Lecture 14 Monday, September 29 Using Newton’s Laws

Harper’s Index Minimum number of U.S. cities that have shortened the yellow light on traffic signals to under the legal limit: 6

Today Equilibrium F NET = 0 Dynamics F NET ≠ 0 Mass and Weight Normal forces Frictional forces Chapter 5: 1-5

Equilibrium Hence a=0 and v=constant If v=0 and is constant, static equilibrium If v≠0 and is constant, dynamic equilibrium

30 0 Find the tensions, T 1 and T 2 in the two cords if the mass of the picture is 11.1 kg.

Procedure Make free body diagram Picture in (static) equilibrium so

Example Macie pulls a 40 kg rolling trunk by a strap angled at 30° from the horizontal. She pulls with a force of 40 N, and there is a 30 N rolling friction force acting on trunk. What is the trunk’s acceleration? Slide 5-17

Mass and Weight Freely falling object

Elevators You are standing on a scale in an elevator. A) The elevator is moving upwards (downwards) at a steady speed. If your mass is 70 kg, what does the scale read? B) The elevator which had been moving upwards, slows to a stop. What does the scale read during this time? Normal force

Quiz An elevator, lifted by a cable, is going up at a steady speed. Identify the forces acting on the elevator. Make a free body diagram. Is T greater than, equal to, or less than w? Or is there not enough information to tell? Slide 4-23

Friction Static friction Opposes motion

Kinetic Friction Opposes motion when object is sliding µ k is the coefficient of kinetic friction

Rolling Friction µ r is the coefficient of rolling friction

Problem 5:24 A 23kg child on slide inclined at What forces act on child? Make a free body diagram Magnitude of normal force? If the coefficient of static friction is 0.62, will the child start to slide?

Problems due Today 4: 13, 14, 17, 18, 20, 25, 29, 30, 31, 33, 35, 38, 65

Problems due Wednesday Reading: 5: 6-8 Drag (skip), applying Newton’s laws Problems: 5: CQ1, CQ3, MC24, MC26, MC30, 1, 2, 4, 7, 11, 13, 16, 19, 21