Newton’s Laws - University of North Floridan00006757/physicslectures/Knight 2e/04_lect_outline/ja... · Newton’s Laws of Motion ... Seat Belts: An Application of Newton’s First

© 2010 Pearson Education, Inc.

PowerPoint® Lectures for

College Physics: A Strategic Approach, Second Edition

Chapter 4

Forces and

Newton’s Laws

of Motion

© 2010 Pearson Education, Inc. Slide 4-2

4 Forces and Newton’s Laws of Motion




Weight is actually not the only one…



Reading Quiz

1. A “net force” is

A. the sum of the magnitudes of all the forces acting on an

object.

B. the difference between two forces that are acting on an

object.

C. the vector sum of all the forces acting on an object.

D. the force with the largest magnitude acting on an object.

Slide 4-7


Answer

1. A “net force” is

A. the sum of the magnitudes of all the forces acting on an

object.

B. the difference between two forces that are acting on an

object.

C. the vector sum of all the forces acting on an object.

D. the force with the largest magnitude acting on an object.

Slide 4-8


Reading Quiz

2. Which of the following is NOT one of the steps used to identify

the forces acting on an object?

A. Name and label each force the object exerts on the

environment.

B. Name and label each contact force acting on the object.

C. Draw a picture of the situation.

D. Identify “the system” and “the environment.”

E. Name and label each long-range force acting on the

object.

Slide 4-9


Answer

2. Which of the following is NOT on of the steps used to identify

the forces acting on an object?

A. Name and label each force the object exerts on the

environment.

B. Name and label each contact force acting on the object.

C. Draw a picture of the situation.

D. Identify “the system” and “the environment.”

E. Name and label each long-range force acting on the

object.

Slide 4-10


Reading Quiz

3. Which of these is not a force discussed in this chapter?

A. The tension force.

B. The normal force.

C. The orthogonal force.

D. The thrust force.

Slide 4-11


Answer

3. Which of these is not a force discussed in this chapter?

A. The tension force.

B. The normal force.

C. The orthogonal force.

D. The thrust force.

Slide 4-12


Reading Quiz

4. An action/reaction pair of forces

A. point in the same direction.

B. act on the same object.

C. are always long-range forces.

D. act on two different objects.

Slide 4-13


Answer

4. An action/reaction pair of forces

A. point in the same direction.

B. act on the same object.

C. are always long-range forces.

D. act on two different objects.

Slide 4-14


What Causes Motion?

Slide 4-15


What Causes Motion?

Slide 4-15


What Causes Motion?

In the absence of any forces acting on it, an object will continue

moving forever. Motion needs no “cause.”

Slide 4-15


Seat Belts: An Application of Newton’s First Law

Slide 4-16


What Is a Force?

A force...

... is a push or a pull. ... acts on an

object.

... requires an

agent.

... is a vector. ... is a contact force or a

long-range force. Slide 4-17


Force Vectors

Slide 4-18


A Short Catalog of Forces: Weight w

Slide 4-19


Spring Force Fsp

Slide 4-20


Tension ForceT

Slide 4-21


Normal Force n

Slide 4-22


Friction fk and fs

Slide 4-23


Drag D and Thrust Fthrust

Slide 4-24


Identifying Forces

Slide 4-25


Example Problem

A block is dragged uphill by a rope. Identify all

forces acting on the block.

Slide 4-26

w

kf

Tn

weight

tension

normal force

kinetic friction forcek

w

T

n

f


Example Problem

Block A hangs from the ceiling by a rope. Another block B hangs

from A. Identify the forces acting on A.

Slide 4-27

Aw

ABT

Block A

Block B

ACT

These are the forces acting on Block A:

weight of Block A

tension force on Block A due to Block B

tension force on Block A due to the ceiling

A

AB

AC

w

T

T


Example Problem



Slide 4-27

Aw

ABT

Block A

Block B

ACT

These are the forces acting on Block A:

weight of Block A

tension force on Block A due to Block B

tension force on Block A due to the ceiling

But there are other forces in the system...

A

AB

AC

w

T

T


Example Problem



Slide 4-27

Aw

Bw

BAT

ABT

Block A

Block B

ACT

CABT

The other forces are:

weight of Block B

tension force on Block B due to Block A

tension force on the ceiling due to Blocks A and B

Since the system is at rest, 0.

B

BA

CAB

net

w

T

T

F


A ball, hanging from the ceiling by a string, is pulled

back and released. Identify the forces acting on it just

after its release.

Example Problem

Slide 4-28

w

T

the ball's weight

tension

There are no other forces such as air resistanceacting on the ball, because "just after its release" it has not yet started moving

w

T


Newton’s Second Law

Slide 4-29


An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator.

• Is T greater than, equal to, or less than w? Or is there not

enough information to tell?

Example Problem

Slide 4-30

w

T the elevator's weight

tension

Since the elevator is moving upward at a"steady speed", it is not accelerating. Thenet force must therefore be zero, and the

magnitude of and must be equal.

w

T

w T

In

other words, .w T


An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator.

• Is T greater than, equal to, or less than w? Or is there not

enough information to tell?

Example Problem

Slide 4-30

w

T

If the elevator moves fast enough, there could beother downward forces, such as the drag force

due to air resistance, shown as in the figure. At the slow speeds most elevators travel it would be u

D

nlikely to be significant, but if present, the

magnitude of would have to exceed that of to maintain a steady speed.

T w

D


Free-Body Diagrams

Slide 4-31


Newton’s Third Law

Slide 4-32


Checking Understanding

An object, when pushed with a net force F, has an

acceleration of 2 m/s2. Now twice the force is applied to an

object that has four times the mass. Its acceleration will be

A. ½ m/s2.

B. 1 m/s2.

C. 2 m/s2.

D. 4 m/s2.

Slide 4-33


Answer

An object, when pushed with a net force F, has an

acceleration of 2 m/s2. Now twice the force is applied to an

object that has four times the mass. Its acceleration will be

A. ½ m/s2.

B. 1 m/s2.

C. 2 m/s2.

D. 4 m/s2.

Slide 4-34


A 40-car train travels along a straight track at 40 mph. A

skier speeds up as she skis downhill. On which is the

magnitude of the net force greater?

A. The train.

B. The skier.

C. The net force is the same on both.

D. There’s not enough information to tell.


Slide 4-35


Answer

A 40-car train travels along a straight track at 40 mph. A

skier speeds up as she skis downhill. On which is the

magnitude of the net force greater?

A. The train.

B. The skier.

C. The net force is the same on both.

D. There’s not enough information to tell.

Slide 4-36



10-year-old Sarah stands on a skateboard. Her older brother

Jack starts pushing her backward and she starts speeding up.

The force of Jack on Sarah is

A. greater than the force of Sarah on Jack.

B. equal to than the force of Sarah on Jack.

C. less than the force of Sarah on Jack.

Slide 4-37


Answer

10-year-old Sarah stands on a skateboard. Her older brother

Jack starts pushing her backward and she starts speeding up.

The force of Jack on Sarah is

A. greater than the force of Sarah on Jack.

B. equal to than the force of Sarah on Jack.

C. less than the force of Sarah on Jack.

Slide 4-38

Then why is it that Sarah speeds up?

It is because the force of Jack on Sarah exceeds the friction force due to the ground on Sarah, so the net force on Sarah is not zero


Summary

Slide 4-39


Summary

Slide 4-40


Summary

Slide 4-41

Newton’s Laws - University of North Floridan00006757/physicslectures/Knight 2e/04_lect_outline/ja... · Newton’s Laws of Motion ... Seat Belts: An Application of Newton’s First

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