Chapter 4 Newton’s First Law of Motion-

InertiaConceptual Physics

Hewitt, 1999

Divided motion into two categories:◦ Natural motion- falling (boulders) or rising

(smoke)◦ Violent motion- applying a force to make

something move, “against its nature”

4.1 Aristotle on Motion(400B.C.)

Heliocentric view- earth around sun Worked in secret to avoid persecution

4.2 Copernicus on Motion (1543A.D.)

Force- any push or pull◦ Friction- force that acts when two

things physically interact◦ Only when friction is present will

something have its state of motion changed

Inertia- every object resists change in its state of motion (velocity)◦ If moving, wants to keep moving◦ If stationary, wants to sit still

4.3 Galileo on Motion(1600’s A.D.)

Friction Ramp

“Every object continues in its state of rest or uniform velocity as long as no net force acts on it.”

4.4 Newton’s Law of Inertia

Example: Kick an empty paint can and it flies. Kick a paint can filled with paint and you break your toe.◦ The can with more “stuff” in it has more inertia: more resistance

to change in motion (or lack of motion). Mass is not volume

◦ Mass- measured in kilograms (kg) and is a statement of how much matter it contains Most fundamental measurement of quantity of matter Independent of location

◦ Volume- measured in milliliters (mL) or liters (L) and is a statement of how much space the matter takes up

4.5 Mass- A Measure of Inertia

Weight- the force of gravity on an object◦ On Earth, Mr. Latham has a mass of 85kg and 185lbs◦ In deep space, far away from anything, he still has a

mass of 85kg, but no weight (weightless)

Weightlessness- your apparent weight is 0N◦ If your “a”=g◦ If your “a”≈0m/s2

◦ Think free falling◦ http://www.youtube.com/watch?v=2V9h42yspbo

Mass is not weight

Mass is not weight

One kg weighs 9.8 Newtons (on Earth)◦ 1kg = 9.8N◦ Force is measured in Newtons (N)

Force of gravity (weight) = (mass)(acceleration due to gravity)◦ Fg=mg

Remember that on Earth a=g=9.8m/s2

On the Earth’s moon, g=1.63m/s2

Mass & Force

Net force- sum of all of the forces acting on the object

Force (free-body) diagram- shows vectors of all forces involved in motion◦ Use arrows, length matters, to show the force(s)

involved◦ Helps us visualize what’s going on

4.6 Net Force

Normal force- a supporting force: a table, the earth, etc.◦ Anything that holds an object (or you) up◦ Typically can support much more than just that one

object We only talk about the force that is used, not what is

available◦ Labeled as FN

Equilibrium- when all of the forces cancel each other out

4.7 Equilibrium- When Net Force Equals Zero

As the angle gets larger, the resultant force is constant, but the actual forces must get larger as well

Hanging from a vertical string is easier than from a horizontal string

4.8 Vector Addition of Forces

F=ma (force) Fg=mg (weight) Fnet=F1+F2

gE=9.8m/s2

gM=1.63m/s2

Review Questions 1-20

Think & Explain 1-12

Conceptual Development 4-1, 4-2, 4-3

Assessments & Formula