Week 4 Assignments: - HW #4 Chp 4: Q2, Q4, P2, P18, P28, P35, P48, P51 || chp 5: Q5, P11

- MasteringPhysics: - Assignment #4

Week 4 Reading: Chapter 5 - Giancoli

This Week’s Announcements:

Lecture 9: Forces (Cont.)

Due: 9/22

* HW #2 will be returned in recitation

* HW #3 due today

Forces - 1) Action at a distance forces (fundamental forces):

- force always towards the center of the earth

- Gravitational Force (Fg) - force linearly dependent of mass of the the object

Name strength range Interaction Electromagnetism ~10-2 infinite +/- charges

Gravity ~10-40 infinite Attracts everything

- Electromagnetic Force - force linearly dependent on charge of the the object - force is between charges - much stronger than Gravity!!!!!!!!!

- Like charges repel, unlikes attract

- unlikes attract: locked up in atoms

Gravitational Force:

Forces --- Quantitative

Fgrav = m g

m

mg

Contact Forces

- force points perpendicular to the contact surface

- Normal Force (N)

2) Contact Forces (just hidden versions of the electromagnetic force):

m

mg

N

Contact Forces

- force points perpendicular to the contact surface

- Normal Force (N)

- force is in the direction of the “pull”

- Tension Force (T)

- a balancing force, remember not to “double count” the magnitude

* Contact Forces (just hidden versions of the electromagnetic force):

1N 1N

N

T

Clicker Question:

1) Compare the situation in a) to that in b). How does the tension at point P compare in each situation. (Assume masses on right are initially at rest.)

P P

a) Tension at P in a) in larger than in b) b) Tension at P in b) in larger than in a) c) Tension at P in a) and b) are equal

Clicker Question:

1)

P P

a) Tension at P in a) in larger than in b) b) Tension at P in b) in larger than in a) c) Tension at P in a) and b) are equal

Compare the situation in a) to that in b). How does the tension at point P compare in each situation. (Assume masses on right are initially at rest.)

Forces --- Quantitative

Spring Tension Force: Fsp = -k Δx

- The spring force is in the opposite direction of the spring stretch

Δx

Fsp

Δx

Fsp

xeq

- Depends linearly on the magnitude of the stretch

(Δx = x – xeq)

Contact Forces

- 2) Contact Forces (just hidden versions of the electromagnetic force):

- force points perpendicular to the contact surface

- Normal Force (N)

- force is in the direction of the “pull”

- Tension Force (T)

-  force parallel to surface and against the direction of motion

- Frictional Force (Fs or Fk)

- magnitude is related to the force against the surface (FN) and its “roughness”

- Drag Force (Fd) -  like friction except is directed perpendicular to surface and its magnitude depends on an objects velocity and surface area

Appr

oxim

atio

ns

N

T

Gravitational Force:

Forces --- Quantitative

Fgrav = m g

Frictional Forces: Fs ≤ µs N

Fk = µk N

Normal Force: N Tension Force: T

Important caveat

Spring Tension Force: Fsp = -k x

N

Fs (fric) v=0

N

Fk (fric) v

Gravitational Force:

Forces --- Quantitative

Fgrav = m g

Frictional Forces: Fs ≤ µs N

Fk = µk N

Normal Force: N Tension Force: T

Centripetal Force: Fcp = m v2/r

Clicker Question:

2) Your kitty likes to jump on you when you enter the room. During its flight through the air, what are the forces operating on the kitty?

a) Force of the kitty’s legs that starts it moving, gravity

c) gravity

b) Normal force, air resistance, gravity

d) gravity, air resistance

e) none, it is in uniform motion

Clicker Question:

2) Your kitty likes to jump on you when you enter the room. During its flight through the air, what are the forces operating on the kitty?

a) Force of the kitty’s legs that starts it moving, gravity

c) gravity

b) Normal force, air resistance, gravity

d) gravity, air resistance

e) none, it is in uniform motion

Forces

- Different types of forces include contact, interaction, action-at-a-distance and tension forces

- Forces are vectors:

Fair Fg

Feng

Flying in a straight line with constant speed:

Accelerating upward:

Fair Fg Feng

Fnet = 0

Fnet ≠ 0

- When there is more than one force involved what matters is the net force:

- ID object of interest - create “free-body diagrams”

- Equate to manet

- Forces sum vectorially

- Solve kinematics give anet

* What is the weight (magnitude of N) a scale would read if a person was in an elevator accelerating upward at anet, due to the tension T of a cable ?

T

Fgr

N Fnet

* What is the weight a scale would read if a person was in an elevator accelerating downward at anet, due to the tension T of a cable ?

T

Fgr

N Fnet

N – Fgr = manet

N – Fgr = -manet

* What if anet is downward and equal in magnitude to g?