physics of technology phys 1800

Conservation of Energy

Introduction Section 0 Lecture 1 Slide 1

Lecture 14 Slide 1

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 14




Lecture 14 Slide 2


Fall 2004


Spring 2009

PHYSICS OF TECHNOLOGY Spring 2009 Assignment Sheet

*Homework Handout

Date Day Lecture Chapter Homework Due Jan 5 6 7 9

M T W F*

Class Admin: Intro.Physics Phenomena Problem solving and math Units, Scalars, Vectors, Speed and Velocity

1 App. B, C 1 2

-

Jan 12 14 16

M W F*

Acceleration Free Falling Objects Projectile Motion

2 3 3

1

Jan 19 21 23

M W F*

Martin Luther King Newton’s Laws Mass and Weight

No Class 4 4

2

Jan 26 28 29 30

M W Th F

Motion with Friction Review Test 1 Circular Motion

4 1-4 1-4 5

3

Feb 2 4 6

M W F*

Planetary Motion and Gravity Energy Harmonic Motion

5 6 6

4

Feb 9 11 13

M W F*

Momentum Impulse and Collisions Rotational Motion

7 7 8

5

Feb 16 17 18 19 20

M Tu W H F*

Presidents Day Angular Momentum (Virtual Monday) Review Test 2 Static Fluids, Pressure

No Class 8 5-8 5-8 9

-

Feb 23 25 27

M W F*

Flotation Fluids in Motion Temperature and Heat

9 9 10

6

Mar 2 4 6

M W F*

First Law of Thermodynamics Heat flow and Greenhouse Effect Climate Change

10 10 -

7

Mar 9-13 M-F Spring Break No Classes Mar 16 18 20

M W F*

Heat Engines Power and Refrigeration Electric Charge

11 11 12

8

Mar 23 25 26 27

M W H F*

Electric Fields and Electric Potential Review Test 3 Electric Circuits

12 13 9-12 13

-

Mar 30 Apr 1 3

M W F

Magnetic Force Review Electromagnets Motors and Generators

14 9-12 14

9

Apr 6 8 10

M W F*

Making Waves Sound Waves E-M Waves, Light and Color

15 15 16

10

Apr 13 15 17

M W F*

Mirrors and Reflections Refraction and Lenses Telescopes and Microscopes

17 17 17

11

Apr 20 22 24

M W F

Review Seeing Atoms The really BIG & the really small

1-17 18 (not on test) 21 (not on test)

No test week 12

May 1 F Final Exam: 09:30-11:20am



Lecture 14 Slide 3


Fall 2004


Spring 2009


PHYS 1800

Lecture 14


Introduction



Lecture 14 Slide 4


Fall 2004


Spring 2009

Describing Motion and Interactions

Position—where you are in space (L or meter)

Velocity—how fast position is changing with time (LT-1 or m/s)

Acceleration—how fast velocity is changing with time (LT-2 or m/s2)

Force— what is required to change to motion of a body (MLT-2 or kg-m/s2)

In this chapter we will develop on of the most useful concepts in science…ENERGY…and learn what it means to conserve energy.



Lecture 14 Slide 5


Fall 2004


Spring 2009

• Work is equal to the force applied times the distance moved.– Work = Force x Distance: W = F d– Work output = Work input

• units: 1 joule (J) = 1 Nm = 1 kg m2 / s2 [ML2T-2]

Defining Work



Lecture 14 Slide 6


Fall 2004


Spring 2009

• Only forces parallel to the motion do work.• Power is the rate of doing work

– Power = Work divided by Time: P = W / t

units: 1 watt (W) = 1 J / s = 1 kg m2 / s3 [ML2T-3]

Work and Power



Lecture 14 Slide 7


Fall 2004


Spring 2009


PHYS 1800

Lecture 14


Kinetic Energy



Lecture 14 Slide 8


Fall 2004


Spring 2009

Kinetic Energy

• Kinetic energy is the energy associated with an object’s motion.– Doing work on an object increases its kinetic energy.– Work done = change in kinetic energy

2

2

1mvKE



Lecture 14 Slide 9


Fall 2004


Spring 2009

Kinetic Energy

• Negative work is the work done by a force acting in a direction opposite to the object’s motion.– For example, a car skidding to a stop– What force is acting to slow the car?



Lecture 14 Slide 10


Fall 2004


Spring 2009


PHYS 1800

Lecture 14


Potential Energy



Lecture 14 Slide 11


Fall 2004


Spring 2009

Potential Energy

• If work is done but no kinetic energy is gained, we say that the potential energy has increased.– For example, if a force is

applied to lift a crate, the gravitational potential energy of the crate has increased.

– The work done is equal to the force (mg) times the distance lifted (height).

– The gravitational potential energy PEgravity=mgh.



Lecture 14 Slide 12


Fall 2004


Spring 2009

Work is done on a large crate to tilt the crate so that it is balanced on one edge, rather than sitting squarely on the floor as it was at first.

Has the potential energy of the crate increased?

a) Yesb) No

Yes. The weight of the crate has been lifted slightly. If it is released it will fall back and convert the potential energy into kinetic energy.

Potential Energy



Lecture 14 Slide 13


Fall 2004


Spring 2009

Potential Energy

• The term potential energy implies storing energy to use later for other purposes.– For example, the

gravitational potential energy of the crate can be converted to kinetic energy and used for other purposes.



Lecture 14 Slide 14


Fall 2004


Spring 2009


PHYS 1800

Lecture 14





Lecture 14 Slide 15


Fall 2004


Spring 2009

Energy: The potential to do work.Conservation of Energy: The total

energy of a closed system remains constant.

– Energy can be converted from one form to another.

– Not all forms of energy can be fully recovered.


Time

Ene

rgy



Lecture 14 Slide 16


Fall 2004


Spring 2009

A lever is used to lift a rock. Will the work done by the person on the lever be greater

than, less than, or equal to the work done by the lever on the rock?

a) Greater thanb) Less thanc) Equal tod) Unable to tell

from this graph

The work done by the person can never be less than the work done by the lever on the rock. If there are no dissipative forces they will be equal. This is a consequence of the conservation of energy.

Work Input ≤ Work Out



Lecture 14 Slide 17


Fall 2004


Spring 2009

– Work done in pulling a sled up a hill produces an increase in potential energy of the sled and rider.

– This initial energy is converted to kinetic energy as they slide down the hill.




Lecture 14 Slide 18


Fall 2004


Spring 2009

– Any work done by frictional forces is negative.– That work removes mechanical energy from the system.

• Conservative forces are forces for which the energy can be completely recovered.– Gravity and elastic forces are conservative.– Friction is not conservative.




Lecture 14 Slide 19


Fall 2004


Spring 2009

A sled and rider with a total mass of 40 kg are perched at the top of the hill shown. Suppose that 2000 J of work is done against friction as the sled travels from the top (at 40 m) to the second hump (at 30 m). Will the sled make it to the top of the second hump if no kinetic energy is

given to the sled at the start of its motion?

a) yesb) noc) It depends.

Yes. The difference between the potential energy at the first point and the second point, plus loss to friction is less than the kinetic energy given at the start of the motion.



Lecture 14 Slide 20


Fall 2004


Spring 2009


PHYS 1800

Lecture 14


Hooke’s Law and Spring Potential Energy



Lecture 14 Slide 21


Fall 2004


Spring 2009

Potential Energy of a Spring

• An elastic force is a force that results from stretching or compressing an object.

• Elastic potential energy is the energy gained when work is done to stretch a spring.– The spring constant, k, is a number describing the

stiffness of the spring.



Lecture 14 Slide 22


Fall 2004


Spring 2009

Hooke’s Law and Potential Energy

• Hooke’s Law: The increase in elastic potential energy is equal to the work done by the average force needed to stretch the spring.

PE work done = average force distance

average force = 1

2kx

PE 1

2kx 2



Lecture 14 Slide 23


Fall 2004


Spring 2009


PHYS 1800

Lecture 14


Energy and Oscillations



Lecture 14 Slide 24


Fall 2004


Spring 2009

• A restoring force is a force that exerts a push or a pull back towards equilibrium.

• A restoring force that increases in direct proportion to the distance from equilibrium results in simple harmonic motion.



Lecture 14 Slide 25


Fall 2004


Spring 2009

Springs and Simple Harmonic Motion

• Simple harmonic motion occurs when the energy of a system repeatedly changes from potential energy to kinetic energy and back again.

Energy added by doing work to stretch the spring is transformed back and forth between potential energy and kinetic energy.



Lecture 14 Slide 26


Fall 2004


Spring 2009

The horizontal position x of the mass on the spring is plotted against time as the mass moves back and forth.

• The period T is the time taken for one complete cycle.

• The frequency f is the number of cycles per unit time. F=1/T

• The amplitude is the maximum distance from equilibrium.

X(t) = A sin (2π f t)



Lecture 14 Slide 27


Fall 2004


Spring 2009

Why does a swinging pendant

return to the same point after each

swing?




Lecture 14 Slide 28


Fall 2004


Spring 2009

The force does work to

move the ball. This increases the ball’s

energy, affecting its

motion.




Lecture 14 Slide 29


Fall 2004


Spring 2009

• Conservative forces are forces for which the energy can be completely recovered.– Gravity and elastic forces are conservative.– Friction is not conservative.



Lecture 14 Slide 30


Fall 2004


Spring 2009


• Conservation of energy means the total energy (the kinetic plus potential energies) of a system remain constant.– Energy is conserved if

there are no forces doing work on the system.



Lecture 14 Slide 31


Fall 2004


Spring 2009


Next Lab/Demo: Energy & OscillationsMomentum and CollisionsThursday 1:30-2:45

ESLC 53 Ch 6 and 7

Next Class: Wednesday 10:30-11:20BUS 318 roomReview Ch 6Read Ch 7

physics of technology phys 1800

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