chapter 3 lecture conceptual integrated science second edition © 2013 pearson education, inc....

Chapter 3 Lecture

ConceptualIntegrated Science

Second Edition

© 2013 Pearson Education, Inc.

Newton's Lawsof 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


Newton's First Law of Motion

• The law of inertia:– Every object continues in a state of rest or of

uniform speed in a straight line unless acted on by a nonzero force.


Newton's First Law of MotionCHECK YOUR NEIGHBOR

A sheet of paper can be quickly withdrawn fromunder a soft-drink can without the can toppling because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

Explain your answer to your neighbor.


Newton's First Law of MotionCHECK YOUR ANSWER

A sheet of paper can be quickly withdrawn from under a soft-drink can without the can toppling because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.



Newton's First Law of MotionCHECK 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.



Newton's First Law of MotionCHECK 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.


Newton's Second Law of Motion

• The law of acceleration:– The acceleration produced

by a net force on an object isdirectly proportional to the netforce, is in the same directionas the net force, and isinversely proportional to themass of the object.


Acceleration net forcemass

a Fm


• Equation for acceleration:

Small net force, large mass small acceleration

Large net force, small mass large

acceleration


Newton's Second Law of MotionCHECK YOUR NEIGHBORConsider 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 will

A. remain the same.B. halve.C. double.D. change unpredictably.



Newton's Second Law of MotionCHECK YOUR ANSWER 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 will

A. remain the same.B. halve.C. double.D. change unpredictably.


Newton's Second Law of MotionCHECK YOUR NEIGHBOR Push a cart along a track so that 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 doubles when the force is doubled.

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

C. The track is not level.

D. Friction reverses the direction.



Newton's Second Law of MotionCHECK YOUR ANSWER

Push a cart along a track so that 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 doubles when the force is doubled.

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

C. The track is not level.

D. Friction reverses the direction.



When acceleration is g—free fall• When the only force acting on a falling object is

gravity (with negligible air resistance), the object is in free fall.

• An object in free fall accelerates toward Earth at 10 m/s per second.


Newton's Second Law of MotionCHECK YOUR NEIGHBOR

At one instant, an object in free fall has a speed of 40 m/s. Its speed one second later will be

A. also 40 m/s.

B. 45 m/s.

C. 50 m/s.




Newton's Second Law of MotionCHECK YOUR ANSWER At one instant, an object in free fall has a speed of 40 m/s. Its speed one second later will be

A. also 40 m/s.B. 45 m/s.C. 50 m/s.D. none of the above

Comment:We assume the object is falling downward.



• When acceleration is g—freefall

Twice the force on twicethe mass

same acceleration as half the force on half the mass



A 5-kg iron ball and a 10-kg iron ball are dropped from rest. With negligible air resistance, the acceleration of the heavier ball will be

A. less.

B. the same.

C. greater.

D. undetermined.



Newton's Second Law of MotionCHECK YOUR NEIGHBORA 5-kg iron ball and a 10-kg iron ball are dropped from rest. When the free-falling 5-kg ball reaches a speed of 10 m/s, the speed of the free-falling 10-kg ball will be

A. less than 10 m/s.B. 10 m/s. C. greater than 10 m/s.D. undetermined.




A 5-kg iron ball and a 10-kg iron ball are dropped from rest. When the free-falling 5-kg ball reaches a speed of 10 m/s, the speed of the free-falling 10-kg ball will be

A. less than 10 m/s.

B. 10 m/s.

C. greater than 10 m/s.

D. undetermined.



• When acceleration is g—free fall– The ratio of weight to mass

is the same for all fallingobjects in the same locality;hence, their accelerations arethe same in the absence ofair resistance.



• When acceleration is g—free fall• Demonstration of a feather and a

coin in a vacuum:• In a vacuum, a feather and a coin

fall together at g—the accelerationdue to gravity.


Newton's Second Law of MotionA situation to ponder…

• When an air-filled glass tube containing a coin and a feather isinverted, the coin falls quickly to thebottom of the tube while the featherflutters to the bottom.


A situation to ponder…CHECK YOUR NEIGHBORWhen the air is removed by a vacuum pump and the activity is repeated,

A. the feather hits the bottom first, beforethe coin hits.

B. the coin hits the bottom first, beforethe feather hits.

C. both the coin and the feather droptogether, side by side.

D. There is not enough information.



A situation to ponder…CHECK YOUR ANSWER

When the air is removed by a vacuum pump and the activity is repeated,

A. the feather hits the bottom first, before the coin hits.

B. the coin hits the bottom first, before the feather hits.

C. both the coin and the feather drop together, side by side.




• When acceleration is less than g—non-free fall

• When two forces act on a falling

object,– a force due to gravity acts

downward.– air resistance acts upward.



When a 20-N falling object encounters 5 N of air resistance, its acceleration of fall is

A. less than g.

B. greater than g.

C. g.

D. terminated.



Newton's Second Law of MotionCHECK YOUR ANSWERWhen a 20-N falling object encounters 5 N of air resistance, its acceleration of fall is

A. less than g.

B. greater than g.

C. g.

D. terminated.

Explanation:

Acceleration of non-free fall is always less than g. The acceleration is actually (20 N – 5 N)/2 kg = 7.5 m/s2.



• When acceleration is less than g—non-free fall• The force exerted by the surrounding air

increases with the increasing falling speed.

• The force of air resistance may continue to increase until it equals the weight. At this point, the net force is zero and no further acceleration occurs. The object has reached terminal velocity and continues to fall with no acceleration—at constant velocity.



If a 50-N person is to fall at terminal speed, the air resistance needed is

A. less than 50 N.

B. 50 N.

C. greater than 50 N.





If a 50-N person is to fall at terminal speed, the air resistance needed is

A. less than 50 N.

B. 50 N.

C. greater than 50 N.


Explanation:

Then F = 0 and acceleration = 0



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

A. increases.

B. decreases.

C. remains the same.





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

A. increases.

B. decreases.


D. There is not enoughinformation.



As the skydiver continues to fall faster and faster through the air, the net force

A. increases.

B. decreases.






As the skydiver continues to fall faster and faster through the air, her acceleration

A. increases.

B. decreases.





Newton's Second Law of MotionA situation to ponder…

• Consider a heavy and a light person with same-size parachutes jumping together from the same altitude.


A situation to ponder…CHECK YOUR NEIGHBOR

Who will reach the ground first?

A. The light person

B. The heavy person

C. Both at the same time




A situation to ponder…CHECK YOUR ANSWER

Who will reach the ground first?

A. The light person

B. The heavy person

C. Both at the same time


Explanation:

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


Forces and Interactions

• Force is simply a push or a pull.Interaction occurs between one thing and another.

Example:

When you push against a wall,you're interacting with the wall.


Newton's Third Law of Motion

• 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.

Example:

When your hand presses on the wall, the wall simultaneously presses on your hand. Hand and wall press on each other with equal and opposite forces.


Newton's Third Law of MotionCHECK YOUR NEIGHBORA soccer player kicks a ball with 1500 N of force. The ball exerts a reaction force against the player's foot of

A. somewhat less than 1500 N.B. 1500 N. C. somewhat greater than 1500 N.D. none of the above



Newton's Third Law of MotionCHECK YOUR ANSWER

A soccer player kicks a ball with 1500 N of force. The ball exerts a reaction force against the player's foot of

A. somewhat less than 1500 N.

B. 1500 N.

C. somewhat greater than 1500 N.




• Simple rule to identify action and reaction

Action:

Object A exerts a force onobject B.

Reaction:

Object B exerts a force onobject A.


Newton's Third Law of MotionCHECK 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 pull the Earth upward.





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 pull the Earth upward.




• Action and reaction on different masses

If the same force is applied to two objects of different masses,

Greater mass object small acceleration

Smaller mass object large acceleration


Newton's Third Law of MotionCHECK YOUR NEIGHBORWhen 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, though theoretically the same, in practice aren't the same.

C. masses are different.

D. ratios of force to mass are the same.




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, though theoretically the same, in practice aren't the same.

C. masses are different.

D. ratios of force to mass are the same.


Newton's Third Law of MotionA situation to ponder…

• Consider a high-speed bus colliding head-on with an innocent bug. The force of impact splatters the unfortunate bug over the windshield.


A situation to ponder…CHECK YOUR NEIGHBOR

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

A. Bug

B. Bus

C. Both forces are the same.

D. Cannot say



A situation to ponder…CHECK YOUR ANSWERWhich is greater: the force on the bug or the force on the bus?

A. BugB. BusC. Both forces are the same.D. Cannot say

Comment:Although the forces are equal in magnitude, the effects are very different. Do you know why?


Newton's Third Law of MotionCHECK 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



Newton's Third Law of MotionCHECK YOUR ANSWERTwo people of equal mass on slippery ice push off from each other. Will both move at the same speed in opposite directions?

A. YesB. Yes, but only if both push equallyC. NoD. No, unless acceleration occurs

Explanation:However they push, the result is equal-magnitude forces on equal masses, which produce equal accelerations and, therefore, equal changes in speed.


Vectors

• A vector is– a quantity whose complete description

involves magnitude and direction.– represented by an arrow drawn to scale.

Examples of a vector quantity:velocity, force, acceleration


Vectors

• A scalar is a quantity described only by magnitude.

Examples of a scalar quantity:mass, volume, speed


Vectors

• Working with vectors– Vectors in the same direction add

– Vectors in opposite directions subtract

– Nonparallel vectors use the parallelogram rule


VectorsCHECK YOUR NEIGHBOR Referring to the figure, which of the following statements are true?

A. The 50-N vector is the resultant of the 30-N 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.

D. All the statements are true.

Explain your answer to your neighbor


VectorsCHECK YOUR ANSWER Referring to the figure, which of the following statements are true?

A. The 50-N vector is the resultant of the 30-N 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.



VectorsCHECK YOUR NEIGHBORReferring to the figure, which of the following statements are true?

A. The 100-km/h vector is the resultant of the 80-km/h and the 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.


Explain your answer to your neighbor. 80 K

m/h


VectorsCHECK YOUR ANSWERReferring to the figure, which of the following statements are true?

A. The 100-km/h vector is the resultant of the 80-km/h and the 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.


80 K

m/h


VectorsCHECK 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.



VectorsCHECK 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.

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.

chapter 3 lecture conceptual integrated science second edition © 2013 pearson education, inc....

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