Page 1. Physics 131: Lecture 16. Today s Agenda. Collisions. Elastic Collision

Transcription

1 Physics 131: Lecture 16 Today s Agenda Elastic Collisions Definition Exaples Work and Energy Definition of work Exaples Physics 01: Lecture 10, Pg 1 Collisions Moentu is alost always consered during as collision (external forces are generally sall copared to collision forces) Two kinds of collisions Elastic-KE is consered Very special case Inelastic-KE is not consered Can be assued if Objects stick together Daage is done Physics 01: Lecture 10, Pg Elastic Collision The balls behae like springs! Physics 01: Lecture 10, Pg 3 Page 1

2 Elastic Collision During an elastic collision both oentu and echanical energy are consered: 11f f 1 1i i (1) f f 1i i () Cobining equation (1) and () we get that speed of approach equals speed of recession : f 1f i 1i Together with equation (1) you can sole for just about any 1-d elastic collision Physics 01: Lecture 10, Pg 4 Basketball Deo. Carefully place a sall tennis ball (ass ) on top of a uch bigger basketball (ass M). Drop these fro soe height. The height reached by the sall ball after they bounce is ~ 9 ties the original height!! (Assues M >> and all bounces are elastic). Understand this using the speed of approach = speed of recession property we just showed. 3 M (a) (b) (c) Physics 01: Lecture 10, Pg 5 Basketball Deo. 3 M (a) (b) (c) Physics 01: Lecture 10, Pg 6 Page

3 Work by a Constant Force Work is the energy transferred to a syste or object caused by a force To do work you need two things A Force Motion I lift y book with force F through a distance of s Work = Fs = Force (Displaceent) Only works with a constant force Only true when force is in sae direction as displaceent Otherwise Work = F s cos() Work N Joule J Physics 01: Lecture 10, Pg 7 Clicker Question 1: A box sits on the floor. A force F = 00 N is then applied to the right but slightly upward at an angle of 30 fro the horizontal (see diagra) such that the box oes 5 eters horizontally to the right. How uch work is done by the force? A. 50 J B. 866 J C J D. 430 J E. 330 J Cos 30 = Sin 30 = 0.5 Physics 01: Lecture 10, Pg 8 Interesting things about work You hae to oe the object to DO work!! I hold up a dubbell, but it doesn t oe I do no work!! Definition of work! If force is perpendicular to otion you do no work! You can hae positie or negatie work Work is a scalar (easier for us!) Physics 01: Lecture 10, Pg 9 Page 3

4 Positie s. negatie work If coponent of force points in sae direction as displaceent, work is positie If coponent of force points in opposite direction as displaceent, work is negatie Good exaple: Friction (often does negatie work) Physics 01: Lecture 10, Pg 10 QuickCheck 11.5 Clicker Question : A constant force pushes a particle through a displaceent. In which of these three cases does the force do negatie work? D. Both A and B. E. Both A and C. Physics 01: Lecture 10, Pg 11 Clicker Question 3: A box is pulled up a rough ( > 0) incline by a ropepulley-weight arrangeent as shown below. How any forces are doing work on the box? (a) 1 (b) (c) 3 (d) 4 (e) 5 Physics 01: Lecture 10, Pg 1 Page 4

5 Clicker Question 3: Draw FBD of box: Consider direction of otion of the box T F N Any force not perpendicular to the otion will do work: F N does no work (perp. to ) f T does positie work f does negatie work g does negatie work 3 forces do work g Physics 01: Lecture 10, Pg 13 Work total Work total is the work on an object by all forces acting. W tot W Work total is the work done by the net force Physics 01: Lecture 10, Pg 14 Kinetic Energy: Motion Apply constant force along x-axis to a point particle. W = F x s = a x s = ½ ( f i ) 1 recall: axs ( xf xi) Work changes ½ This is the Kinetic Energy K = ½ W = K For Point Particles Physics 01: Lecture 10, Pg 15 Page 5

6 The Work-Energy Theore W tot = ½ f -½ i Put positie work into an object, its speed increases! Put negatie work into an object, its speed decreases! Be careful, we are interested in total work done!!! We ust look at all forces acting, not just one! Physics 01: Lecture 10, Pg 16 Clicker Question 4: Two identical arrows, one with twice the speed of the other, are fired into a bale of hay. Assuing the hay exerts a constant frictional force on the arrows, the faster arrow will penetrate how uch further than the slower arrow? (A) twice as uch as the slower arrow (B) four ties as uch the slower arrow (C) six ties as uch the slower arrow (D) eight ties as uch the slower arrow Physics 01: Lecture 10, Pg 17 Clicker Question 5: You are towing a car up a hill with constant elocity. The total work done on the car by all forces is: A. positie B. negatie C. zero V T Physics 01: Lecture 10, Pg 18 Page 6

7 Clicker Question 6: Two blocks of ass 1 and ( 1 > ) slide on a frictionless floor and hae the sae elocity when they hit a long rough stretch ( > 0), which slows the down to a stop. Which one goes farther? a) 1 b) c) they will go the sae distance Physics 01: Lecture 10, Pg 19 Clicker Question 7: A light plastic cart and a heay steel cart are both pushed with the sae force for a distance of 1.0, starting fro rest. After the force is reoed, the kinetic energy of the light plastic cart is that of the heay steel cart. A. greater than B. equal to C. less than D. Can t say. It depends on how big the force is. Physics 01: Lecture 10, Pg 0 Exaple A block of ass kg is pulled up a frictionless incline by a tension T. The block starts at rest and after being pulled up the incline 4 has a speed of 15 /s. (a) What is the net work done on the block? (b) What is the force of tension? Physics 01: Lecture 10, Pg 1 Page 7