FORCES…

Chapter 2

Force Mechanics: Branch of physics that considers how interactions between objects affect the motion of those objects. Force... is a push or a pullhttp://www.physicsclassroom.com/Class/newtlaws/U2L2a.cfm

Section 2.1

2.1 con’t

Physical interactions change the behavior (motion, temp. etc…) of matter.

Ex. Of interaction forces… kick a soccer ball… force of foot on ball…force of

ball on foot

Long Range forces…

Do not have to be touching ex. Gravity Terms.. magnitude- “how much” weight- result of both mass and

gravity

Other examples of long range forces…

electric, magnetic

2.1 con’t

Contact forces… as you would think… must be

touching hence “contact” ex. Foot on a soccer ball

Recognizing forces…link

http://www.physicsclassroom.com/Class/newtlaws/u2l2b.cfm

Measuring forces

In U.S. - calibrated to measure in lbs. SI unit is the Newton

Conversion factor 1 lb = 4.448 N or 1 N = .2248

lbs

In general… a force is measured by the deformation which is the change in the size or shape it produces.

Force is a vector quantity Vectors- have both magnitude and

direction Other examples of vectors… a) position b) displacement c) displacement d) acceleration e) momentum f) angular momentum g) torque h) electric and magnetic field

Section 2.1 cont

Scalars and vectors…

Scalar- only magnitude…NO direction

ex. Mass 300 kg + 200 kg 500 kg

Link to scalars and vectors

http://www.physicsclassroom.com/Class/vectors/U3L1a.cfm

http://www.physicsclassroom.com/Class/1DKin/U1L1b.cfm

Mops…

Minds on Physics problems http://www.physicsclassroom.com/m

op/topics.cfm

Kinematic concepts assignments… 1 and 2

Section 2.1 cont

Ex. Problem 2.1 Practice prob. 2.1

Section 2.2

Graphical Vector Addition

Head to tail method

Link to vector addition

http://www.physicsclassroom.com/Class/vectors/U3l1b.cfm

Mops… for section 2.2

Minds on Physics problems.. http://www.physicsclassroom.com/m

op/topics.cfm

Vectors/projectiles Assignment 1, 2, 3, 4, and 5

Section 2.2 cont

Ex. Problem 2.2 practice problem 2.2

Section 2.3

Vector addition using components…Link http://www.physicsclassroom.com/Cl

ass/vectors/U3L1d.cfmThe resultant vector is the sum of the X and the Y components. http://www.physicsclassroom.com/Cl

ass/vectors/U3L1eb.cfm

Section 2.3

Mops for section 2.3http://www.physicsclassroom.com/mop/topics.cfm

Forces in two dimension assignment # 1Vector/projectiles assignment # 7

Ex. Problem 2.3 and 2.4 practice prob 2.3 and 2.4

Section 2.4

Inertia and Equilibrium… Newton’s first law of motion

For motion to occur…must have net force?

True or False?

To identify motion… must have a frame of reference.

2.4 cont

Net force…is vector sum of all the forces acting on an object.

First law states… (also called law of inertia)

An object in motion will stay in motion unless a net force acts upon it….and

*** if in constant motion… net force is zero

An object at rest will stay at rest unless a net force acts upon it.

Link Mops…Newton’s laws Assignment 1 - 5

http://www.physicsclassroom.com/mop/topics.cfm

Newton combined ideas of the past for his complete idea Galileo- Inclined plane/ball…based

on no friction. Idea Contradicted Aristotle who

thought everything is at rest so motion would require a net force

Descartes added “in a straight line”

Section 2.4 cont

Ex. 2.5 and practice 2.5

Know free body diagrams… Link http://www.physicsclassroom.com/Cl

ass/newtlaws/U2L2c.cfm

Section 2.4 cont

Translational equilibrium has a net force of zero

Spring scales measure weight…equal and opposite of gravity.

http://www.exo.net/~pauld/activities/physics/springscales.html

Section 2.4 cont

Ex. 2.6 and 2.7 And Practice prob. 2.6 ; 2.7 and 2.8

Section 2.5 Interaction pairs Newton’s 3rd law Force- Always in pairs…called

interaction pairs. Ex. Push the door… the door pushes you

3rd law states- interaction partners always have the same magnitude and in opposite direction.

Link http://

www.physicsclassroom.com/Class/newtlaws/U2l4b.cfm

2.5 cont

Ex. Problems 2.9 practice prob. 2.9 Checkpoint 2.5

***Sometimes two equal and opposites acting on same object… not interactive.

2.5 cont.

Internal and External forces

Internal-Both interacting objects are part of the system, and forces add to zero

External- not so Link… (Review all sections) http://

www.physicsclassroom.com/Class/energy/u5l2a.cfm

Mops… Newton’s laws Assignment… 12

Section 2.6 Gravity Forces Newton’s law of universal gravitation F= Gm1 *m2

r2

W=mg Weight = mass* gravity g = 9.81N/kg Link (Review all sections) http://

www.physicsclassroom.com/Class/circles/u6l3c.cfm

2.6 cont

Ex. Problem 2.11 practice prob. 2.11

Checkpoint 2.6

Mops… Newton’s laws Assignments 6; 7; and 8

Section 2.7 Contact forcesNormal Force… Contact force which is perpendicular To the contact surface…prevents two objects from passing through one another.

Checkpoint 2.7Examples are in types of forces link

2.7 cont.

Friction… a constant force parallel to the contact surface.

a) static friction… “stationary” b) kinetic friction…”moving”

Fsmax is proportional to N force better traction in snow??? How fs= us N Fk= uk N

Section 2.7 cont

Ex. 2.12 and 2.13 Practice prob. 2.12 and 2.13

Friction is explained on previous link

Microscopic origin of Friction…Skip!

Sect 2.7 cont.

Equilibrium on an inclined plane

Link http://

www.physicsclassroom.com/Class/vectors/U3L3e.cfm

Ex. Prob. 2.14 Practice prob. 2.14

2.8 Tension

Tension and pulley systems

Link http://

www.physicsclassroom.com/Class/vectors/U3L3f.cfm

Ex. Problems 2.15 and 2.16 Practice problems 2.15 and 2.16 Plus… Many others!!!

2.8 cont.

Mops…newton’s laws Assignment… # 4

Section 2.9

Fundamental Forces Forces have 4 different types of

interactions… a) Gravity… the weakest. Gets weaker

as the distance increases, but never gets to zero

b) Electromagnetism…acts on particles with electric charge…binds electrons to nucleus. Ex. Solids, liquids, gasses, also friction and normal force. Springs; muscles; and wind. **much stronger than gravity

2.9 cont.

c) Strong … atoms d) weak force Radioactive

decay particles

End of Chapter 2 concepts

Link is in chapter 2 HW/Lab folder