PreClass Notes: Chapter 4, Sections 4.5,4.6

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1 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 Harlow, University of Toronto This video is meant for University of Toronto students taking PHY Pearson Education, Inc. Slide 1-1 Outline 4.5 Solving Force Problems Using Newton s second law with multiple forces is easier if we draw a free-body diagram. R.Wolfson 4.6 Newton s Third Law, The Normal Force Hooke s Law Image from Pearson Education, Inc. Slide 1-2 1

2 Drawing a Free-Body Diagram When solving a problem in which multiple forces act on an object, you should: 1. Identify the object of interest. 2. List all the forces that are acting on that object. 3. Represent the object as a dot. 4. Draw the vectors for only those forces acting on the object, with their tails all starting on the dot Pearson Education, Inc. Slide 1-3 Example 4.3 A 740-kg elevator accelerates upward at 1.1 m/s 2, pulled by a cable of negligible mass. Find the tension force in the cable Pearson Education, Inc. Slide 1-4 2

3 Forces and Interactions Interaction is between one thing and another. requires a pair of forces acting on two objects. Example: interaction of hand and wall pushing on each other Force pair you push on wall; wall pushes on you Pearson Education, Inc. Slide 1-5 Newton s Third Law of Motion If object A exerts a force on object B, then object B exerts an oppositely directed force of equal magnitude on A Pearson Education, Inc. Slide 1-6 3

4 Action and reaction forces one force is called the action force; the other force is called the reaction force. are co-pairs of a single interaction. neither force exists without the other. are equal in strength and opposite in direction. always act on different objects Pearson Education, Inc. Slide 1-7 Reexpression of Newton s third law: To every action there is always an opposed equal reaction Pearson Education, Inc. Slide 1-8 4

5 Got it? If you exert a force F on a bookcase, the force which the bookcase exerts on you A. is always F. B. depends on your mass. C. depends on the mass of the bookcase. D. depends on the acceleration of the bookcase. E. depends on the speed of the bookcase Pearson Education, Inc. Slide 1-9 Not every pair of equal and opposite forces form a third-law pair. The two forces on the block act on the same object, so they cannot be a third-law pair. They happen to be equal and opposite, but not because of Newton s Third Law Pearson Education, Inc. Slide

6 Below is the same block on a tilted table. In this case those same two forces are clearly not equal and opposite. The forces do not cancel, and the block accelerates Pearson Education, Inc. Slide 1-11 The gravitational forces on you and Earth do form a third-law pair. [Figure not to scale.] 2012 Pearson Education, Inc. Slide

7 Example What is the net force on the 3 kg block? 2012 Pearson Education, Inc. Slide 1-13 Elasticity Hooke s law: The extension of a spring is directly proportional to the force applied to it. Force ~ extension or F ~ Dx 2012 Pearson Education, Inc. Slide

8 Tension and Compression When something is pulled it is in tension. squashed it is in compression. [Image retrieved Jan from ] 2012 Pearson Education, Inc. Slide 1-15 Hooke s Law A stretched or compressed spring produces a restoring force in a direction is opposite that of the stretch or compression. In an ideal spring, the stretch or compression is directly proportional to the force exerted by the spring: F s = kx k is called the spring constant of the spring [N/m] The negative sign is there to remind you that the force F s exerted by the spring on the object is opposite in direction to the displacement x of the end of the spring from its normal position. Springs provide convenient devices for measuring force Pearson Education, Inc. Slide

9 Chapter 4 Big Ideas Force causes change in motion, not motion itself. Uniform motion (straight line, constant speed) needs no cause or explanation. Any deviation in speed or direction requires a net force. Forces always come in pairs: Every interaction between two objects consists of two forces which are equal in magnitude but opposite in direction Pearson Education, Inc. Slide