second Law of Motion In another mathematically equivalent form: Acceleration net force mass Examples: a = F net m (Newton s 2 nd Law) If net force acting on object is doubled object s acceleration will be doubled. If mass of object is doubled object s acceleration will be halved Pearson Education, Inc. Slide

Newton s Second Law of Motion 2012 Pearson Education, Inc.

6 Newton s Second Law of Motion 2012 Pearson Education, Inc. Slide 1-11 Inertial Reference Frames Newton s laws are only valid in reference frames that are not accelerating. A reference frame that is not accelerating is called an inertial reference frame. A physics student cruises at a constant velocity in an airplane A ball placed on the floor stays at rest relative to the airplane This airplane is an inertial reference frame Pearson Education, Inc. Slide

Inertial Reference Frames A physics student is standing up in an airplane during takeoff A ball placed on the floor rolls toward the back of the plane There are no horizontal forces on the ball, and

7 Inertial Reference Frames A physics student is standing up in an airplane during takeoff A ball placed on the floor rolls toward the back of the plane There are no horizontal forces on the ball, and yet the ball accelerates in the plane s reference frame Newton s first law is violated, therefore this airplane is not an inertial reference frame Pearson Education, Inc. Slide 1-13 Got it? If an object moves with constant velocity in a straight line, which of the following statements is true? A. There are no forces acting on the object. B. The net force on the object is zero. C. There is a constant force in the direction of motion. D. There is a constant force in the direction opposite of motion Pearson Education, Inc. Slide

Long-range forces are forces that act on an object without physical contact A coffee cup released from your hand is pulled to

Slide 1-15 The Fundamental Forces Physicists now recognize three fundamental forces: Gravity The strong force The electroweak

8 What is a force? Contact forces are forces that act on an object by touching it at a point of contact The bat must touch the ball to hit it Long-range forces are forces that act on an object without physical contact A coffee cup released from your hand is pulled to the earth by the long-range force of gravity 2012 Pearson Education, Inc. Slide 1-15 The Fundamental Forces Physicists now recognize three fundamental forces: Gravity The strong force The electroweak force All common forces fall under these three categories. Nearly all everyday forces, except gravity, are electromagnetic forces, which is one aspect of the electroweak force. A goal of physics is to unify all forces in a Theory of Everything Pearson Education, Inc. Slide

9 Mass, Weight, and Gravity Weight is the force of gravity on an object: w mg Mass doesn t depend on the presence or strength of gravity. Weight depends on gravity, so varies with location: Weight is different on different planets. Near Earth s surface, g has magnitude 9.8 m/s 2 or 9.8 N/kg, and is directed downward Pearson Education, Inc. Slide 1-17 Got it? If you were to move to the Moon, A. your mass would change, but your weight would not. B. your weight would change, but your mass would not. C. both your weight and mass would change. D. neither your weight nor your mass would change Pearson Education, Inc. Slide

10 2012 Pearson Education, Inc. Slide Pearson Education, Inc. Slide

PreClass Notes: Chapter 4, Sections 4.5,4.6

PreClass Notes: Chapter 4, Sections 4.5,4.6 From Essential University Physics 3 rd Edition by Richard Wolfson, Middlebury College 2016 by Pearson Education, Inc. Narration and extra little notes by Jason