Homework See Supplemental Chapter 1. Read pages 3-end. #7-14, 16, 18, on pg 8 Due Tonight’s HW Ch. 4 Notes – read pages 43-55. As you read, only fill out the BULLET portion using the Cornell Notes handout. (you will get a copy of this at end of class) Do Now EXTRA INFO: o We will do the KEY POINTS and SUMMARY sections in class o If you need more paper, you can download from my website, or alter loose leaf, to make more. If you pull a crate across a factory floor at a constant speed in a constant direction… - What is the size of the force of friction on the crate compared with your pull if your pull force is 50 N? - What is the net force? - Is it in mechanical equilibrium? If so, which type? (static or dynamic?) If you pull a crate across a factory floor at a constant speed in a constant direction… - What is the size of the force of friction on the crate compared with your pull if your pull force is 50 N? - What is the net force? - Is it in mechanical equilibrium? If so, which type? (static or dynamic?) 50 N of pull force ???? of friction force Box is moving at a constant speed and direction to the left. 15. If you pull a crate across a factory floor at a constant speed in a constant direction, what is the size of the force of friction on the crate compared with your pull if your pull force is 50 N? What is the net force? Is it in mechanical equilibrium? If so, which type? Both the pull force and the friction force are equal in magnitude, but are in opposite directions. Thus, the net force is zero and the object is in dynamic equilibrium. 50 N of pull force 50 N of friction force Box is moving at a constant speed and direction to the left (NET FORCE = 0….so it is in Dynamic equilibrium) “The Equilibrium Rule” Take a copy of “The Equilibrium Rule” and READ through the solved sample problem. As you read, please HIGLIGHT, make NOTES, or COMMENT with any “aha” moments, questions, or areas that you don’t understand. We will discuss as a class and then complete…finished reading /commenting? Try to do some of the problems! Take out the HW that was due today… We are going to GO OVER it quickly via PowerPoint If you have questions, STOP and ask! Use a different color pen (if you would like) when making corrections 7. How much tension is in a rope that holds up a 20-N bag of apples at rest? Since the bag is at rest, it is in mechanical equilibrium. Therefore, there must be 20N of tension force to balance the 20N of weight force due to gravity. This is referred to in the book as static equilibrium. 8. What does F = 0 mean? The sum of all forces acting on a body are balanced (or net force is equal to zero) 9. What is the net force on an object at rest? Zero. All forces are balanced. 10. When you do pull-ups and you hang at rest, how much of your weight is supported by each arm? Each arm supports one-half of your weight. 11. What is the angle between the support force and the surface an object rests on? 90 degrees 12. What two forces compress a spring inside a weighing scale when you weight yourself? Your weight (your mass accelerated by gravity) and the support force of the floor (pushing upward). 13. When you are at rest and supported by a pair of scales, how does the sum of the scale readings compare with your weight? The sum of the readings is = to your weight 14. Can an object be moving and still be in (mechanical) equilibrium? Yes. If the object moves at a constant speed in a straight line, then the net force on the object is equal to zero. This is referred to in the book as dynamic equilibrium. 16. Distinguish between static equilibrium and dynamic equilibrium. An object at rest is said to be in static equilibrium. An object moving in a straight line at a constant speed is said to be in dynamic equilibrium. For each case, the net force = 0, thus the forces acting are balanced…in other words, the object is NOT changing its state of motion 18. Consider Nellie in Fig. 2.11. Name the 3 forces that act on her. What is your evidence they cancel to zero? Tension of 1 rope, tension of the second rope, and her weight She is in mechanical equilibrium (not changing her state of motion!!!), so the resultant of the rope tensions must equal / cancel out her weight….net force must be zero.