newton’s laws introduction section 0 lecture 1 slide 1 lecture 7 slide 1 introduction to modern...

Newton’s Laws

Introduction Section 0 Lecture 1 Slide 1

Lecture 7 Slide 1

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 7

Newton’s Laws

Newton’s Laws


Lecture 7 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

Newton’s Laws


Lecture 7 Slide 3


Fall 2004


Spring 2009


PHYS 1800

Lecture 6

Newton’s Laws

Introduction and Significance

Newton’s Laws


Lecture 7 Slide 4


Fall 2004


Spring 2009

Describing Motion

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

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

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

Question: How do we get things to accelerate?

Newton’s Laws


Lecture 7 Slide 5


Fall 2004


Spring 2009

Acceleration

Acceleration is the rate at which velocity changes.– Our bodies don’t feel velocity, if the velocity is constant.– Our bodies feel acceleration.

• A car changing speed or direction.• An elevator speeding up or slowing down.

Acceleration can be either a change in the object’s speed or direction of motion.

t1VV

intervalTime velocityinChange

accelerationAverage 2

2smtVa

In this Chapter acceleration is a variable, caused by FORCE.

Newton’s Laws


Lecture 7 Slide 6


Fall 2004


Spring 2009

Acceleration Due to Gravity

• Earth exerts a gravitational force on objects that is attractive (towards Earth’s surface).

• Near Earth’s surface, this force produces a constant acceleration downward.– To measure this acceleration, we need to slow down the

action.– Galileo was the first to accurately measure this

acceleration due to gravity.– By rolling objects down an inclined plane, he slowed the

motion enough to establish that the gravitational acceleration is uniform, or constant with time.

Newton’s Laws


Lecture 7 Slide 7


Fall 2004


Spring 2009

How does this trajectory happen?

Key: - resolve motion into its HORIZONTAL and VERTICAL components.

But we know VG increases with time due to gravity acceleration!

At any instant the total velocity is vector sum of VH and VG

Resultant TRAJECTORY STEEPENS with increasing time.

As NO horizontal acceleration the ball movesequal distances horizontally in equal time (assuming NO air resistance).

VH = constant

VG (due to gravity)VTOTAL

Uniform increasein VG

withtime

VH (constant)

Newton’s Laws


Lecture 7 Slide 8


Fall 2004


Spring 2009

One Heck of a Ball Team!!!

Hart’s list of most influential people in the history of the world:Newton (2)* Einstein (10)Galileo Galilei (12)*Aristole (13)***Copernicus (19) *Kepler (75) *

*(even though they got the wrong answer on the test)

Simmon’s list of most influential scientists in the history of the world Newton (1)* (and 2 and 6 and 40)Einstein (2)Galileo Galilei (7)*Copernicus (9)Kepler (10)Tyco Brahe (22) Aristole (an honorable mentioned)***

Newton’s Laws


Lecture 7 Slide 9


Fall 2004


Spring 2009

Newton’s Contribution

• Newton built on Galileo’s work, expanding it.

• He developed a comprehensive theory of motion that replaced Aristotle’s ideas.

• Newton’s theory is still widely used to explain ordinary motions.

Newton’s Laws


Lecture 7 Slide 10


Fall 2004


Spring 2009

Aristotle’s View

• A force is needed to keep an object moving.

• Air rushing around a thrown object continues to push the object forward.

Newton’s Laws


Lecture 7 Slide 11


Fall 2004


Spring 2009

Galileo’s Contribution

• Galileo challenged Aristotle’s ideas that had been widely accepted for many centuries.

• He argued that the natural tendency of a moving object is to continue moving.– No force is needed to keep an object

moving.– This goes against what we seem to

experience.

Newton’s Laws


Lecture 7 Slide 12


Fall 2004


Spring 2009

Newton’s First and Second Laws

Put Galileo's notions of motion on a mathematical footing with calculus

Set up the framework to explain motion. How do forces affect the motion of an object? What exactly do we mean by force? Is there a difference between,

say, force, energy, momentum, impulse? What do Newton’s first and second laws of motion tell us, and how

are they related to one another?

Developed the first hints of a concervation law— Newton’s 3rd Law of Motion.

Developed the first formulation of a force—the gravitational force.

Made seminal contributions in thermodynamics and optics.

Newton’s Laws


Lecture 7 Slide 13


Fall 2004


Spring 2009

Inconsistencies in Physics cira 1900

Electricity & Magnetism• Medium for propagation of light• Obeys Lorentz transformation

Mechanics (Gravity) Obeys Galilean transformation

• Blackbody radiation• Wein’s Law• Photoelectric effect• Diffraction of x rays

Statistical Mechanics• Boltzmann Distribution• Entropy and counting states

• Brownian motion

• Discrete atomic spectra• Radioactive decay

• Existence of Atoms!

Newton’s Laws


Lecture 7 Slide 14


Fall 2004


Spring 2009

Newton’s Law of Universal Gravitation

• Newton recognized the similarity between the motion of a projectile on Earth and the orbit of the moon.

• If a projectile is fired with enough velocity, it could fall towards Earth but never reach the surface.

• The projectile would be in orbit.

• Newton’s law of universal gravitation says the gravitational force between two objects is proportional to the mass of each object, and inversely proportional to the square of the distance between the two objects.

• G is the Universal gravitational constant G.

221

r

mGmFgravity

Newton’s Laws


Lecture 7 Slide 15


Fall 2004


Spring 2009

Dennison’s Laws of Motion

1. Stuff happens (or not).

2. The bigger they are the harder they fall.

3. You get what you give.

Newton’s Laws


Lecture 7 Slide 16


Fall 2004


Spring 2009

Newton’s First Law of Motion

An object remains at rest, or in uniform motion in a straight line, unless it is compelled to change by an externally imposed force.

Newton’s Laws


Lecture 7 Slide 17


Fall 2004


Spring 2009

Newton’s Second Law of Motion

The acceleration of an object is directly proportional to the magnitude of the imposed force and inversely proportional to the mass of the object.

The acceleration is the same direction as that of the imposed force.

Newton’s Laws


Lecture 7 Slide 18


Fall 2004


Spring 2009

Newton’s Second Law of Motion

Note that a force is proportional to an object’s acceleration, not its velocity.

Precise definitions of some commonly used terms:

The mass of an object is a quantity that tells us how much resistance the object has to a change in its motion. This resistance to a change in motion is called inertia.

F ma

units : 1 newton = 1 N = 1 kgm s2

Force has dimensions of (MLT-2)

Newton’s Laws


Lecture 7 Slide 19


Fall 2004


Spring 2009

It is the total force or net force that determines an object’s acceleration.

If there is more than one vector acting on an object, the forces are added together as vectors, taking into account their directions.

Fstring 10 N (to the right)

ftable 2 N (to the left)

Fnet 10 N 2 N

8 N (to the right)

a Fnet

m8 N

5 kg

1.6 m s2 (to the right)

Net Forces

Newton’s Laws


Lecture 7 Slide 20


Fall 2004


Spring 2009

Two equal-magnitude horizontal forces act on a box. Is the object accelerated horizontally?

a) Yes.b) No.c) You can’t tell from

this diagram.

Since the two forces are equal in size, and are in opposite directions, they cancel each other out and there is no acceleration.

Newton’s Laws


Lecture 7 Slide 21


Fall 2004


Spring 2009

Is it possible that the box is moving, since the forces are equal in size but opposite in

direction?

a) Yes, it is possible for the object to be moving.

a) No, it is impossible for the object to be moving.

Even though there is no acceleration, it is possible the object is moving at constant speed.

Newton’s Laws


Lecture 7 Slide 22


Fall 2004


Spring 2009

Two equal forces act on an object in the directions shown. If these are the only

forces involved, will the object be accelerated?

a) Yes.b) No.c) It is impossible to determine

from this figure.

The vector sum of the two forces results in a force directed toward the upper right corner. The object will be accelerated toward the upper right corner.

Newton’s Laws


Lecture 7 Slide 23


Fall 2004


Spring 2009

Two forces act in opposite directions on a box. What is the mass of the box if its acceleration is

4.0 m/s2?

a) 5 kgb) 7.5 kgc) 12.5 kgd) 80 kge) 120 kg

The net force is 50 N - 30 N = 20 N, directed to the right.

From F=ma, the mass is given by: m = F/a

= (20 N) / (4 m/s2) = 5 kg.

Newton’s Laws


Lecture 7 Slide 24


Fall 2004


Spring 2009

A 4-kg block is acted on by three horizontal forces. What is the net horizontal force acting on the

block?

a) 10 Nb) 20 Nc) 25 Nd) 30 Ne) 40 N

The net horizontal force is:

5 N + 25 N - 10 N = 20 N

directed to the right.

Newton’s Laws


Lecture 7 Slide 25


Fall 2004


Spring 2009

A 4-kg block is acted on by three horizontal forces. What is the horizontal acceleration of the block?

a) 10 Nb) 20 Nc) 25 Nd) 30 Ne) 40 N

From F=ma, the acceleration is given by:

a = F/m = (20 N) / (4 kg) = 5 m/s2

directed to the right.

Newton’s Laws


Lecture 7 Slide 26


Fall 2004


Spring 2009

A ball hangs from a string attached to the ceiling. What is the net force acting on the

ball?

a) The net force is downward.

b) The net force is upward. c) The net force is zero.

Since the ball is hanging from the ceiling at rest, it is not accelerating so the net force is zero. There are two forces acting on the ball: tension from the string and force due to gravitation. They cancel each other.

Newton’s Laws


Lecture 7 Slide 27


Fall 2004


Spring 2009

Two masses connected by a string are placed on a fixed frictionless pulley. If m2 is larger

than m1, will the two masses accelerate?

a) Yes. b) No. c) You can’t tell

from this diagram.

The acceleration of the two masses will be equal and will cause m2 to fall and m1 to rise.

Newton’s Laws


Lecture 7 Slide 28


Fall 2004


Spring 2009

Newton’s Third Law

• Where do forces come from?

• If we push on an object like a chair, does the chair also push back on us?

• If objects do push back, who experiences the greater push, us or the chair?

• Does our answer change if we are pushing against a wall?

• How does Newton’s third law of motion help us to define force, and how is it applied?

Newton’s Laws


Lecture 7 Slide 29


Fall 2004


Spring 2009

Newton’s Third Law (“action/reaction”)

For every action (force), there is an equal but

opposite reaction (force).

Newton’s Laws


Lecture 7 Slide 30


Fall 2004


Spring 2009

Identifying the forces acting on an object.

• The forces acting on the book are W (gravitational force from Earth) and N (normal force from table).

• Normal force refers to the perpendicular force a surface exerts on an object.

Newton’s Laws


Lecture 7 Slide 31


Fall 2004


Spring 2009

An uncompressed spring and the same spring supporting a book. The compressed spring exerts an upward force on the book.

Third-Law Action/Reaction Pair

Newton’s Laws


Lecture 7 Slide 32


Fall 2004


Spring 2009


If the cart pulls back on the mule equal and opposite to the mule’s pull on the cart,

how does the cart over move?

Newton’s Laws


Lecture 7 Slide 33


Fall 2004


Spring 2009


The car pushes against the road, and the road, in turn, pushes against the car.

Newton’s Laws


Lecture 7 Slide 34


Fall 2004


Spring 2009

Newton’s Laws in Review

• 2nd Law (and 1st Law)—How motion of a object is aeffected by a force.

• 3rd Law—Forces come from interactions with other objects.

• Two branches of Mechanics: • Statics using the 1st Law with a=0 or Fnet=0• Dynamics using the 2nd Law with a≠0 or Fnet ≠0

• Note: At the most fundamental level, there are only 4 forces in nature, gravity, electricity and magnetism, tweak nuclear force and strong nuclear force.

Newton’s Laws


Lecture 7 Slide 35


Fall 2004


Spring 2009

Dennison’s Laws of Motion

1. Stuff happens (or not).

2. The bigger they are the harder they fall.

3. You get what you give.

Newton’s Laws


Lecture 7 Slide 36


Fall 2004


Spring 2009

Free Body Diagrams

• Fancy Science: Vector analysis of complex force problems is facilitated by use of a free body diagram.

• Common Sense: A picture is worth a 100 words. (A scale picture is worth an A!)

• Key is to:• Isolate a single body and draw all the forces acting on it. • Add up all the arrows (vectors).• What’s left is the net force.• Net force (and masses) a.• A plus initial conditions motion!

Newton’s Laws


Lecture 7 Slide 37


Fall 2004


Spring 2009


Next Lab/Demo: Forces

Thursday 1:30-2:45 ESLC 53

Ch 3

Next Class: Wednesday 10:30-11:20

BUS 318 roomRead Ch 4

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