Chapter 4-6 Newton’s Laws of MotionMrs. Pagar

• Gravity is a very important force in nature, it literally holds the universe together. Yet it is also one of the weakest force in nature.

• I Corinthians 1:27 “ But God hath chosen the foolish things of the world to confound the wise; and God hath chosen the weak things of the world to confound the things that are mighty.”

• Though we are weak, God can do mighty things with us. Perhaps the greatest power in the universe in manifested when a Christian who loves God with all of his/her heart shares burdens with the Lord in prayer. What appears to be weak and foolish has a greater influence than the most powerful physical agents in the universe.

Mass• Mass• -amount of matter in an object.• It is measured in kilograms.

• It is a measure of the inertia of an object.

• Inertia • is the natural tendency of a body to resist changes

in motion. It is the reluctance of any material object has to change in its state of motion.

• Weight• -gravitational force acting on the object.• It depends on the location of the object.• Unit: Newton

History

• Aristotle• - foremost Greek scientist, studied motion and

divided it into natural motion and violent motion.

• Natural motion-straight up, down, circular motion.

• Violent motion-imposed motion, with an external cause

• Nicolaus Copernicus(1473-1543)• First person to publicly state that the earth

revolves around the sun. • It was extremely controversial at the time. He

worked in secret to escape persecution. His printed work,De Revolutionibus reached him on the day of his death.

• Galileo Galilei• Foremost Scientist of late-Rennaisance Italy,

supported Copernicus theory• -demolished the notion that a force is

necessary to keep an object moving.• He found that a ball rolling down one inclined

plane would roll to nearly the same height as where it originally started.

•1st Law of Motion : •An object remains at a constant speed in a straight path until a net force acts on it.

NEWTON’s LAWS

Newton’s First Law• Law of Inertia

• “A body remains at rest or moves in a straight line at a constant speed unless acted upon by a force.”

Force• …the agency of change.• … a push or a pull.

• …changes the velocity.

• …is a vector quantity.

• ...measured in Newton’s.

NEWTON’s2nd Law of Motion : 

F = m•a

acceleration of the club

force of the club

mass of the club

Force of gravity

Force of musclesNet force

•Net force is the total amount of Force (minus the forces that cancel each other out).

• Net force• -vector sum of all forces acting on an object;

affects the object’s state of motion.• When an object is at rest, its weight is

balanced by an equal and opposite support force.

• An object is in equilibrium when it is at rest, with zero net force acting on it.

• Newton’s 2nd Law:• The acceleration produced by a net force on

an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object

• F=ma• Newton(N)- unit of force

Newton’s Second LawThe Sum of the Forces acting on a body is proportional to the acceleration that the body experiences

SF a

S F = (mass) a

amF

xx maF

yy maF

Net Force

zz maF

• F = ma• a=F/m• a=acceleration : m/s 2

• m= mass of the object : kg• F=force:N• The acceleration is proportional to the Force• The acceleration is inversely proportional to

the mass of the object• Acceleration is in the same direction as the

net force.

When the net force is Zero.-> NO movement

When the net force is NOT Zero.-> movement

• Pressure• -force per unit area• P=F/A• Unit is Pascal• 1 Pascal= 1 N/m 2

• The bigger the force, the bigger the pressure.• The smaller the area, the bigger the pressure

• The acceleration of all objects in free fall is the same, regardless of their mass. a=g=9.8 m.s.s

• When air resistance is present, a falling object accelerates(increase in speed) only until it reaches its terminal speed

• (constant speed where air resistance balances the force of gravity).

• NEWTON’s 3rd Law of Motion:• For every action there is an equal and opposite reaction.

NEWTON’s 3rd Law of Motion:  

For every action, there is an equal and opposite reaction.

• An interaction between two things produces a pair of forces.

• Interacting things exert forces on each other.• The two interacting forces are equal in

strength and opposite in direction.• Forces always occur in • pairs.

• Things fall toward the ground because the earth attracts objects towards itself. This attraction is called the earth’s

• Gravity• All objects in the universe attract each other.• The more massive the object, the greater the

attraction• These attractions are called action-at-a-

distance forces because they act on objects without touching them.

• Weight• - measure of the force of gravity• W=mg g= 10 m/s2

• 1 kg=2.2 lb• If you are 120 lb, 120 /2.2= 55 kg• m = 55 kg• W= mg = 55 ( 10)= 550 N

Tension (Tensile Force)

• Tension • is the force in a string, chain or tendon that is

applied tending to stretch it.• FT

• Example: A 20 N picture frame hangs supported by 2 strands of rope. How much force is supported by each strand?

• Answer : 10 N

Normal Force

• The normal force on an object that is being supported by a surface is the component of the supporting force that is perpendicular to the surface.

• FN

Coefficient of Friction

Kinetic Friction• Ff = mk FN

• Static Friction• Ff ms FN

• In most cases, mk < ms.

#21 p. 57 Conceptual Physics• If a woman has a mass of 50 kg, calculate her weight in

Newtons.• Problem: Fg=?• Given: m= 50 kg; g = 10 m/s2

• Formula: F= ma; a= g; Fg =mg

• Solution: Fg = mg

• Fg =(50)(10)

• Fg = 500 N• Final Answer: Fg =500 N

#19 p. 72 Conceptual Physics• Calculate the acceleration of a 2000 kg single-engine

airplane just before takeoff when the thrust of its engine is 500 N.

• Problem: a=?• Given: m= 2000 kg ; F= 500 N• Formula: F =ma ; a=F/m• Solution: a=F/m

• a=500/2000• a= 0.25 m/s2

• Final answer: a= 0.25 m/s2