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General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces
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General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Dec 16, 2015

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Kurt Dempster
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Page 1: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

General Physics I: Day 16Elastic Potential Energy & Nonconservative Forces

Page 2: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Conservative Forces & Potential Energy

Conservative forces can “store” energy. Technically defined in terms of “path-independent” work.

ALL forces conserve energy. Conservative forces also conserve mechanical energy.

Potential energy:

• Only associated with conservative forces

• Potential energy is associated with a system

• Depends on an arbitrary (usually) reference point

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Page 3: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

3WarmUp: Gravity vs. FrictionA trunk of mass m is lifted along a curved path of length L to a height h. Another trunk with twice the mass is slid across a level floor () along a curved path also having length L. Which is greater, the work done against friction or the work done against gravity?

~51% → More work is done against friction.

~27% → More work is done against gravity.

~16% → The work done against friction is the same as the work done against gravity.

~6% → Cannot be determined from the given information.

Page 4: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Calculating Potential Energy

For conservative forces, potential energy equals how much work must be done against that force to achieve a certain configuration.

For gravity, to move an object from yi to yf:

Thus, gravitational potential energy:

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for a force done against force done by forceU W W

against grav.W mg y against i grav.o r, if =0 y W mgy

gU mgy

Page 5: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Cons. of Mech. Energy (Gravity only)

We define mechanical energy:

For an isolated system with only conservative forces, the total mechanical energy cannot change!

This means that any potential energy gained comes from kinetic energy lost, and vice versa.

or

or

5

.mechE U K

U K

mech., initial mech., finalE E

i i f fU K U K

Page 6: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Applying Conservation of Mech. Energy

If you decide to use conservation of energy (or of mechanical energy) you must

• Choose your system!

• Decide on initial and final situations

–One should be as simple as possible, or should be one you know a lot about

–The other should be what you are trying to learn about

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Page 7: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

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Warm-Up: Work & PE

You throw a ball straight up in the air. On its way up is the work done by gravity positive or negative? Is the change in potential energy positive or negative during this same period?

~10% → Positive W, negative ΔUg

~0% → Positive W, positive ΔUg

~65% → Negative W, positive ΔUg

~15% → Negative W, negative ΔUg

~15% → Didn’t answer both parts… lost a point.

Page 8: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

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Warm-Up: Work & PE

“Potential energy is positive, the higher something is the more kinetic energy that can be produced The work is negative, due to the fact that the force of gravity is opposing the current motion of the ball”

Some missed the word “change”!

Page 9: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

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Warm-Up: Work & PE

“The work done by gravity is negative, that is because the work is done in the opposite direction of motion. The change in potential energy is positive, because the gravitational potential energy of the ball increases as the height increases.”

“The change in potential energy is positive but since work= -change in potential energy. Work done by gravity is negative.”

Page 10: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

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Two marbles, one twice as heavy as the other, are dropped to the ground from the roof of a building. Just before hitting the ground, the heavier marble has

A) as much kinetic energy as the lighter one.

B) twice as much kinetic energy as the lighter one.

C) half as much kinetic energy as the lighter one.

D) four times as much kinetic energy as the lighter one.

E) Cannot be determined from what is given.

Page 11: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

A coconut is thrown at a speed of 8 m/s from the top of a coconut tree and lands on the ground. Rank the following directions in order of which will result in the largest speed when the coconut hits the ground.

1) Almost straight up 2) 45° above horizontal

3) Horizontal 4) 45° below horizontal

5) Straight down

A) 1 > 2 > 3 > 4 > 5 B) 1 = 5 > 2 = 4 > 3

C) 1 = 2 = 3 = 4 = 5 D) 5 > 4 > 3 > 2 > 1

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Page 12: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Energy: The easy way!

A block slides down a frictionless ramp, starting out 1.5 meters above the ground (height).

How fast is it going at the bottom of the ramp?

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Page 13: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Two balls will be released from rest at the top of the apparatus shown. Ball 1 travels on the straight track, while ball 2 travels on the bent track. If we measure the speed of each ball as it leaves the track, what will we find?

A) Ball 1 will have a larger final speed.

B) Ball 2 will have a larger final speed.

C) They will have the same final speed.

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Page 14: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Two balls will be released from rest at the top of the apparatus shown. Ball 1 travels on the straight track, while ball 2 travels on the bent track. If they are released at the same time, which ball will reach the end first?

A) Ball 1 will win the race!

B) Ball 2 will win the race!

C) They will tie!

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Page 15: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Sample Problem

A Hot Wheels car will roll down a track as shown on the board. Assume there is no friction in the motion. From what height should the car be released so that it is going 0.8 m/s when it reaches point B which is 11 cm above the bottom of the track?

An aside: On a homework question you have to figure out how fast something must go to make it through a loop-the-loop…

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Page 16: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

16Worked-Example: Spring Ball Launch

A vertical spring with a spring constant k = 150 N/m is compressed down 1.5 m. A 2-kg ball is placed on the compressed spring and released from rest. What height does the ball reach after it is released?

Page 17: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

17Worked-Example: Spring Ball Launch

Page 18: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

18Worked-Example: Spring Ball Launch

Page 19: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

19Worked-Example: Spring Ball Launch

Page 20: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

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Elastic Potential Energy

Since

Where x is measured from the relaxed position.

x

F

21by spr. 2W kx

F=kx

for a force done by that forceU W 212springU kx

Page 21: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Difficult/Interesting

“I thought it was interesting that unlike gravitational potential energy, for elastic potential energy we can't just choose x to be 0. It has to be at the point where the spring is neither compressing or stretching.”

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Page 22: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Cons. of Mech. Energy (Grav. + Elast.)

We define mechanical energy:

For an isolated system with only conservative forces, the total mechanical energy cannot change!

This means that any potential energy gained comes from kinetic energy lost, and vice versa.

or

or

22

.mechE U K

U K

mech., initial mech., finalE E

i i f fU K U K

Page 23: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

Sample Problem

A block (m = 4.5 kg) slides, from rest, down a 12 meter frictionless ramp, which is angled at 30° above the horizontal. The block then slides along a frictionless level surface until it encounters a spring

(k = 220 N/m) whose other end is attached to a wall. What is the maximum distance that the spring will be compressed by?

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Page 24: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

A 2.0 kg block is moving at 10 m/s along a horizontal frictionless table. It encounters and compresses a spring whose other end is attached to a wall. How much potential energy is stored in the spring at the moment when the block stops moving?

A) 10 J B) 20 J

C) 100 J D) Not enough information

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Page 25: General Physics I: Day 16 Elastic Potential Energy & Nonconservative Forces.

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Coming up…Thursday (10/16) → 7.4 – 7.5

Chapter 7 Homework due Sunday by 11:59 PM

Warm-Up due Wednesday by 10:00 PM

In-progress grades are posted!