Chapter 6Chapter 6Force and MotionForce and Motion

An object that experiences a push or a pull An object that experiences a push or a pull has a has a FORCEFORCE exerted on it. exerted on it.

The object is called the The object is called the SYSTEM.SYSTEM.

The world around the object that exerts The world around the object that exerts forces on it is called the forces on it is called the ENVIRONMENT.ENVIRONMENT.

Forces have both Forces have both magnitudemagnitude and and directiondirection and are therefore _____________.and are therefore _____________.

Contact vs. Long-Range ForcesContact vs. Long-Range Forces

Contact forcesContact forces – Act on an object only by – Act on an object only by touching it.touching it.

Long-Range ForcesLong-Range Forces – Exerted without – Exerted without contact. Ex. The force of gravity.contact. Ex. The force of gravity.

Each force has a specific, identifiable, Each force has a specific, identifiable, immediate cause called the immediate cause called the AgentAgent. . Ex. Gravity – Earth’s massEx. Gravity – Earth’s mass

VECTORS.VECTORS.

Newton’s Second Law of MotionNewton’s Second Law of Motion

FFnetnet = = mmaa

or ∑ or ∑ F = F = mmaa

Or Or aa = = FFnetnet/m/m

Recall: vectors are added and subtracted Recall: vectors are added and subtracted in one direction at a time only. in one direction at a time only. Acceleration is a vector quantity. Mass is Acceleration is a vector quantity. Mass is a scalar quantity.a scalar quantity.

Newton’s First LawNewton’s First Law

Known as the Known as the Law of InertiaLaw of Inertia..

EquilibriumEquilibrium – Net forces are zero! – Net forces are zero!

Free-Body DiagramsFree-Body Diagrams– Net Forces = sum of all forcesNet Forces = sum of all forces

Constructing Free-Body DiagramsConstructing Free-Body Diagrams

1.1. Sketch the Problem.Sketch the Problem.

2.2. Choose a coordinate system.Choose a coordinate system.

3.3. Locate EVERY point where the Locate EVERY point where the environment touches the system.environment touches the system.

4.4. Draw a motion diagram including the Draw a motion diagram including the velocity and acceleration vectors.velocity and acceleration vectors.

5.5. Draw the Free-Body diagram.Draw the Free-Body diagram.

6.6. Check your answer. –vector addition!Check your answer. –vector addition!

Review Ch. 6 Sec. 1Review Ch. 6 Sec. 1

For the following ten items, Identify as:For the following ten items, Identify as:

a. Contact Forcea. Contact Force

b. Long-Range Forceb. Long-Range Force

c. Not a Forcec. Not a Force

Yes this is a quiz!Yes this is a quiz!

1.1. WeightWeight

2.2. MassMass

3.3. InertiaInertia

4.4. Push of a handPush of a hand

5.5. Air dragAir drag

6.6. Spring forceSpring force

7.7. AccelerationAcceleration

8.8. FrictionFriction

9.9. TensionTension

10.10. Mass times accelerationMass times acceleration

6.2 Using Newton’s Laws6.2 Using Newton’s Laws

Newton’s second law gives us a connection Newton’s second law gives us a connection between the net force exerted on an object and between the net force exerted on an object and its acceleration. The law identifies the its acceleration. The law identifies the causecause of of a change in velocity and the resulting a change in velocity and the resulting displacement.displacement.

Aristotle’s followers believed that the heavier an Aristotle’s followers believed that the heavier an object the faster the fall. Galileo hypothesized object the faster the fall. Galileo hypothesized that all objects, no matter their weight, gain that all objects, no matter their weight, gain speed at the same rate.speed at the same rate.

What is the weight force What is the weight force FFgg, exerted , exerted on an object of mass m?on an object of mass m?

FFgg = m = mgg

ScalesScales – what do they measure? – what do they measure?

Apparent Weight:Apparent Weight:What would happen to the What would happen to the readings on the bathroom scale if readings on the bathroom scale if you took readings in an elevator?you took readings in an elevator?What would happen to the What would happen to the readings if the cable holding the readings if the cable holding the elevator were to break?elevator were to break?Does this mean you have no Does this mean you have no weight?weight?

Friction:Friction:

Imagine pushing on a crate and it not Imagine pushing on a crate and it not moving. You push harder and it still moving. You push harder and it still does not move across the floor.does not move across the floor.

Describe the forces acting on the Describe the forces acting on the crate.crate.

The force The force opposingopposing you moving the you moving the crate crate from restfrom rest is called is called STATIC STATIC FRICTIONFRICTION..

Static friction:Static friction:– http://www.physclips.unsw.edu.au/jw/weight_ahttp://www.physclips.unsw.edu.au/jw/weight_a

nd_friction.htm#coefficientsnd_friction.htm#coefficients

Kinetic friction:Kinetic friction:– http://www.physclips.unsw.edu.au/jw/weight_ahttp://www.physclips.unsw.edu.au/jw/weight_a

nd_friction.htm#coefficientsnd_friction.htm#coefficients

FFff kinetickinetic = = μμkk FFNN

0 ≤ 0 ≤ FFff static static ≤ ≤ μμss FFN N

Some common coefficients are in table 6-3, pg 131Some common coefficients are in table 6-3, pg 131

Causes of friction:Causes of friction:

When two surfaces touch, they temporarily When two surfaces touch, they temporarily bond. In order to move the object one bond. In order to move the object one must break this bond. When objects are must break this bond. When objects are moving past each other, there is still an moving past each other, there is still an electrostatic attraction at the atomic level electrostatic attraction at the atomic level and this is the weaker kinetic friction.and this is the weaker kinetic friction.

6.3 Interaction Forces6.3 Interaction Forces

Identifying Interaction Forces.Identifying Interaction Forces.

System and Environment. Consider two System and Environment. Consider two systems whose motions you want to study. systems whose motions you want to study. Recall that the environment is all the other Recall that the environment is all the other systems whose motions are not being studied.systems whose motions are not being studied.

Ex. – Catcher catching a baseball. There are Ex. – Catcher catching a baseball. There are external forces acting on both systems and also external forces acting on both systems and also interactions between the two systems.interactions between the two systems.

F F handhand on ballon ball F F ballball on handon hand

OROR

FFAA on Bon B F FB on AB on A

Question?Question?

Does this mean the ball causes the hand Does this mean the ball causes the hand to exert a force?to exert a force?

The two forces either exist together or not The two forces either exist together or not at all.at all.

Newton’s Third LawNewton’s Third Law

An interaction pair is two forces that are An interaction pair is two forces that are equal in magnitude but opposite in equal in magnitude but opposite in direction.direction.

**ALL FORCES COME IN PAIRS.ALL FORCES COME IN PAIRS.

Fundamental ForcesFundamental Forces

Four Fundamental Forces.Four Fundamental Forces.GravityGravity

Electromagnetic Ex. Static cling, molecular bonds.Electromagnetic Ex. Static cling, molecular bonds.

Strong Nuclear – acts between protons and Strong Nuclear – acts between protons and neutrons.neutrons.

Weak Nuclear – some kinds of radioactive decay.Weak Nuclear – some kinds of radioactive decay.

Goal: To show that at some level, all four Goal: To show that at some level, all four interactions are really one!interactions are really one!

Forces on Ropes and StringsForces on Ropes and Strings

Identify the Force Pairs.Identify the Force Pairs.

If the rope breaks, the bucket will fall, so If the rope breaks, the bucket will fall, so there must be a force holding the rope there must be a force holding the rope together. The force that the top part of the together. The force that the top part of the rope exerts on the bottom part isrope exerts on the bottom part is

F F TOP ON BOTTOMTOP ON BOTTOM

Newton’s Third Law states that this is part Newton’s Third Law states that this is part of an interaction pair. So….of an interaction pair. So…. F F BOTTOM ON TOPBOTTOM ON TOP

wouldwould be the other half.be the other half.

These forces are equal in magnitude but These forces are equal in magnitude but opposite in direction.opposite in direction.

If the bucket is in equilibrium, the net force If the bucket is in equilibrium, the net force must be zero! So the tension on the top of must be zero! So the tension on the top of the rope must be the weight of the bucket.the rope must be the weight of the bucket.

Tug of war example.Tug of war example.

6.4 Pulleys6.4 Pulleys

Examine the following diagram and label Examine the following diagram and label the forces.the forces.

m1 m2

Frictionless pulley

If mIf m11 = 0.5 kg and m = 0.5 kg and m22 = 0.2 kg what = 0.2 kg what

happens to the system?happens to the system?

What does the pulley do?What does the pulley do?

m1 m2

Frictionless pulley

PSSPSS

1.1. Separate the system from the environment.Separate the system from the environment.

2.2. Draw a sketch of the problem with a coordinate Draw a sketch of the problem with a coordinate system.system.

3.3. Identify all forces on the system and add them Identify all forces on the system and add them to your sketchto your sketch

4.4. Draw a free-body diagramDraw a free-body diagram

5.5. Use Newton’s Second Law to equate the Use Newton’s Second Law to equate the forcesforces

6.6. Solve the problemSolve the problem

Online practice using FBDOnline practice using FBD

http://www.glenbrook.k12.il.us/gbssci/physhttp://www.glenbrook.k12.il.us/gbssci/phys/shwave/fbd.html/shwave/fbd.html

http://www.physicsclassroom.com/Class/nhttp://www.physicsclassroom.com/Class/newtlaws/newtltoc.htmlewtlaws/newtltoc.html