Newton’s First Law of Motion – The Law of Inertia

Conceptual Physical Science

Chapter 2

Aristotle’s Classification of Motion

• Greek scientist (Born 384 BCE)

• Classified motion into two categories– Natural motion– Unnatural motion

• Natural motion occurs without force.

• Unnatural motion required a force.

Aristotle’s Classification of Motion

• Aristotle believed that– All objects have resting places that they naturally seek. – All motion on the Earth is linear.– All motion in the heavens (outer space) is curved. – The speed at which an object falls is directly related to the mass

of an object.– Motion continues so long as there is only an applied motion

(force) to an object. Removing the motion (force) stops the object.

• Aristotle’s ideas lasted almost 2000 years…• It would take a man named Galileo to start the wheels of

change in this field of knowledge called physics.

Concept Check:

• According to Aristotle, what tendency of moving objects governed their motions?

• According to Aristotle, what kinds of motion required no forces?

Galileo’s Concept of Inertia

• Galileo Galilei was an Italian scientist (1564-1642 CE).

• Believed in experimentation

• Blew away Aristotle’s notions of motion

Galileo’s Concept of Inertia

• Galileo found that– Except for the effects of friction, objects fall at

the same rate… regardless of size.– Force is required to start an object moving,

but not to keep it moving.– Tested with this experimental design.

Galileo’s Concept of Inertia• The tendency of things is to remain as they

are…– If moving, they tend to keep moving.– If at rest, they tend to stay at rest.

• is called inertia.

Concept CheckA ball rolling on a pool table

slowly comes to a stop. • How would Aristotle explain

this behavior?• How would Galileo explain it?• How would you explain it?

What two main ideas of Aristotle did Galileo discredit?

What is the name of the property of objects to maintain their states of motion?

Galileo Formulates Speed & Velocity

• Until Galileo motion was described as either fast or slow.

• Galileo measured speed by considering distance and the time it took to cover it.

Distance

• Speed = Time

Galileo Formulates Speed & Velocity

Approximate speeds in different units

12 mph = 20 km/h = 6 m/s (bowling ball)

25 mph = 40 km/h = 11 m/s (super sprinter)

37 mph = 60 km/h = 17 m/s (sprinting rabbit)

50 mph = 80 km/h = 22 m/s (tsunami)

62 mph = 100 km/h = 28 m/s (cheetah)

75 mph = 120 km/h = 33 m/s (batted softball)

100 mph = 160 km/h = 44 m/s (batted baseball)

Galileo Formulates Speed & Velocity

• Velocity differs from speed in that we also know the direction of the moving object.

• Velocity is both speed and direction.

• Velocity is a vector quantity.

• Speed is a scalar quantity.

• Constant speed doesn’t mean constant velocity… the opposite is true though.

Concept Check• What is the average speed of

cheetah that sprints 100 m in 4 s?• The speedometer on a bicycle

moving east reads 50 km/h. It passes another bicycle moving west at 50 km/h. Do both bikes have the same speed? Do they have the same velocity?

• “She moves at a constant speed in a constant direction.” Say the same sentence in fewer words.

Motion is Relative

• Everything is always moving!

• Our speed relative to the sun is 100,000 km/h. Faster relative to center of our galaxy!

• Discussing motion always involves a reference point.

And then there was Newton!

• Aristotle valued logic to figure things out.

• Galileo used experiments.

• Galileo showed that experiments are better than logic in testing knowledge.

• The path was clear then for Isaac Newton…

Newton

• Sir Isaac Newton (1642-1727)

• Mathematician and physicist

• Discovered many things:– Laws of motion– Optics– Gravity– Calculus

Newton’s 1st Law of Motion

• “An object tends to remain in a constant state (at rest or in motion) until an unbalanced force acts upon it.”

• The larger the mass, the larger the inertia.

• Mass and inertia are similar, but not equivalent.

• Inertia depends on the distribution of mass.

Concept Check

• How does the law of inertia account for removing dirt from your shoes by stamping on the porch before entering a house or removing dust from a coat by shaking it?

Net Force

• The Net Force (Fnet) is the sum of all the forces acting on an object.

• Force is measured in Newtons (N).

5 N

5 N

10 N

Equilibrium for Objects at Rest

• Objects at rest are said to be in static equilibrium.

• Static objects have Fnet = 0 N. Even though there may be many forces acting on that object they all cancel each other out.

5 N 5 N

Fnet = 0 N

Equilibrium rules

• Forces are vector so they have direction and magnitude.

• Upward forces +, downward forces -, forward forces +, backward forces -.

Equilibrium

• Weight is a force (-) that acts in a downward direction (towards the center of the Earth).

• For an object at rest, there must be an opposite but equal force acting on it to have a net force = 0 N.

• That force is called the Normal force (Fn).

• It’s always perpendicular to the surface.

Concept Check

• As you stand at rest on a floor, does the floor exert an upward force on your feet? If so, what exactly is this force?

Equilibrium for moving objects

• Moving objects are in dynamic equilibrium, not static.

• Sum of forces must be = 0 N if its velocity is constant. Remember: Newton’s First Law!

• The “balancing a deck of cards in your hand while on a moving train,” example… cards don’t know the difference.

Am I moving or standing

still?

Earth’s Rotation

• If the Earth came to a sudden stop, and I jumped up into the air at that exact moment, wouldn’t I land safely back on the ground?

Earth’s Rotation & Inertia

• The Earth is rotating very rapidly (even as you write this down). Do you feel it?

• Could you travel from San Francisco to New York by staying stationary in the air for 3 hours?

Hard Work!

Humankind struggled for nearly 2000 years in developing the ideas of this section. You should be patient with yourself if it takes a few days or weeks to achieve as much!

“Thinking” by Broken Persona