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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