conceptual physics fundamentals - srjcsrjcstaff.santarosa.edu/~lwillia2/11/11ch4.pdf · when a...

Conceptual Physics

Fundamentals

Chapter 4:

NEWTON’S LAWS OF MOTION

This lecture will help you

understand:

• Newton’s First Law of Motion

• Newton’s Second Law of Motion

• Forces and Interactions

• Newton’s Third Law of Motion

• Vectors

• Summary of Newton’s Laws of Motion

Newton’s Laws of Motion

“I was only a scalar until you came along and

gave me direction.”

—Barbara Wolfe

Newton’s First Law of Motion

Newton’s First Law (the law of inertia)

• a restatement of Galileo’s concept of inertia

• states that every object continues in a state of rest

or of uniform speed in a straight line unless acted

on by a nonzero force


example: dishes remain in their initial state of rest

when a tablecloth is whipped from beneath them

Inertia is the property of objects to resist

changes in motion.

Newton’s First Law(Law of Inertia)

An object will remain at rest or in a

constant state of motion unless acted upon

by net external forces.

0F

Newton’s First Law

0 => No Change in MotionIf F

Dynamic Equilibrium Static Equilibrium

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley

A sheet of paper can be quickly withdrawn from under a

soft-drink can without the can toppling, because

A. gravity pulls harder on the can than on the paper.

B. the can has weight.

C. the can has inertia.

D. none of the above


CHECK YOUR NEIGHBOR


A sheet of paper can be quickly withdrawn from under a

soft-drink can without the can toppling, because

A. gravity pulls harder on the can than on the paper.

B. the can has weight.

C. the can has inertia.



CHECK YOUR ANSWER


If you swing a stone overhead in a horizontal circle and the

string breaks, the tendency of the stone is to follow a

A. curved path.

B. straight-line path.

C. spiral path.

D. vertical path.


CHECK YOUR NEIGHBOR


If you swing a stone overhead in a horizontal circle and the

string breaks, the tendency of the stone is to follow a

A. curved path.

B. straight-line path.

C. spiral path.

D. vertical path.


CHECK YOUR ANSWER

Net Forces cause Acceleration

Newton’s 2nd Law

The acceleration of object is directly

related to the net force acting on it and

inversely proportional to its mass.

netF ma

netFa

m

Newton’s 2nd Law

2

mkg N

s

netF ma


When a 10-kg block is simultaneously pushed eastward

with 20 N and westward with 15 N, the net force on the

block is

A. 35 N west.

B. 35 N east.

C. 5 N west.

D. 5 N east.

Conceptual Physics Fundamentals—Chapter 4

netF ma


When a 10-kg block is simultaneously pushed eastward

with 20 N and westward with 15 N, the acceleration of the

block is

A. 0.5 m/s2 east.

B. 0.5 m/s2 west.

C. 0.5 m/s2 east–west.

D. none of the above.

Conceptual Physics Fundamentals—Chapter 4

netF ma

Acceleration ~ Net Force

netFa

m

Acceleration ~ 1/Mass

netFa

m

Consider a cart pushed along a track with a certain force. If the

force remains the same while the mass of the cart decreases to

half, the acceleration of the cart

A. remains the same.

B. halves.

C. doubles.

D. changes unpredictably.

Newton’s Second Law of Motion

CHECK YOUR NEIGHBOR

netFa

m

Consider a cart pushed along a track with a certain force. If the

force remains the same while the mass of the cart decreases to

half, the acceleration of the cart

A. remains the same.

B. halves.

C. doubles.

D. changes unpredictably.


CHECK YOUR ANSWER

netFa

m

Push a cart along a track so twice as much net force acts on it. If

the acceleration remains the same, what is a reasonable

explanation?

A. The mass of the cart doubled when the force doubled.

B. The cart experiences a force that it didn’t before.

C. The track is not level.

D. Friction reversed direction.


CHECK YOUR NEIGHBOR

Push a cart along a track so twice as much net force acts on it. If

the acceleration remains the same, what is a reasonable

explanation?

A. The mass of the cart doubled when the force doubled.

B. The cart experiences a force that it didn’t before.

C. The track is not level.

D. Friction reversed direction.


CHECK YOUR ANSWER

(Work Book 16)

Frictional ForcesStatic Friction

Sliding Friction

Frictional ForcesStatic Friction

Sliding Friction

netFa

m

F fa

m

Question: Frictional ForcesStatic Friction

Sliding Friction

What net force does a

sliding crate experience

when you exert a force of

110 N and the sliding

friction between the crate

and the floor is 110 N?

What can you say about

the motion of the crate?

Is it moving?

Is it accelerating?

F fa

m

(Workbook pg 21)

Force of Gravity: Weight

F ma

W mg

29.8 /g m s

Rock & FeatherA rock and feather fall with the same acceleration due to gravity.

Is the force of gravity acting on them the same?

Rock & FeatherNO! The force of gravity acting on them is their weight!

Rock & FeatherWhy does the rock and feather fall with the same acceleration

due to gravity?

Rock & FeatherThe acceleration due to gravity is the same for all object regardless of

weight. More massive objects resist their change in motion more than

less massive objects. The ratio F/m always turns out to be g.

Free Fall Acceleration

Air ResistanceAir resistance is proportional to the size and speed of an object.

WHY?

Air ResistanceAir resistance is proportional to the size and speed of an object.

The bigger the object, the more air it has to push aside.

The faster the object, the faster it has to push air aside.

Terminal VelocityWhen the air resistance balances the weight (R = W),

the object stops accelerating and it falls with constant velocity

called the Terminal Velocity.

W Ra

m

W W

m

0

netFa

m

Terminal VelocityWhich person will have the greatest terminal velocity.

Why?

Terminal VelocityThe heavier an object, the greater the terminal velocity.

WHY?

Terminal VelocityThe heavier object requires greater air resistance to balance its

weight and therefore more speed so the heavier object accelerates

longer and has a greater speed and greater terminal velocity.

(Work Book page 22!)


As the skydiver falls faster and faster through the air, air

resistance

A. increases.

B. decreases.

C. remains the same.

D. not enough information


CHECK YOUR NEIGHBOR


As the skydiver falls faster and faster through the air, air

resistance

A. increases.

B. decreases.




CHECK YOUR ANSWER


As the skydiver continues to fall faster and faster through

the air, net force

A. increases.

B. decreases.




CHECK YOUR NEIGHBOR



the air, net force

A. increases.

B. decreases.




CHECK YOUR ANSWER



the air, her acceleration

A. increases.

B. decreases.




CHECK YOUR NEIGHBOR



the air, her acceleration

A. increases.

B. decreases.




CHECK YOUR ANSWER


Consider a heavy and light person jumping together with

the same-size parachutes from the same altitude.

Who will reach the ground first?

A. the light person

B. the heavy person

C. both will reach at the same time


A situation to ponder…

CHECK YOUR NEIGHBOR


Consider a heavy and light person jumping together with

the same-size parachutes from the same altitude.

Who will reach the ground first?

A. the light person

B. the heavy person

C. both will reach at the same time


Explanation:

The heavier person has a greater terminal velocity. Do you know

why?


CHECK YOUR ANSWER

Newton’s Third Law

Newton’s First Law(Law of Inertia)

An object will remain at rest or in a

constant state of motion unless acted upon

by net external forces.

0F

Newton’s 2nd Law

The acceleration of object is directly

related to the net force acting on it and

inversely proportional to its mass.

netF ma

netFa

m

Newton’s 3rd Law

To every force there is an equal but

opposite reaction force.

hand on wall wall on handF F

Newton’s 3rd Law

You can’t TOUCH without being

TOUCHED back!!


Newton’s 3rd Law

This is an INTERACTIVE Universe.


An interaction requires a pair of

forces acting on two objects.

Gravity is an Interaction

Earth on Rock Rock on EarthF F

Gravity is an Interaction

The Earth pulls on you, you pull on the Earth.

You fall to the Earth, the Earth Falls to you.

You accelerate towards the Earth with g = 10m/s2.

With what acceleration is the Earth falling towards you?

Earth on You You on EarthF F

E Emg M a

E

E

mga

M

22 2

24

6621.1 10 /

5.98 10E

Na x m s

x kg

Force is not Acceleration

The forces are equal but the accelerations are not!

Earth on You You on EarthF F

Free FallIs the acceleration due to gravity zero?

Free FallYou feel weightless in free fall because there is no floor

pushing up against you.

Bug SplatA bug and bus have a head on collision.

Compared to the FORCE that acts on the bug,

how much force acts on the bus?

More Same Less

bus bug bug busF F

Newton’s 3rd Law:

Bug SplatWhich undergoes the greater acceleration?

Bug Same BusFa

m

Which suffers the

greatest damage?

Bug Same Bus

Action Reaction Pairs

Gun Pushes Bullet out.

Bullet Pushes back on Gun (& Man)

kick

Rocket Thrust

Rocket Pushes Gas Out.

Gas Pushes Back on Rocket.

Action-Reaction Pairs

If ACTION is A acting on B, then REACTION is B acting on A.

Action-Reaction Pairs

(Work Book page 23 & 24!)

Action-Reaction

You push a heavy car by hand. The car, in turn,

pushes back with an opposite but equal force on

you. Doesn’t this mean the forces cancel one

another, making acceleration impossible?

Why or Why not?

Action-Reaction pairs act on different objects.

For F = ma, all the forces act on the same object.

When you step off a curb, Earth pulls you downward. The

reaction to this force is

A. a slight air resistance.

B. nonexistent in this case.

C. you pulling Earth upward.


Newton’s Third Law of Motion

CHECK YOUR NEIGHBOR

When you step off a curb, Earth pulls you downward. The

reaction to this force is

A. a slight air resistance.

B. nonexistent in this case.

C. you pulling Earth upward.



CHECK YOUR ANSWER


Action and Reaction on Different Masses

cannonball: F/m = a

cannon: F/m = a

• the same force exerted on a small mass produces a

large acceleration

• the same force exerted on a large mass produces a

small acceleration

When a cannon is fired, the accelerations of the cannon and

cannonball are different because the

A. forces don’t occur at the same time.

B. forces, although theoretically the same, in practice are not.

C. masses are different.

D. ratios of force to mass are the same.


CHECK YOUR NEIGHBOR

When a cannon is fired, the accelerations of the cannon and

cannonball are different because the

A. forces don’t occur at the same time.

B. forces, although theoretically the same, in practice are not.

C. masses are different.

D. ratios of force to mass are the same.


CHECK YOUR ANSWER

Consider a high-speed bus colliding head-on

with an innocent bug. The force of impact

splatters the unfortunate bug over the

windshield.



Which is greater, the force on the bug or the force on the bus?

A. bug

B. bus

C. both are the same

D. cannot say


CHECK YOUR NEIGHBOR

Which is greater, the force on the bug or the force on the bus?

A. bug

B. bus

C. both are the same

D. cannot say

Comment:

Although the forces are equal in magnitude, the effects are very

different. Do you know why?


CHECK YOUR ANSWER

Two people of equal mass on slippery ice push off from each

other. Will both move at the same speed in opposite directions?

A. yes

B. yes, but only if both push equally

C. no

D. no, unless acceleration occurs


CHECK YOUR NEIGHBOR

Two people of equal mass on slippery ice push off from each

other. Will both move at the same speed in opposite directions?

A. yes

B. yes, but only if both push equally

C. no

D. no, unless acceleration occurs

Explanation:

However they push, the result is equal-magnitude forces on equal

masses, which produces equal accelerations; therefore, there are equal

changes in speed.


CHECK YOUR ANSWER

VectorsVector quantity

• has magnitude and direction

• is represented by an arrow

example: velocity, force, acceleration

Scalar quantity

• has magnitude

example: mass, volume, speed

Vectors

Resultant

• the sum of two or more vectors

– for vectors in the same direction, add arithmetically

– for vectors in opposite directions, subtract

arithmetically

– two vectors that don’t act in the same or opposite

direction

• use parallelogram rule

– two vectors at right angles to each other

• use Pythagorean theorem: R2 = V2 + H2

Vectors

Vector Components

• vertical and horizontal components of a

vector are perpendicular to each other

• determined by resolution

(Work Book page 25, 26, 29!)

Referring to the figure, which of the following are true

statements?

A. 50 N is the resultant of the 30 and 40-N vectors.

B. The 30-N vector can be considered a component of the 50-N vector.

C. The 40-N vector can be considered a component of the 50-N vector.

D. All of the above are correct.

Vectors

CHECK YOUR NEIGHBOR


statements?

A. 50 N is the resultant of the 30 and the 40-N vectors.

B. The 30-N vector can be considered a component of the 50-N vector.

C. The 40-N vector can be considered a component of the 50-N vector.


Vectors

CHECK YOUR ANSWER


statements?

A. 100 km/h is the resultant of the 80 and 60-km/h vectors.

B. The 80-km/h vector can be considered a component of the

100-km/h vector.

C. The 60-km/h vector can be considered a component of the

100-km/h vector.


Vectors

CHECK YOUR NEIGHBOR


statements?

A. 100 km/h is the resultant of the 80 and 60-km/h vectors.

B. The 80-km/h vector can be considered a component of the

100-km/h vector.

C. The 60-km/h vector can be considered a component of the

100-km/h vector.


Vectors

CHECK YOUR ANSWER

You run horizontally at 4 m/s in a vertically falling rain that falls

at 4 m/s. Relative to you, the raindrops are falling at an angle of

A. 0.

B. 45.

C. 53.

D. 90.

Vectors

CHECK YOUR NEIGHBOR

You run horizontally at 4 m/s in a vertically falling rain that falls

at 4 m/s. Relative to you, the raindrops are falling at an angle of

A. 0.

B. 45.

C. 53.

D. 90.

Explanation:

The horizontal 4 m/s and vertical 4 m/s combine by the parallelogram

rule to produce a resultant of 5.6 m/s at 45.

Vectors

CHECK YOUR ANSWER

Summary of Newton’s Three

Laws of Motion• Newton’s first law of motion (the law of inertia)

– An object at rest tends to remain at rest; an object in motion tends

to remain in motion at constant speed along a straight-line path.

• Newton’s second law of motion (the law of acceleration)

– When a net force acts on an object, the object will accelerate. The

acceleration is directly proportional to the net force and inversely

proportional to the mass.

• Newton’s third law of motion (the law of action and

reaction)

– Whenever one object exerts a force on a second object, the second

object exerts an equal and opposite force on the first.

conceptual physics fundamentals - srjcsrjcstaff.santarosa.edu/~lwillia2/11/11ch4.pdf · when a...

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