Physical Science 2Chapter 2.3 & Chapter 3 Forces

NEWTON

• If I have seen farther than other men, it is because I have stood on the shoulders of giants."

-Isaac Newton• When Newton made that famous

statement, he was referring to such giants as Tycho Brahe, Johannes Kepler, and, most specifically, Galileo Galilei

History behind the First Law of Motion

• Prior to Galileo and Newton's work on motion, it was believed that the natural tendency of objects was to come to rest. – Sooner or later, moving objects would stop moving. – Meaning that some force was necessary to keep an object in

motion.– Aristotle and Ptolemy.

• Galileo boldly asserted the exact opposite. • He reasoned that moving objects eventually stopped

moving due to a force called friction. • Galileo's Experiment • Galileo developed the concept of inertia.

Galileo developed the concept of inertia

• Galileo's Experiment • After placing a marble at any height on

one plane, Galileo observed that, when released, the marble rolled down that plane and up the opposite plane to approximately the same height. If he sanded the planes to be smoother, he noted that the marble rolled up the opposite plane even closer to its original height

• Galileo concluded that the difference between the marble's initial and final heights could be attributed to a force he called friction. He theorized that without this force, the marble would reach its original height exactly.

Newton’s First Law of Motion - INERTIA• Objects at rest stay at rest unless acted on by another force.• Objects in motion stay in motion unless acted on by another force

• There is a natural tendency for objects to resist changes in their state of motion.

• INERTIA – The resistance of an object to a change in its state of motion.

• The more mass an object has, the greater inertia.

Shooting Objects at RestThe object’s inertia causes it to remain still

even when a bullet passes through it.

FORCE

– is a push or a pull – can give energy to an

object causing the object to

• start moving, • stop moving, • change its motion.

Balanced vs. Unbalanced

• Balanced forces – They are equal in size and

opposite in direction – Do NOT cause a change in

motion. – Result in a zero net force

• Net force is the result of all the forces acting on the object

• Unbalanced forces – They are unequal in size

and or in the same direction

– Cause a change in motion

Tugboat Problems• Two tugboats are moving a barge. Tugboat A exerts a force of 3000 Newtons

on the barge. Tugboat B exerts a force of 5000 Newtons in the same direction. What is the combined force on the barge? Draw arrows showing the individual and combined forces of the tugboats.

• Now suppose that Tugboat C exerts a force of 2000 Newtons on the barge and Tugboat D exerts a force of 4000 Newtons in the opposite direction. What is the combined force on the barge? Draw arrows showing the individual and combined forces of the tugboats.

• Could there ever be a case when Tugboat E and Tugboat F are both exerting a force on the barge but the barge doesn't move? Draw arrows showing the individual and combined forces in such a situation.

Newton’s Second Law

• The acceleration of an object is dependent upon the force acting upon the object and the mass of the object.

• Newton’s Second Law can be written as the equation:

F = m x a    • F = force (N)  Newton is the SI Unit of

force• m = mass (kg)    N = kg m / s2

• a = acceleration (m/s/s)

NEWTON’S 3rd LAW

For every action, there is an equal and opposite reaction

• “Action” and “Reaction” are names of forces• Forces ALWAYS occur in pairs • Single forces NEVER happen • action reaction pairs

“Equal & Opposite”• In Newton’s Third Law,

“equal” means:

• Equal in size.

– The action and reaction forces are EXACTLY the same size.

• Equal in time.

– The action and reaction forces occur at EXACTLY the same time.

• In Newton’s Third Law, “opposite” means:

• Opposite in direction– The action and reaction

forces are EXACTLY 180o apart in direction.

Why don’t the forces cancel each other out?

• Only forces pushing or pulling on an object affect the object’s motion.

• Only forces that act on the same object can cancel.

• Newton’s Third Law action and reaction forces act on different objects, so they don’t cancel.

Writing Action Reaction Forces

• Action force: “A action verb B”

• Reaction force: “B action verb A”

– ACTION

Bowling ball hits the pin to the left.

– REACTION

Pin hits the bowling ball to the right

• The action and reaction forces don’t cancel since they push on different objects.

Friction• A force that opposes motion between two

surfaces that are touching each other

• The amount of friction is dependant upon– Surfaces– Force pressing the surfaces together

Types of Friction

• Static Friction– Friction between two objects that are

NOT moving.• Sliding Friction

– Friction that between two surfaces moving past each other.

• Rolling Friction– Friction between a rolling object & the

surface it is rolling on.• Fluid Friction

– Friction between an object and a gas or liquid

• Encarta Friction Animation

Air Resistance• Collisions of the object's leading surface with air

molecules. • Dependent upon a variety of factors, most

importantly– the speed of the object

• Increased speeds = increased air resistance. – the cross-sectional area of the object

• Increased cross-sectional area = increased air resistance.

FIND the Friction – 5 sentences

Gravity• Universal law of Gravity

– Any two objects will exert an attractive force on each other

– The size of the force is dependant on

• Mass of both objects• Distance between the

objects

– The range of gravity never disappears

• Therefore it is not possible for an object with mass to have no weight.

expanding universe

Weight vs. Mass• Weight

– Force of gravity pulling you toward the earth

– F = ma becomes W = mg • W means weight • g means acceleration due to gravity

~10m/s2

– SI Base Unit = Newtons– Weight can change with a change in

location.

• Mass – A measure of how much matter any

object has– You know an object has mass

because it has inertia– SI Base Unit = Kilograms

Falling Objects on Earth

• Near Earth’s surface• Acceleration due to gravity

– 9.8 m/s2 or ~ 10 m/s2

• If a bowling ball and a marble are dropped from the same height, which would hit the ground first?

Simultaneous drop of two objects with different massesdropping objects on the moon

Momentum

• A property related to the how much force is required to change the objects motion

• The product of the objects mass and velocity• p = mv

– p = momentum– m = mass– v = velocity

• SI UNIT = Kg m / s

                                             

    

Law of Conservation of Momentum

• Momentum may be transferred to another object

• The total momentum before a collision is equal to the total momentum after the collision

• m1v1 = m2v2

• interactive

Types of Collisions• Elastic collisions

– the total kinetic energy before the collision is equal to the total kinetic energy after the collision.

– Usually the objects bounce off of each other.

• Inelastic collisions– the total kinetic energy

before the collision is NOT equal to the total kinetic energy after the collision.

– Usually the objects stick together

Monkey and the Zookeeper• The zookeeper must shoot the

banana from the banana cannon to the monkey who hangs from the limb of a tree.

• This particular monkey has a habit of dropping from the tree the moment that the banana leaves the muzzle of the cannon.

• The zookeeper is faced with the dilemma of where to aim the banana cannon in order to hit the monkey.

• If the monkey lets go of the tree the moment that the banana is fired, then where should she aim the banana cannon?

Monkey and the ZookeeperWhere should the zookeeper aim?

Taking Gravity into effect The zookeeper aims above the monkeyDoes the speed of the banana change the effect?

The zookeeper aims at the monkey and shoots the banana very fast The zookeeper aims at the monkey, yet shoots the banana very slow.

ZOO in SPACEIn a Gravity free environment, where should the zookeeper shoot the banana?