Physics 111. Help this week: Wednesday, 8-9 pm in NSC 118/119 Sunday, 6:30-8 pm in CCLIR 468. Response rate: 23 out of 33

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ics day, ember 30, 2004 Mid-term survey results Ch 5: Newton s 3rd Law Ch 6: Examples Help this week: Wednesday, 8-9 pm in NSC 118/119 Sunday, 6:30-8 pm in CCLIR 468 Response rate: 23 out of 33

1 ics day, ember 30, 2004 Mid-term survey results Ch 5: Newton s 3rd Law Ch 6: Examples Help this week: Wednesday, 8-9 pm in NSC 118/119 Sunday, 6:30-8 pm in CCLIR 468 Response rate: 23 out of 33 Several students provided little to no feedback on the free-response page Don t forget to read over the lab write-up and be ready for the quiz. I need to hear from you if you have suggestions or comments! Most questions had mean of 3.0 Least favorite activities: lets Mastering ics Favorite activities: Interactive Exercises Average time on class: ~7 hours/week What can you do? Lots of recommended problems including Mastering ics One exam problem will be MP One exam problem will be Walker What I will do in the future: Make sure we cover what you need before Mastering ics assignment is due. However, lets will cover material before we cover it in class. 1

2 We ve looked at a lot of problems with but a single system of interest. Let s now formalize the way to handle problems with multiple systems. In the process, we ll encounter Newton s 3rd Law. Example of me pushing I can huff and puff and push on the wall, but it doesn t seem to be accelerating. And neither am I!!! Just because neither I nor the wall seem to be accelerating, however, does NOT mean that there are no forces acting. In fact, there are several relevant forces involved in this process. Let s just look at the forces acting on me. Let s just look at the forces acting on me. N normal force of wall on me A STATIC System: all the forces are in balance. Nothing accelerates! (wall) (floor) friction f frictional force of floor on me N normal force of floor on me W force of Earth s gravity on me weight All forces on hero -FBD C contact force of wall on me Let s now look at the forces acting A STATIC System: all the forces are in balance. Nothing accelerates! friction of floor on wall force of floor on wall Let s now look at the forces acting of the me pushing f frictional force of floor on wall N normal force of floor on wall All forces on wall - picture W force of Earth s gravity on wall weight of wall All forces on wall -FBD 2

C mf contact force of me on floor C wf contact force of wall on floor N normal force of the Earth on the floor Let s just look at the forces acting on the floor.

3 C mf contact force of me on floor C wf contact force of wall on floor N normal force of the Earth on the floor Let s just look at the forces acting on the floor. f mf frictional force of me on floor f wf frictional force of wall on floor W force of Earth s gravity on the floor Friction (me) weight of floor of me on the floor Let s just look at the forces acting on the floor. Friction (wall) of wall on the floor (Earth) Finally, let s look at the forces acting in this problem. Finally, let s look at the forces acting in this problem. C of floor on the Earth F gw Gravitational of wall EARTH of floor on the Earth Gravitational of wall Gravitational of Me F gf Gravitational of floor F ghero Gravitational of Me Gravitational of floor EARTH All forces on Earth - FBD If one object exerts a force on a second object, the second object necessarily exerts an equal but oppositely directed force on the first. We re talking about TWO DIFFERENT FORCES HERE!!! NOT a of the wall pushing on me. of the me pushing Interaction Pair -- Newton s Third Law always act on DIFFERENT objects Law 3 Super Hero: normal & contact 3

4 NOT a of the wall pushing on me. of the me pushing Interaction Pair -- Newton s Third Law always appear in DIFFERENT FBDs. Several notes: The NORMAL force acts perpendicular to some surface. It is NOT NECESSARILY equal to mg! Each and every object in a problem has its own free body! Draw each one separately. Third Law pairs act on DIFFERENT OBJECTS. They NEVER act on the same object! I m going to jump off a chair. Worksheet Problem #1 1) 2) 3) > F Henry on Earth F Henry on Earth < F Henry on Earth F Henry on Earth Watch as the Earth rushes up to meet me! Do you want to see that again? What s going on here? W1: Henry & Earth Demo: my gravity on Earth Ch 4: Newton s Laws I need two student volunteers skip Exp. #1: Each of you take one scale. Push on each other through the scales and call out the readings. Exp. #2: One of you sit in the rolling chair. The other push through the scale. Both call out readings on the scales. Demo: students and 3rd Law Our avant-guarde socialite pulls on the rope that s wrapped around the tree. Nothing happens. 4

5 here here here Let s examine this piece more carefully... What must be true about the forces acting... The forces balance -- The rope does NOT accelerate. Remember this one? In fact, no matter which little segment of the rope I examine in this case, the tension forces balance in either direction, and the rope remains stationary. Okay, let s look at tension in a rope that results in the acceleration of an object... balance What exactly is it that causes the green block to accelerate? Meter Frictionless pond of ice block & ice Let s look at the for the green block. What forces are acting on the green block? What is the acceleration of the green block? a block F contact m block The contact force of the rope on the block results in the block accelerating. Weight force Weight force block accelerates block accelerates 5

6 What if we look at a piece of the rope in this case? a a Remember, the whole system is accelerating at the same rate, a. F net m r T 2 T 1 m r Some mass m r Some mass m b 1 2 Some mass m b And clearly the block s acceleration will be dictated by the magnitude of the contact force at the end of the rope (which in magnitude is equal to the tension in the rope at that end of the rope) connected to the block... a F net m b F contact m b T end m b Back to the rope for a minute... left T left right Frictionless pond of ice > T right However, what happens if m r 0? 1 2 F net T 2 T 1 m r a 0 a 0 T 2 T 1 Worksheet Problem #2 left right T left T right Frictionless pond of ice equal tensions CQ2 6

7 y N x A worker drags a crate across a factory floor by pulling on a rope tied to the crate. The worker exerts a force of 450 N on a rope that is inclined at 38 o to the horizontal, and the floor exerts a horizontal force of N that opposes the motion. (a) Calculate the acceleration of the crate if its mass is 310 kg. (b) Calculate the normal force of the floor on the crate. Problem Sheet #1 a 38 o A worker sits in a bosun s chair that is supported by a massless rope that runs over a massless, frictionless pulley and back down to the man s hand. The combined mass of the man and the chair is 95.0 kg. (a) With what force must the man pull on the rope for him to rise at a constant speed? (b) How would the force be different if it were exerted by a second man on the ground instead of the man in the chair? This looks like a pretty complex problem And it can be tricky so, let s be careful and use Newton s Laws explicitly for each moving object. Problem Sheet #2 Two blocks of mass 3.50 kg and 8.00 kg are connected by a massless string passing over a frictionless pulley. The inclines are also frictionless. Find (a) the magnitude of the acceleration of each block and (b) the tension in the string. m kg m kg Problem Sheet #3 θ 1 35 o θ 2 35 o? Inclined plane & newton s laws (W5) 7

Thurs Sept.23. Thurs Sept. Phys .23. Why is it moving upwards after I let go? Don t forget to read over the lab write-up and be ready for the quiz.

Thurs Sept.23. Thurs Sept. Phys .23. Why is it moving upwards after I let go? Don t forget to read over the lab write-up and be ready for the quiz. ics Announcements day, ember 23, 2004 Ch 5: Newton s 1st and 2nd Laws Example Problems Ch 6: Intro to Friction static kinetic Help this week: Wednesday, 8-9 pm in NSC 118/119 Sunday, 6:30-8 pm in CCLIR