Mechanics Friction. Lana Sheridan. Oct 23, De Anza College

Transcription

1 Mechanics riction Lana heridan De Anza College Oct 23, 2018

2 Last time Types of forces and new scenarios contact forces tension pulleys

3 Overview finish Atwood machine friction

4 Recap: Pulleys and the Atwood Machine over a frictionless pulley of s called an Atwood machine. rmine the value of g. Deterjects and the tension in the ure 5.14a in action: as one ward. Because the objects erations must be of equal + m 1 m 2 subject to the gravitational a b nected We to them. cantherefore, consider the motion for each mass separately. In the rticles under y-directions: a net force. igure 5.14 (Example 5.9) The Atwood machine. (a) Two objects are shown in igure 5.14b. connected by a massless inextensible net,1,y = string T over ma 1 g frictionless = m 1 a pulley. (up positive) (1) exerted by the string and (b) The free-body diagrams for the as this one in which the net,2,y = mtwo 2 gobjects. T = m 2 a (down positive) (2) sion in the string on both or is subject to friction, the tensions on either side are not the arn in Chapter Be careful 10. about the signs! Both masses must accelerate h as this together one. In igure - one 5.14a, up, notice onethat down. if object 1 accelerates + T m 1 m 1 g T m 2 m 2 g

5 Recap: Pulleys and the Atwood Machine inding acceleration and tension in the rope: Take eq (1) + eq (2): net,1,y = T m 1 g = m 1 a (1) net,2,y = m 2 g T = m 2 a (2) m 2 g m 1 g = m 1 a + m 2 a a = (m 2 m 1 )g m 1 + m 2 T = 2m 1m 2 g m 1 + m 2

6 Question You push on a heavy crate and moves it across the floor. However, even as you push it does not accelerate and if you stop pushing, the box stops moving. Why?

7 ome Types of orces: riction riction is a force that opposes motion. Tiny defects in the surface of the floor and the crate catch on one another as the crate is pushed. 1 igure from boundless.com

8 riction friction The force caused by small-scale roughness of surfaces or even electrostatic attractions between surfaces. It resists motion. There are actually two types of friction: kinetic (moving across surface) static (stationary on surface)

9 riction Kinetic riction Kinetic friction is a resistive force that dissipates energy. fk n mg Motion kinetic friction normal force f k = µ k n µ k is the coefficient of kinetic friction The kinetic friction force always acts to oppose motion of the surfaces relative to each other. That means the kinetic friction f k always points opposite to the velocity vector.

10 riction tatic riction tatic friction acts when there is some component of a force on an object parallel to the surface it rests on, but the object does not move relative to the surface. fs n max. static friction normal force f s µ s n µ s is the coefficient of static friction mg f s,max = µ s n

11 riction magnitude of the applied force. can breaks free and accelerates to the right. n n Motion fs fk mg mg a f b f s,max f s f k m k n O tatic region Kinetic region c

12 riction Example or waxed wood on wet snow µ s = 0.14 and µ k = You pull on a sled of mass 10 kg that is at rest initially. How much force do you need to apply to get the sled moving? If you continue to apply that force, what will the magnitude of sled s acceleration be once it is moving?

13 riction Example or waxed wood on wet snow µ s = 0.14 and µ k = You pull on a sled of mass 10 kg that is at rest initially. How much force do you need to apply to get the sled moving? If you continue to apply that force, what will the magnitude of sled s acceleration be once it is moving? To get the sled moving app f s f s = µ s n = (0.14)(10 kg)g = N = 14 N (2 sig figs)

14 riction Example or waxed wood on wet snow µ s = 0.14 and µ k = You pull on a sled of mass 10 kg that is at rest initially. How much force do you need to apply to get the sled moving? If you continue to apply that force, what will the magnitude of sled s acceleration be once it is moving? To get the sled moving app f s f s = µ s n = (0.14)(10 kg)g = N = 14 N (2 sig figs) app f k = µ k n = (0.1)(10 kg)g = 3.92 N a = 3.92 N = 0.39 ms 2 10 kg

15 riction Question Quick Quiz You are playing with your daughter in the snow. he sits on a sled and asks you to slide her across a flat, horizontal field. You have a choice of: (A) pushing her from behind by applying a force downward on her shoulders at 30 below the horizontal or (B) attaching a rope to the front of the sled and pulling with a force at 30 above the horizontal. Which would be easier for you and why? 2 erway & Jewett, page 132. a b igure 5.17 (Quick Quiz 5.7) A father slides his daughter on a sled either by (a) pushing down on her shoulders or (b) pulling up on a rope. Rubb tee Alum Glas Cop Woo Wax Wax Meta Teflo Ice o yno Note: can e Q u yo b Q u a ch sh th (

16 riction Question Quick Quiz You are playing with your daughter in the snow. he sits on a sled and asks you to slide her across a flat, horizontal field. You have a choice of: (A) pushing her from behind by applying a force downward on her shoulders at 30 below the horizontal or (B) attaching a rope to the front of the sled and pulling with a force at 30 above the horizontal. Which would be easier for you and why? 2 erway & Jewett, page 132. a b igure 5.17 (Quick Quiz 5.7) A father slides his daughter on a sled either by (a) pushing down on her shoulders or (b) pulling up on a rope. Rubb tee Alum Glas Cop Woo Wax Wax Meta Teflo Ice o yno Note: can e Q u yo b Q u a ch sh th (

17 the tile surface if she is wearing (a) footwear meeting riction Example ntal een iniat is m s 5 surset e of contion the 0-kg seat the netic m/h er a the the Postal ervice minimum and (b) a typical athletic shoe? 60. A woman at an airport is towing W her 20.0-kg suitcase at constant speed by pulling on a strap at an angle u above the horizontal (ig. P5.60). he pulls on the u strap with a 35.0-N force, and the friction force on the suitcase is 20.0 N. (a) Draw a freebody diagram of the suitcase. igure P5.60 (b) What angle does the strap make with the horizontal? (c) What is the magnitude of the normal force that the ground exerts on the suitcase? 61. Review. A 3.00-kg block starts from rest at the top of a M 30.0 incline and slides a distance of 2.00 m down the incline in 1.50 s. ind (a) the magnitude of the acceleration of the block, (b) the coefficient of kinetic friction between block and plane, (c) the friction force acting

18 the tile surface if she is wearing (a) footwear meeting riction Example ntal een iniat is m s 5 surset e of contion the 0-kg seat the netic m/h er a the the Postal ervice minimum and (b) a typical athletic shoe? 60. A woman at an airport is towing W her 20.0-kg suitcase at constant speed by pulling on a strap at an angle u above the horizontal (ig. P5.60). he pulls on the u strap with a 35.0-N force, and the friction force on the suitcase is 20.0 N. (a) Draw a freebody diagram of the suitcase. igure P5.60 (b) What angle does the strap make with the horizontal? (c) What is the magnitude of the normal force that the ground exerts on the suitcase? 61. Review. A 3.00-kg block starts from rest at the top of a Answers: M 30.0 incline and slides a distance of 2.00 m down the b) θ = incline 55.2 in 1.50 s. ind (a) the magnitude of the acceleration 167 Nof the block, (b) the coefficient of kinetic friction c) n = between block and plane, (c) the friction force acting

19 ummary finish Atwood machine friction Homework Ch 6 Ques: 1, 3; Probs: 1, 5, 7, 9, 17, 27, 29