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Announcements. Exam solutions on CULearn . Scores hopefully by tomorrow and booklets returned next week. Read Ch. 6 (Work & Energy): Entire chapter. Read ahead Ch. 7 (Momentum): Ch. 7.1 – 7.6, 7.8. CAPA assignment #8 is due Friday at 10 pm

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Presentation Transcript
slide1

Announcements

  • Exam solutions on CULearn. Scores hopefully by tomorrow and booklets returned next week.
  • Read Ch. 6 (Work & Energy): Entire chapter.
  • Read ahead Ch. 7 (Momentum): Ch. 7.1 – 7.6, 7.8.
  • CAPA assignment #8 is due Friday at 10 pm
  • This week in Section, Assignment #5: Work & Energy Print and bring to your Lab Section.
  • Standard Wednesday evening review session tonight
  • Nagle help room hours today 1:45 – 3:45 pm
  • Kinney help room hours Thursday 11 am - noon
slide2

1. Work:

Component of “specific” force along displacement x displacement

2. Kinetic Energy:

KE =

3. Work – Energy Principle:

Wnet = WFnet = ΔKE = KEf - KEi

(No change in PE, no dissipation.)

slide3

4. Potential energy:

PE is the amount of work done on a system by an external force when KE does not change and

no heat flows (no friction):

ΔPE = Wext when ΔKE = 0

Example: PEgrav= mgy

5. Conservation of “Mechanical Energy”:

Emechanical = KE + PE = constant

(isolated system, no dissipation)

slide4

Announcements

  • Exam solutions on CULearn. Scores hopefully by tomorrow and booklets returned next week.
  • Read Ch. 6 (Work & Energy): Entire chapter.
  • Read ahead Ch. 7 (Momentum): Ch. 7.1 – 7.6, 7.8.
  • CAPA assignment #8 is due Friday at 10 pm
  • This week in Section, Assignment #5: Work & Energy Print and bring to your Lab Section.
  • Standard Wednesday evening review session tonight
  • Nagle help room hours today 1:45 – 3:45 pm
  • Kinney help room hours Thursday 11 am - noon
slide5

1. Work:

Component of “specific” force along displacement x displacement

2. Kinetic Energy:

KE =

3. Work – Energy Principle:

Wnet = WFnet = ΔKE = KEf - KEi

(No change in PE, no dissipation.)

slide6

4. Potential energy:

PE is the amount of work done on a system by an external force when KE does not change and

no heat flows (no friction):

ΔPE = Wext when ΔKE = 0

Example: PEgrav= mgy

5. Conservation of “Mechanical Energy”:

Emechanical = KE + PE = constant

(isolated system, no dissipation)

slide7

Clicker Question

Room Frequency BA

Consider mass mswinging attached to a string of length L.

The swing is released from rest at a height h.

What is the speed v of the swing when it reaches height h=0?

KE = ½ mv2

PEgrav = mgy

slide8

Clicker Question

Room Frequency BA

A projectile is fired with an initial speed voat

an angle θ from the horizontal.

What is the KE of the projectile when it is on the way down at a height h above the ground?

(Assume no air resistance)

Use PE=0 at ground level

½ mvo2 + mgh

mgh

½ mvo2 – mgh

Impossible to tell.

slide9

Clicker Question

Room Frequency BA

A mass slides down a frictionless ramp of height h.

Its initial speed is zero.

Its final speed at the bottom of the ramp is v.

h

As the mass descended, its KE

A) increased B) decreased C) remained constant

As the mass descended, its PE

A) increased B) decreased C) remained constant

As the mass descended, its (KE + PE) = total mechanical energy

A) increased B) decreased C) remained constant.

slide10

Clicker Question

Room Frequency BA

Mechanical Energy of a system is conserved if there is no dissipation.

A physical system with no “dissipation”

is one in which:

There is no friction.

There is friction.

No external forces are applied.

External forces are applied.

Energy is not conserved.

Friction results in energy transferred to heat and thus loss of mechanical energy.

slide11

Clicker Question

Room Frequency BA

Mechanical Energy of a system is conserved if the system is isolated.

An “isolated” physical system is one in which

There is no friction.

There is friction.

No external forces are applied.

External forces are applied.

Energy is not conserved.

External forces can do work on the system.

Thus energy is transferred into

or out of the system.

slide13

For the bowling ball to hit me, it would

be an example of violation of conservation of energy.

PE = ME, KE = 0 (velocity = 0 at turning point)

PE = 0, ME = KE (maximum velocity)

slide15

Clicker Question

Room Frequency BA

A block initially at rest is allowed to slide down a frictionless ramp and attains a speed v at the bottom.

How is the initial height h related to the

speed at the bottom of the ramp?

PE = 0

slide16

Clicker Question

Room Frequency BA

A hockey puck slides without friction along a frozen lake toward an ice ramp and plateau as shown. The speed of the puck is 4 m/s and the height of the plateau is 1 meter.

+y

0

Will the puck make it all the way up the ramp?

Yes B) No C) Depends on the puck mass

ME(initial) = KEi + PEi = ½ mv2 + 0

= (8 x mass) Joules

ME(on the plateau) = KEp + PEp = ½ mvp2 + mgh

>= (9.8 x mass) Joules

Not enough energy to get up the ramp!