Aristotle

• 4th Century

• Natural Motion

• Up or down

• Violent Motion

• Pushed or pulled

2.32.3Newton’s 1st LawNewton’s 1st Law

Nicholas Copernicus

(Mikolaj Kopernik)

• 1473-1543

• De Revolutionibus

• Earth goes around the Sun

• Feared Persecution

2.32.3Newton’s 1st LawNewton’s 1st Law

Galileo Galilei• 1564-1642

• Force is a push or pull

• Friction is the force between materials- caused by irregularities in surface

• Inertia

2.32.3Newton’s 1st LawNewton’s 1st Law

• Galileo explains his discoveries to the pope

Inertia

• Inertia (ih NUR shuh) is the tendency of an object to resist any change in its motion.

2.32.3Motion and ForcesMotion and Forces

• It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.

Inertia• The velocity of the object remains constant

unless a force changes it.

2.32.3Motion and ForcesMotion and Forces

• If an object is at rest, it tends to remain at rest. Its velocity is zero unless a force makes it move.

Galileo Galilei: Spheres on inclines2.32.3Newton’s 1st LawNewton’s 1st Law

•Horizontal Surface:

•How High:

Without Friction

With Friction

Newton's Laws of Motion

• The British scientist Sir Isaac Newton (1642–1727) was able to state rules that describe the effects of forces on the motion of objects.

2.32.3Motion and ForcesMotion and Forces

• These rules are known as Newton's law's of motion.

Newton's First Law of Motion

• Newton's first law of motion is that an object stays at rest or stays at a constant velocity unless acted upon by a net force

2.32.3Motion and ForcesMotion and Forces

• This law is sometimes called the law of inertia.

• ( you might know this as an object at rest stays at rest, an object in motion stays in motion unless acted upon by an unbalanced force)

Newton's First Law of Motion

• Newton's first law of motion is that an object stays at rest or stays at a constant velocity unless acted upon by a net force

2.32.3Motion and ForcesMotion and Forces

• Does this law pertain to moving objects, objects at rest or both?

Forces

• Push or pull

• Can be

• At-a-distance

• Contact

2.32.3Motion and ForcesMotion and Forces

Unbalanced Forces 2.32.3

Motion and ForcesMotion and Forces

• The forces are considered to be unbalanced forces because the forces do not cancel each other perfectly.

Unbalanced Forces • The students are pushing on the box in the

same direction.

2.32.3Motion and ForcesMotion and Forces

• These forces are combined, or added together, because they are exerted on the box in the same direction.

Balanced Forces

• The net force on the box is zero because the two forces cancel each other.

• Forces on an object that are equal in size and opposite in direction are called balanced forces.

2.32.3Motion and ForcesMotion and Forces

Possible Motions2.32.3

Motion and ForcesMotion and Forces

• What are the possible motions for an object with

• Unbalanced forces Balanced Forces

Newton's First Law of Motion

2.32.3Motion and ForcesMotion and Forces

• Demonstrations

• Table clothhttp://www.stevespanglerscience.com/content/science-video/tablecloth-trick

http://images.google.com/imgres?imgurl=http://www.boingboing.net/images/ricardocloth.jpg&imgrefurl=http://www.boingboing.net/2007/12/19/old-table-cloth-tric.html&usg=__wHCnH5iUI2NuMys86fXQ6wWUU4g=&h=200&w=296&sz=24&hl=en&start=6&tbnid=Y6wH3EH6oHOE-M:&tbnh=78&tbnw=116&prev=/images%3Fq%3Dtablecloth%2Btrick%26gbv%3D2%26hl%3Den

• Seatbelts• http://www.youtube.com/watch?v=knCgF8K8xcY&feature=channel

2.32.3

Question 1

A force is a push or pull. Forces, such as theforce of the atmosphere against a person’s body,are not always noticeable.

A force is a __________.

Section CheckSection Check

Answer

2.32.3

Question 2

When are forces on an object balanced?

Section CheckSection Check

Answer

When forces are equal in size and opposite in direction, they are balanced forces, and the net force is zero.

2.32.3

Question 3

Inertia is __________.

Section CheckSection Check

A. the tendency of an object to resist any change in its motionB. the tendency of an object to have a positive acceleration

2.32.3Section CheckSection Check

C. The tendency of an object to have a net force of zero.D. The tendency of an object to change in speed or direction.

2.32.3

Answer

Inertia is the tendency of an object to resist any change in its motion. An unbalanced force must act upon the object in order for its motion to change.

Section CheckSection Check

• Newton’s second law of motion states that a net force results in an acceleration

Newton’s Second Law•3.1•3.1

•Newton’s Second Law•Newton’s Second Law

Unbalanced Forces • net force is the combination of forces acting

on an object

• F Newtons

3.13.1Motion and ForcesMotion and Forces

• When two students are pushing with unequal forces in opposite directions, a net force occurs in the direction of the larger force.

Mass• NOT

• Volume

IS

Number of atoms

Measured in kilograms

Related to inertia

M kg

3.13.1Motion and ForcesMotion and Forces

• Newton’s second law of motion states that an unbalanced force results in an acceleration

Newton’s Second Law•3.1•3.1

•Newton’s Second Law•Newton’s Second Law

Fnet

m a

Net Forces 3.13.1

Motion and ForcesMotion and Forces

Large mass vs. Small mass

•What is gravity? • Gravity is an attractive force between any

two objects that depends on the masses of the objects and the distance between them.

•3.2•3.2•Gravity•Gravity

• The gravitational force exerted on an object is called the object’s weight.

•3.2•3.2•Gravity•Gravity

•Weight

•Symbol: W

•Unit : Newton

Mass IS

Number of atoms

Measured in kilograms

Related to inertia

Symbol: M

Unit: kg

3.23.2Motion and ForcesMotion and Forces

• Close to Earth’s surface, the acceleration of a falling object in free fall is about 9.8 m/s2.

•3.2•3.2•Gravity•Gravity

•Earth’s Gravitational Acceleration

• This acceleration is given the symbol g and is sometimes called the acceleration of gravity.

•3.2•3.2•Gravity•Gravity

Weight

• Because the weight of an object on Earth is equal to the force of Earth’s gravity on the object, weight can be calculated from this equation:

W

m g

• The table shows how various weights on Earth would be different on the Moon and some of the planets.

•3.2•3.2•Gravity•Gravity

•Weight and Mass

• What happens on the Moon if you drop a feather and hammer?

• http://er.jsc.nasa.gov/seh/feather.html

•3.2•3.2•Gravity•Gravity

•Hammer and Feather

•3.2•3.2•Gravity•Gravity

•Weightlessness and Free Fall• at 400-km altitude the force of Earth’s

gravity is about 90 percent as strong as it is at Earth’s surface.

• So an astronaut with a mass of 80 kg still would weigh about 700 N in orbit, compared with a weight of about 780 N at Earth’s surface.

• If you and the scale were in free fall, then you no longer would push down on the scale at all.

•3.2•3.2•Gravity•Gravity

•Floating in Space

• The scale dial would say you have zero weight, even though the force of gravity on you hasn’t changed.

• The amount of air resistance on an object depends on the speed, size, and shape of the object.

•Air Resistance •3.1•3.1

•Newton’s Second Law•Newton’s Second Law

• Air resistance, not the object’s mass, is why feathers, leaves, and pieces of paper fall more slowly than pennies, acorns, and apples.

• As an object falls, the downward force of gravity causes the object to accelerate.

•Terminal Velocity •3.1•3.1

•Newton’s Second Law•Newton’s Second Law

• However, as an object falls faster, the upward force of air resistance increases.

• This causes the net force on a sky diver to decrease as the sky diver falls.

• The terminal velocity is the highest speed a falling object will reach.

•Terminal Velocity •3.1•3.1

•Newton’s Second Law•Newton’s Second Law

• The terminal velocity depends on the size, shape, and mass of a falling object.

• Happens when

• Weight = air resistance

•3.1•3.1•Section Check•Section Check

•Question 1

• A. acceleration

• B. momentum

• C. speed

• D. velocity

•Newton’s second law of motion states that _________ of an object is in the same direction as the net force on the object.

•3.1•3.1•Section Check•Section Check

•Answer•The answer is A. Acceleration can be calculated by dividing the net force in newtons by the mass in kilograms.

•3.1•3.1•Section Check•Section Check

•Question 2

• A. joule

• B. lux

• C. newton

• D. watt

•The unit of force is __________.

•3.1•3.1•Section Check•Section Check

•Answer•The answer is C. One newton = 1 kg · m/s2

•3.1•3.1•Section Check•Section Check

•Answer•Friction results from the sticking together of two surfaces that are in contact.

•Question 3•What causes friction?

•3.2•3.2•Gravity•Gravity

•Centripetal Force

• An unbalanced force that cause a change in direction is a centripetal force.

• Anything that moves in a circle is doing so because a centripetal force is accelerating it toward the center.

•3.2 •3.2 •Gravity•Gravity

•Centripetal Force and Traction

•3.2•3.2•Gravity•Gravity

•Centripetal Force and Traction• This centripetal force is the

frictional force between the tires and the road surface.

• This centripetal force is the sides of the washer

•Newton’s Third Law

• Newton’s third law of motion describes action-reaction pairs this way. When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction.

•3.3•3.3•The Third Law of Motion•The Third Law of Motion

•Action and Reaction

• When a force is applied in nature, a reaction force occurs at the same time.

• When you jump on a trampoline, for example, you exert a downward force on the trampoline.

•3.3•3.3•The Third Law of Motion•The Third Law of Motion

• Simultaneously, the trampoline exerts an equal force upward, sending you high into the air.

•Action and Reaction Forces Don’t Cancel

• According to the third law of motion, action and reaction forces act on different objects.

• Thus, even though the forces are equal, they are not balanced because they act on different objects.

•3.3•3.3•The Third Law of Motion•The Third Law of Motion

• For example, a swimmer “acts” on the water, the “reaction” of the water pushes the swimmer forward.

• Thus, a net force, or unbalanced force, acts on the swimmer so a change in his or her motion occurs.

•3.3•3.3The Third Law of MotionThe Third Law of Motion

•Action and Reaction Forces Don’t Cancel

•Rocket Propulsion• In a rocket engine, burning fuel produces

hot gases. The rocket engine exerts a force on these gases and causes them to escape out the back of the rocket.

• By Newton’s third law, the gases exert a force on the rocket and push it forward.

•3.3•3.3•The Third Law of Motion•The Third Law of Motion

•3.3•3.3•Section Check•Section Check

Question 1

According to Newton’s law, the second object exerts a force on the first that is equal in strength and opposite in direction.

According to Newton’s third law of motion, what happens when one object exerts a force on a second object?

Answer

Force Pressure Area

• http://www.stevespanglerscience.com/content/science-video/the-can-crusher

•3.3•3.3•The Third Law of Motion•The Third Law of Motion

•Pressure = Force/ Area F

P A