ForcesForcesandandNewton’sNewton’sLaws of Laws of MotionMotion

ForcesForcesandandNewton’sNewton’sLaws of Laws of MotionMotion

Causes of MotionCauses of Motion

Aristotle (384-322 BC) believed that all objects had a (384-322 BC) believed that all objects had a ““natural placenatural place” and that the tendency of an object ” and that the tendency of an object was to reside in its “natural place.”was to reside in its “natural place.”

All objects were classified into All objects were classified into categories of earth, water, air, or fire.categories of earth, water, air, or fire.

““Natural motionNatural motion” occurred when an object ” occurred when an object sought to return to its “natural place” after beingsought to return to its “natural place” after beingmoved from it by some type of “moved from it by some type of “violent motionviolent motion.”.”

The natural state of an object was The natural state of an object was to be “at rest” in its “natural place.”to be “at rest” in its “natural place.”

To keep an object moving would require a To keep an object moving would require a forceforce..

These views remained widelyThese views remained widelysupported until the 1500ssupported until the 1500swhen when Galileo Galilei (1564-1642)(1564-1642)popularized experimentation.popularized experimentation.

Isaac Newton (1642–1727)(1642–1727)proposed that the tendency ofproposed that the tendency ofan object was to maintain itsan object was to maintain itscurrent state of motion.current state of motion.

INERTIAINERTIAthe tendency of an objectthe tendency of an object

to resist any change in its motionto resist any change in its motion

Inertia is a property of matter and does notInertia is a property of matter and does notdepend on the position or location of the object.depend on the position or location of the object.

MASSMASSa quantitative measure of inertiaa quantitative measure of inertia

FORCEFORCE “ “a push or pull”a push or pull”

““Law of Inertia”Law of Inertia”An object at rest remains at rest,An object at rest remains at rest,

and an object in motion continues in motionand an object in motion continues in motionat a constant speed in a straight line,at a constant speed in a straight line,

unless acted on by a nonzero net force.unless acted on by a nonzero net force.

* * OROR * *

The velocity of an object remains constant The velocity of an object remains constant

unless acted on by an unbalanced force.unless acted on by an unbalanced force.

The The net forcenet force acting on an object is the acting on an object is thevector sumvector sum of all the forces acting on it. of all the forces acting on it.

Examples:Examples:

If an object is remaining at rest, itIf an object is remaining at rest, itis incorrect to assume that thereis incorrect to assume that thereare are nono forces acting on the object. forces acting on the object.

We can only conclude that theWe can only conclude that thenet forcenet force on the object is zero. on the object is zero.

?

8 lb

12 lb

3 lb

8 lb

7 lb

4 lb

6 lb

5 lb

9 lb

4 lb 4 lb

7 lb

8 lb

EQUILIBRIUMEQUILIBRIUMthe condition of an objectthe condition of an object

such that it is not acceleratingsuch that it is not accelerating

The first condition for equilibrium is thatthere must be no net force acting on

the object. All forces must be balanced.

F F = 0 0FFxx = 0 0 and FFyy = 0 0

FFleftleft = FFrightright and FFupup = FFdowndown

““Law of Acceleration”Law of Acceleration”

A net force causes an object to accelerateA net force causes an object to acceleratein the direction of the net force.in the direction of the net force.

The acceleration is directly proportionalThe acceleration is directly proportionalto the net force and inversely to the net force and inversely

proportional to the object’s mass.proportional to the object’s mass.

aa=FFnetnet

mmFFnetnet=mmaa

The SI unit of force is the Newton, namedThe SI unit of force is the Newton, namedin honor of in honor of Isaac NewtonIsaac Newton..

One Newton of forceOne Newton of forceis the amount of forceis the amount of forceneeded to cause a oneneeded to cause a one

kilogram mass to acceleratekilogram mass to accelerateat a rate of 1 m/sat a rate of 1 m/s22..

WEIGHTWEIGHTa measure of the gravitational forcea measure of the gravitational force

that a massive object,that a massive object,such as a star or planet,such as a star or planet,puts on another massputs on another mass

FF == mmaaweightweight == massmass xx acceleration of gravityacceleration of gravity

WW == mmggAn object’s weight on planet Earth inAn object’s weight on planet Earth in NewtonsNewtonsis equal to itsis equal to its mass in kilogramsmass in kilograms timestimes 9.8 m/s9.8 m/s22.

Why do all objects with negligibleWhy do all objects with negligibleair resistance accelerate towardair resistance accelerate towardthe Earth at the same rate?the Earth at the same rate?

How can Newton’s 2How can Newton’s 2ndnd Law be used Law be usedto explain how air resistanceto explain how air resistanceaffects the acceleration of anaffects the acceleration of anobject in free fall?object in free fall?

How does Newton’s 2How does Newton’s 2ndnd Law relate to Law relate towhat a weight scale would read as youwhat a weight scale would read as you

move up and down on an elevator?move up and down on an elevator?

““Law of Interaction”Law of Interaction”Short VersionShort Version

““For every action there isFor every action there isan equal and opposite reaction.”an equal and opposite reaction.”

Longer VersionLonger VersionWhen one object exerts a force on aWhen one object exerts a force on a

second object, the second exerts a forcesecond object, the second exerts a forceon the first that is equal in magnitude,on the first that is equal in magnitude,

but opposite in direction.but opposite in direction.

Newton’s Third Law deals with Newton’s Third Law deals with

acting on objects.acting on objects.

two forcestwo forces

two differenttwo different

Newton’s Third Law pairs of forcesNewton’s Third Law pairs of forces

alwaysalways, , sometimessometimes, , nevernever

cancel each other out.cancel each other out.

The The only wayonly way for two forces to cancel for two forces to canceleach other out is for them to be each other out is for them to be equalequal

and oppositeand opposite and act on the and act on the samesame object! object!

Learn more aboutLearn more aboutNewton’s Laws of MotionNewton’s Laws of Motion

at these links:at these links:

link1, link2, link3, link4link1, link2, link3, link4

How do Newton’s Laws of motion How do Newton’s Laws of motion apply to these situations?apply to these situations?

• an object rests in your handan object rests in your hand• a ball is tossed upwarda ball is tossed upward• a car windshield hits a buga car windshield hits a bug• a person sits on a tablea person sits on a table• a person jumps up from the floora person jumps up from the floor• a baseball bat hits a baseballa baseball bat hits a baseball• a truck and car hit head-ona truck and car hit head-on

• Others?Others?