Isaac Newton (1642-1727)

Galileo Galilei(1564-1642)

Albert Einstein(1879-1955)

How Things Work The Physics of Everyday Life

3rd Edition

By Louis A. Bloomfield (ISBN 0-471-46886-X)

John Wiley & Sons, Inc.

The working of familiar objects andapparatus will illuminate a number of

fundamental principles in Physics.

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We’ll discuss the concept of FORCEas a demonstration of the importanceof both

precise DEFINITIONS andcareful OBSERVATIONS

in science.

study falling objects as an example of a scientific investigation

and

a combination ofExperience ObservationIntuition Logic

to frame the basicdescription

using common sense

to apply these ideas to new situations…draw conclusions

…make predictions

Imagine set down at rest:

a glass on the counter

a ball on the billiards table

a book on a desk

if left undisturbedwould remain at rest!

No poking, prodding,pushing, or pulling

Just as obviously, common experience

shows us that we move objects

by force.

An object subject to NOexternal forces will

A. be at rest.B. eventually come to rest.C. move at constant speed.D. move with increasing speed.

When released and nolonger in contact with the bowstring (and propelled by its tension) the arrow does not stop!

We tend to take the force of friction for granted

It is an external force

and in its absenseit is naturalto COAST!

An object subject to NO external forces will

A. be at rest.B. eventually come to rest.C. move at constant speed.D. move with increasing speed.

An object subject to a constant net external force will

A. resist moving and remain at rest. B. move but eventually come to rest.C. move at constant speed as long as the force continues to be applied.D. move with increasing speed.

In an evenly matched arm-wrestling bout

or tug of war match

enormous forces may be involved, but little motion!

Tension in supporting cable

T

Weight, theforce due to

gravity

W

T = W

The tensionand weightare exactlyequal andopposite!

T + W = 0

Theybalance!

In this case there is no NET force!

An object at rest tends to remain at rest

and an object in motion tends

to remain in motion with the same speed

and in the same direction

unless acted upon by an unbalanced force.

Newton’s 1st Law“The Law of Inertia”

constantvelocity

constant velocity of ?

In the absence of a net force, a object’s velocity remains constant!

A hockey puck slides across theice at constant velocity. The total force acting on the puck must be

A) zero

B) greater than zero

Since the puck moves with constant velocity, the net force on the puck must be zero. This is just Newton’s First Law!

You set your book on the bus seat next to you. When the bus stops suddenly, the book slides forward off the seat. Why?

A) a net force acted on it

B) no net force acted on it

The action of the brakes acted to stop the bus itself.Nothing other than the tiny friction with the plastic seat

upholstery acted to slow or stop the book.

A brief pushstarts this

kid off from rest.

An unbalanced force changes histangential speed from 0 to v.

introducing the mathematical conventionof “delta” representing

a small change.

If each timehe swings past

you giveabout the

same PUSH

you will boost his speed by an additional v.

His speed will grow and grow witheach successive push.

What if you keep up byracing alongside

and continue to push?

Isn’t that like a bunchof successive pushes

following uninterrupted?

Easy topush

Hard topush

To produce the same amount of acceleration, need to push the car much harder!

How much?

Twice as much mass requires twice the force!

100 as much mass requires 100 the force!

F m

If a lot of others join in, each contributingan additional force, the net force will

be greater, and the acceleration greater.

F = ma

F a F mtogether with

can be combined consistently to give

Newton’s 2nd Law

The Toyota FT-HS is a 400-horsepower hybrid sports car

 its 3.5-liter V6 engine provide an acceleration of

0-to-60-miles-an-hour in 4 seconds

Unveiled at last spring’s International Auto Show in Detroit:

acceleration, a = v t

+60 miles/hour4 seconds

= 15 miles/hoursec

If X = Y then = ?XY

A. 0 C. 1 E. XYB. ½ D. 2 F. infinity

If 1 mile = 5280 ft

then = ?1 mile5380 ft

+60 miles/hour4 seconds

= 15 miles/hoursec

Notice 60 miles/hour is the same as

60 = 316800 mileshour

5280 feet1 mile

feethour

316800 =feethour

1 hour3600 seconds

feetsec

88

+60 miles/hour4 seconds

= 22 feet/secsec

So alternately

or 22 ft/sec2

present velocity = initial velocity + at

More generally, if an object is alreadyin motion before it starts accelerating:

atvv +=0

starting velocityat time=0

A car traveling 35 mi/hr accelerates

4 over the next 5 seconds. What final speed does it attain?

mi/hrsec

v = 35 mi/hr + (4 ) 5sec mi/hrsec

= 35 mi/hr + 20 mi/hr

= 55 mi/hr

The Space Shuttle touches down with a landing speed of ~100 m/sec (224 mi/hour), and comes to rest at

the end of its runway within 40 seconds. What average acceleration does it undergo?

atvv +=0

starting velocityat time=0

)40(/1000 secasecm +=)40(/100 secasecm =−

a sec

secm =−40

/1002.5m/sec2

Softball6.5-7.0 oz.

Tennis ball2 ounces

(57 grams)

14 lb. medicine ball

32 timesheavier than a

softball!

112 timesheavier than a tennisball!

If all 3 balls were released simultaneously, from the same height, they would reach ground A. tennis ball first, medicine ball last. B. medicine ball first, tennis ball last. C. together at about the same time.

A large boat whose maximum speedin still water is v1, tows a smaller boat

whose maximum speed is the smaller v2.across the lake. If both outboard motorsrun together at full bore, the speed thatthey travel together with will be

A. the smaller value v1. B. the larger value v2.

C. somewhere between v1 and v2.

An object subject to NO external forces will

C. move at constant speed.

Some answers:

Notice that being at rest is just a special case of constant velocity (= zero). An object’s inertia will keep it from spontaneously changing its velocity: whether that’s starting to move from rest, or slowing down from a non-zero speed. It takes force to start something moving, but also to slow or stop it once its moving!

An object subject to a constant net external force will

D. move with increasing speed.

If any unbalanced force can start an object moving…then a continuously applied force can only make it move faster and faster.

People are confused when friction is high enough that an object slows to rest shortly after they stop pushing. In that case, the unbalanced force (friction alone) slows it to a stop. Pushing hard enough to just make up for (match) friction means all the forces have been balanced, and then there is no net (or unbalanced) force left over to speed the object up or slow it down.

Any push beyond the amount needed to overcome friction will cause the object to accelerate. The net force would be your push minus the pull of friction.