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Physics 103 Lecture 8Application of Newton s Laws Survey results Leftovers from Chapter 4 How to solve problems.

Physics 103 Lecture 8 Pg 1 Survey Results78 With 80 approvalAre the preflights helpful 22 NO rating the web.

0 20 40 60 80 format is here tostay for this course Is the interactive learning helpful 0 20 40 60 80 .

Are the homeworks on the web helpful YES0 20 40 60 80 100 Has the course web site been problematic 84 0 20 40 60 80 100 Physics 103 Lecture 8 Pg 2.

But all is not well Most requests Please work out problems in class fully Will address this to some extent in class Better forum for that is the discussion.

Please ask questions Some requests Slow down and explain more Most complaints There is too much work for the course.

Homework is too hard and takes too much time Some complaints We need more problems assigned for practice Some comments Don t go over pre flights in lecture.

Clarify the concepts from pre flights in lecture Thanks for taking time to answer I will try to balance the requests and address complaintsPhysics 103 Lecture 8 Pg 3 From Lecture 1 Course Philosophy.

Basic Course Philosophy read about it text untangle it lectures play with it labs challenge yourself homework .

close the loop discussion Do make use of discussion office hours etc Try the homework during the weekend so that youcan ask questions during the weekHomework is posted Friday and is due the following Friday .

Exam worries Will post practice exams by MondayYou can bring a single 8 5 x11 sheet with equations to examPhysics 103 Lecture 8 Pg 4 Preflight 8 Question 1.

Consider a person standing in an elevator that is acceleratingupward The upward normal force N exerted by the elevator flooron the person isa larger thanb identical to.

Example 4 11 in the book c less thanthe downward weight W of the person 0 20 40 60 Person is accelerating upwards net upwards force is non zero.

Physics 103 Lecture 8 Pg 5 Preflight 8 Question 3You are pushing a wooden crate across the floor atconstant speed You decide to turn the crate onend reducing by half the surface area in contact.

with the floor In the new orientation to push thesame crate across the same floor with the samespeed the force that you apply must be abouta four times as greatb twice as great.

c equally as great fs sNd half as greate one fourth as greatas the force required before you changed the crateorientation .

Frictional force does not depend on the area of contact It depends only onthe normal force and the coefficient of friction for the contact Physics 103 Lecture 8 Pg 6Tension can be transmitted aroundTension is a force along the.

corners If there is no friction in thelength of a mediumpulleys T remains the samePhysics 103 Lecture 8 Pg 7 Example 8 Pulley Problem I.

What is the tension in the string C W T 2W WSame answer W force WPhysics 103 Lecture 8 Pg 8 Example 9 Pulley Problem II.

What is the tension in the string Physics 103 Lecture 8 Pg 9 Preflight 8 Question 5In the 17th century Otto von Guricke a physicist inMagdeburg fitted two hollow bronze hemispheres together.

and removed the air from the resulting sphere with a pump Two eight horse teams could not pull the halves apart eventhough the hemispheres fell apart when air was readmitted Suppose von Guricke had tied both teams of horses to oneside and bolted the other side to a heavy tree trunk In this.

case the tension on the hemisphere would bea twice what it wasb exactly what it wasc half what it wasPhysics 103 Lecture 8 Pg 10.

Reminder Procedure for Solving Problems Identify force using Free Body Diagram the most crucial step Set up axes Write Fnet ma for each axis.

Calculate acceleration components Use kinematic equations Solve Physics 103 Lecture 8 Pg 11 Problem 4 27.

What force does a trampoline have to apply to a 45 kggymnast to accelerate her up at 7 5 m s2 Fnet ma FtFnet Ft mgFt Fnet mg m a g .

Physics 103 Lecture 8 Pg 12 Problem 4 21 A flea jumps by exerting a force of 1 2 x 10 5 N straightdown on the ground A breeze blowing on the flea parallelto ground exerts a force of 0 5 x 10 6 N on the flea Find.

the direction and magnitude of the acceleration of the flea ifits mass is 6 x 10 7 kg Fynet F mgFxnet f Fy 1 F mg .

tan net tandirection f Fx f F net Fxnet 2 Fynet 2magnitude .

acceleration Physics 103 Lecture 8 Pg 13 Problem 4 33Suppose you have a 120 kg wooden crate resting on a wood floor a What maximum force can you exert horizontally on the crate.

without moving it What kind of friction is impeding movement static kineticfs sN fs smg b If you continue to exert this force once the crate starts to slip.

what will its acceleration be What is the net force acting on the crate Crate starts to slipF fk maForce exertedForce exerted Static friction.

F fk smg kmgForce exerted Kinetic friction a g s k Physics 103 Lecture 8 Pg 14 Example 3 k 0 2.

Similar to example 4 8 in bookFind acceleration of blockResolve T in x and y componentsNormal force is smaller than when pulling along horizontalTension component along horizontal is also smaller.

Fxnet Tx kN Tx k Mg Ty T cos k Mg T sin ax Answer 6 0 m s2Physics 103 Lecture 8 Pg 15 Example 6.

mg cos mg sin Find acceleration and normal forceChoose axis and calculate componentsAnswers a g sin FN mg cos .

Physics 103 Lecture 8 Pg 16 Problems 4 59 and 4 60A contestant in a winter games event pushes a 45 kg block of ice across afrozen lake Calculate the minimum force F he must exert to get the bockmoving What is the force F he needs to exert if he were to pull the.

bock with a rope over his shoulder at the same angle above the horizontal Is F F F cos25o 1 Yes F cos25oF F sin25o 2 No F sin25o F s ice on ice 0 1.

Physics 103 Lecture 8 Pg 17 Problem 4 59 and 4 60 Continuedi e F cos25o fsIce block starts to move when friction is just overcome Case A fs sN s mg F sin25 F .

cos25o s sin25oCase B fs sN s mg F sin25o F cos25o s sin25oF cos25o F cos25oF F sin25o F sin25o F .

s ice on ice 0 1Physics 103 Lecture 8 Pg 18Force exerted Force exerted - Static friction Force exerted - Kinetic friction F fk T=50 N M = 5 kg k = 0.2 Find acceleration of block =500 Hints: Draw FBD Resolve T in x and y components Normal force is smaller than when pulling along horizontal Tension component along horizontal is also smaller Answer: 6.0 m/s2 Similar to example 4.8 in book ...

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