For me Delete before class Inertia Laziness in

For me: Delete before class • Inertia = Laziness in Latin • Let students give examples … work towards car and seatbelt. • Importance of wearing a seatbelt An object in motion tends to remain in motion… • Demos: Coins on elbow, knock out bottom weights, index card on cup with quarter on top, roll ball, bed of nails, mass and cars and pulleys, showing acceleration.

• Start class with Kristen’s chain videos? Can do clicker… • http: //www. youtube. com/watch? v=1 er. U-Cwcl 2 c • http: //www. youtube. com/watch? v=X-QFAB 0 g. Et. E?

When turned on do you expect the “fan cart” to go a) b) c) d) e) to the left at const v. to the right at const v. to the left at const a. to the right at const a. nowhere.

Now with the sail attached the cart will move a) b) c) d) e) to the left faster. to the right faster. to the left slower. to the right slower. nowhere.

Which of these is a proper statement of Newton’s Second Law 1. 2. 3. 4. 5. A) F = ma B) SF = ma NONE are C) SFnet = ma correct! D) Fnet = ma E) S|Fnet| = ma

Class Notes 25 September • For HW due Wednesday § Direction on Forces & Acceleration!!! • No direction, half credit at best! § Did you draw a diagram showing all forces? • Today: § Newton’s Second Law - example § Newton’s Third Law § Weight, Tension, Normal Force

Newton’s First Law

Newton’s Second Law FSF=ma net NEVER write “F=ma”. That F is the net force and you will misuse the eqn. Only write SF=ma !

Example • A particle of mass 3. 00 kg is subjected to three forces: F 1 = 1. 00 N left, F 2 = 1. 00 N up, F 3 = 3. 00 N at 30. 0º. If it starts from rest, find its velocity after 2. 00 s. 2. 57 m/s at 65. 5°

Example • Calculate the required force to lb propel a 140. (63. 5 kg) man in a cannon such that he achieves a (maximum) 194 -ft range. Take the cannon depth to be 18. 0 ft and the force constant. Fave = 3370 N, or 760 pounds!

Newton’s Third Law • To every action there is an equal & opposite reaction …

• Barely got here. • Demos for inertia: Coins on elbow, knock out bottom weights, index card on cup with quarter on top • Demo for N 2: Used wind-up green truck to show accel is inversely proportional to mass. Simply wind up the same, but add 150 g to the bed.

When I drop a full cup with two holes the water will a. b. c. d. Shoot out both sides Shoot out one side Not come out Come out from the top while falling

Newton’s Third Law • To every action there is an equal & opposite reaction … FAB = –FBA …Each member of this force pair always acts on a DIFFERENT body. • Law of interaction.

Which of the masses weighs about 1 Newton on the earth? Rank 1 2 3 Responses 1 1000 100 4 5 6 4 5 Other Values: 6, {6, 5. 99} Value Matches: 0

Three Particular Forces • Force of gravity: § Mass: The INTRINSIC amount of substance.

Three Particular Forces • Force of gravity: § Mass: The INTRINSIC amount of substance. § Weight: The FORCE due to gravity on an object FG = W = mg (magnitude) Direction: With ag, down!

Weight vs. Mass N. W. stands for net weight. Not quite correct!

Three Particular Forces • Weight: On Earth it is Fg = mg • Tension… FT • The so-called Normal Force: § When an object is against a surface it experiences a contact force that is perpendicular to the surface. § The magnitude of the force varies, and will be only as large as it needs to be.

Normal Force: Various What is the normal force acting on the block? M m FT q FN=? =(m+M)g m =mg–FTsinq FN=? a=FTcosq/m a=?

Putting it all together • FREE BODY DIAGRAMS! § A diagram separating the ONE object of interest and showing ALL the forces acting on it. • Some things to remember § Strings can only pull § Pulleys have no friction § Draw FBDs, coordinate system § Write equations (SF=ma) (x & y) § Solve!

Example • A force pulls on a 4. 00 -kg block at an angle of 30. 0°, with a force of 20. 0 N. What is the normal force? What is the acceleration? What is its velocity and how far has it traveled after 5. 00 s? 29. 2 N, 4. 33 2 m/s , 21. 7 m/s, 54. 1 m

If a Mack truck and Honda Civic have a head-on collision, upon which vehicle is the impact force greater? a) b) c) d) e) Car Truck Same Neither – they both explode!

Homework • Read Chap 4 sec. 7 • Do problems – Worksheet #15 § (In each one draw Free Body Diagram or diagrams showing ALL forces!!!!) • Rework all of test 2 on Web. Assign by 10: 00 AM Friday • Formal Lab due today