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ForcForcesesForcForceses

Isaac Newton (1642–1727)(1642–1727)proposed that proposed that the tendency the tendency ofofan object was an object was to maintain in to maintain in itsitscurrent state current state of motion.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.

MASS (kg)MASS (kg)a quantitative measure of inertiaa quantitative measure of inertia

FORCEFORCE “ “a push or a push or pull”pull”that may cause an that may cause an object to speed up, object to speed up, slow down or change slow down or change directions.directions.

““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.

* OR ** OR *

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

constant unless acted on by an constant unless acted on by an

unbalanced force.unbalanced force.

Identifying a force:

• Source, agent exerting the force

• Type: “contact” or “field”– “Contact”- the 2 objects HAVE to be touching

• EX. push, normal, pull, thrust, tension, friction, etc.

– “Field”- the 2 objects DO NOT have to be touching• EX. gravitational, electric, magnetic

The force of friction

• The push things give to each other when they rub together.

• Friction is the force that acts in the opposite direction of motion of the object

• In Newton’s world, if you could eliminate friction, then you would see more things keep on moving in a straight line.

• Newton’s first law only makes sense when all forces, including friction, are accounted for.

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 N

12 N

3 N

8 N

7 N

4 N

6 N

5 N

9 N

4 N 8 N

7 N

8 N

Zero Net ForceZero Net Forcethe condition of an objectthe condition of an objectthat is not acceleratingthat is not accelerating

If an object is at rest or moving at constant speed,

there will be no net force acting onthe object. All forces will be balanced.

Newton’s 1st Law- Inertia

• In the absence of “a net force,” an object moves with constant velocity.

• In the absence of “the thing that will cause a “change in velocity,” an object moves with constant velocity.

• The cause of constant velocity is NOTHING that will change the velocity.

• The cause of motion is NOTHING!• Motion Happens!

““Law of Interaction”Law of Interaction”

Short VersionShort Version““For every action there isFor every action there is

an 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.

Example

• When you bump into something, the size of the bruise is determined by Newton's 3rd Law. When you hit the object it hits you back with the same force. You only get back what you put into it.

Newton’s 3rd Law, the Law of Interaction

If (object 1) exerts a (type) force in the (direction) direction on (object 2), then (object 2) is exerting a (same type) force directed (opposite direction) on (object 1) simultaneously.

Consider the interaction depicted below between foot A, ball B, and foot C. The three objects interact simultaneously (at the same time). Identify the two pairs of action-reaction forces. Use the notation "foot A", "foot C", and "ball B" in your statements.

http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l4b.html

Identify at least six pairs of action-reaction force pairs in the following diagram.

http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l4b.html

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!

““Law of Acceleration”Law of Acceleration”

A net force causes an object to A net force causes an object to accelerateaccelerate

in the direction of the net force.in the direction of the net force.

The The accelerationacceleration is isdirectly proportional directly proportional to theto the net force net force

andand inversely proportional inversely proportional to the object’sto the object’s massmass..

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 acceleration of

gravitygravity

FFgg == mmaaggAn 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.

1. As a baseball is being caught it’s velocity goes from 30.0 m/s to 0.0 m/s in about 0.0050 s. The mass of the baseball is 0.145kg.

• What is the baseball’s acceleration?

a = ΔV / Δt

• What is the magnitude and direction of the force acting on the ball?

• What is the magnitude and direction of the force acting on the player who caught the ball?

2. A 65 kg boy and a 45 kg girl use an elastic rope while playing tug – of – war. The two are on an icy frictionless surface. If the acceleration of the girl is 3.0 m/s2, then find the magnitude of and direction of the acceleration of the boy toward the girl.

• FBD

• Newton’s 3rd Law

• F g on b = -F b on g

m = 65 kg m = 45 kg

a = 3.0 m/s2a = ? m/s2

F b on g = -F g on b

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?