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

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

Conceptual Physical Science

Chapter 2

- 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 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.

- According to Aristotle, what tendency of moving objects governed their motions?
- According to Aristotle, what kinds of motion required no forces?

- Galileo Galilei was an Italian scientist (1564-1642 CE).
- Believed in experimentation
- Blew away Aristotle’s notions of motion

- 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.

- 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.

A 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?

- 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

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)

- 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.

- 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.

- 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.

- 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…

- Sir Isaac Newton (1642-1727)
- Mathematician and physicist
- Discovered many things:
- Laws of motion
- Optics
- Gravity
- Calculus

- “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.

- 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?

- The Net Force (Fnet) is the sum of all the forces acting on an object.
- Force is measured in Newtons (N).

5 N

10 N

5 N

- 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.

Fnet = 0 N

5 N

5 N

- Forces are vector so they have direction and magnitude.
- Upward forces +, downward forces -, forward forces +, backward forces -.

- 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.

- 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?

- 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?

- 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?

- 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?

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