ub, phy101: chapter 6, pg 1 physics 101: chapter 6 work and kinetic energy l new stuff: chapter 6,...

UB, Phy101: Chapter 6, Pg 1

Physics 101: Physics 101: Chapter 6 Chapter 6Work and Kinetic EnergyWork and Kinetic Energy

New stuff: Chapter 6, sections 6.1 - 6.7


Work & EnergyWork & Energy

One of the most important concepts in physicsAlternative approach to mechanics

Many applications beyond mechanicsThermodynamics (movement of heat)Quantum mechanics...

Very useful toolsYou will learn new (sometimes much easier)

ways to solve problems


Work/Kinetic Energy Theorem:Work/Kinetic Energy Theorem:{NetNet WorkWork done on object}

={changechange in kinetic energy kinetic energy of object}

KWnet

12 KK

21

22 mv

21mv

21

Also works for a variable force

KE Demo


Work and Kinetic Energy:

KE = Wnet

• WF = |F| |S| cos

• KE = 1/2 mv2

• Work-Kinetic Energy Theorem:


Chapter 6, Preflight Chapter 6, Preflight

You are towing a car up a hill with constant velocity. The work done on the car by the normal force is:1. positive2. negative3. zero

W

T

FN V

The normal force is perpendicular to the displacement, hence, does no work

correct


Chapter 6, PreflightChapter 6, Preflight

You are towing a car up a hill with constant velocity. The work done on the car by the gravitational force is:1. positive2. negative3. zero

with the surface defined as the x-axis, the x component of gravity is in the opposite direction of the displacement, therefore work is negative.

W

T

FN V

correct


Chapter 6, Preflight Chapter 6, Preflight

You are towing a car up a hill with constant velocity. The work done on the car by the tension force is:1. positive2. negative3. zero

Tension is in the same direction of the displacement

W

T

FN V

correct


Chapter 6, PreflightChapter 6, Preflight

You are towing a car up a hill with constant velocity. The total work done on the car by all forces is:1. positive2. negative3. zero

The car is not accelerating, so it has a net force of zero and since work=Fcos0(s), work too will equal zero.

W

T

FN V

correct

the initial KE equils the final KE and so the difference is zero


Work Done by GravityWork Done by Gravity

Example 1: Drop ball

Yi = h

Yf = 0

Wg = (mg)(S)cosS = h

Wg = mghcos(00) = mghy = yf-yi = -h

Wg = -mgy

mg S

y

x

Yi = h

Yf = 0

mg S

y

x


Work Done by GravityWork Done by Gravity Example 2: Toss ball up

Wg = (mg)(S)cosS = h

Wg = mghcos(1800) = -mgh

y = yf-yi = +h

Wg = -mgy

Yi = h

Yf = 0

mg S

y

x


Work Done by GravityWork Done by Gravity Example 3: Slide block down incline

Wg = (mg)(S)cos S = h/cos

Wg = mg(h/cos)cosWg = mgh

y = yf-yi = -hWg = -mgy

h

mg S


Summary: Work Done by GravitySummary: Work Done by Gravity

Independent of path

Wg = -mg(yf - yi) = -mgy

If you end up where you began, Wg = 0

We call this a “Conservative Force” because we candefine a “Potential Energy” to go with it.


Chapter 6, PreflightChapter 6, Preflight(who read section 6.4 in the book ?)

Which of the following statements correctly define a Conservative Force: 1. A force is conservative when the work it does on a moving object is independent of the path of the motion between the object's initial and final positions. 2. A force is conservative when it does no net work on an object moving around a closed path, starting and finishing at the same point. 3. Both of the above statements are correct. 4. Neither of the above statements is correct.

correct


Chapter 6, PreflightChapter 6, PreflightImagine that you are comparing three different ways of having a ball move down through the same height. In which case does the ball get to the bottom first?

1. Dropping2. Slide on ramp (no friction)3. Swinging down4. All the same

It seems logical because A has the shortest distance to go, although the answer is probably all 3, for some crazy physics reason.

1 2 3

correct


Chapter 6, PreflightChapter 6, PreflightImagine that you are comparing three different ways of having a ball move down through the same height. In which case does the ball reach the bottom with the highest speed?

1. Dropping2. Slide on ramp (no friction)3. Swinging down4. All the same

In all three experiments, the balls fall from the same height and therefore the same amount of their gravitational potential energy is converted to kinetic energy. If their kinetic energies are all the same, and their masses are the same, the balls must all have the same speed at the end.

1 2 3correct


Friction Demo


Modified Work-Kinetic Energy Modified Work-Kinetic Energy TheoremTheorem

• Work-Kinetic Energy Theorem: •WNC = KE + PEg = KE + mgy

• E = total energy = KE + PEg

• WNC = E = Ef - Eih

• Conservation of Energy:•If WNC = 0, then Ef = Ei

(KE+PEg)initial = (KE+PEg)final

NC: all forces except gravity

Friction Demo 2


Chapter 6, PreflightChapter 6, PreflightSuppose the initial kinetic and potential energies of a system are 75J and 250J respectively, and that the final kinetic and potential energies of the same system are 300J and -25J respectively. How much work was done on the system by non-conservative forces? 1. 0J 2. 50J 3. -50J 4. 225J 5. -225J

correct

Work done by non-conservative forces equals the difference between final and initial kinetic energies plus the difference between the final and initial gravitational potential energies.

W = (300-75) + ((-25) - 250) = 225 - 275 = -50J.


Conservation of Energy Demo

ub, phy101: chapter 6, pg 1 physics 101: chapter 6 work and kinetic energy l new stuff: chapter 6,...

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