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Inertia and mass

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Newton’s first law of motion: Inertia

Every object continues in it’s state of rest,
or of uniform velocity,
as long as no net force acts on it.

If at rest, objects require force to start moving

If moving, objects require a force to stop moving

Inertia balloon pops leaves water behind

A quick summary.
newtons first law motion

An object at rest, stays at rest, unless accelerated by some external force, and tow trucks see this more often than you might imagine!
Car falling off flatbed Inertia

An object at rest, stays at rest, unless accelerated by some external force
Elegantly illustrated by the leaves staying behind here (until gravity accelerates them!)


2 different definitions of mass

    a measure of inertia
the quantity of matter

Don’t confuse mass with volume

Here are five cylinders of different metals:
they all are different volumes, yet all are of equal mass.
Lead, copper, brass, zinc, and aluminum.

How is this possible?
Somehow, more matter can be crammed into the same volume with denser materials
Less matter takes up the same volume in less-dense materials

Mass is not weight

Weight is how much a mass is pulled down by gravity.
This girl has the same mass on both worlds, yet her weight varies.

Mass is the quantity of matter in an object.

Weight is the force of gravity on an object.

One kilogram weighs (approximately) 10 Newtons

The gravity of Earth gives a downward acceleration,
g = 9.8 m/s2, to objects.
We often approximate this as g ≅ 10 m/s2

Because the object is being accelerated down, we feel this as “weight”.

Can we convert between mass and weight?

Strictly speaking – no. Why not?
The same mass will have different weights, when placed on different planets.

Can we convert between mass and weight, assuming that the object is here on Earth?

Oh, that’s different. Sure – for that one planet alone, we can convert between mass and weight. Here’s a conversion that’s valid only on Earth.

1 kg × g = 9.8 N     (more exact)

1 kg × g ≅ 10N       (approximation)

So these approximate conversions are useful.

  • 1 kg         is about 10 N

  • 1/10 kg  is about 1 N

  • 100 kg   is about a kN

Learning standards

New York Physics: The Physical Setting Core Curriculum

Key Idea 5: Energy and matter interact through forces that result in changes in motion.
5.1 Explain and predict different patterns of motion of objects (e.g., linear and uniform circular motion, velocity and acceleration, momentum and inertia)
5.1i According to Newton’s First Law, the inertia of an object is directly proportional to its mass. An object remains at rest or moves with constant velocity, unless acted upon by an unbalanced force

Appendix D: AP Physics Enduring Understandings

Enduring Understanding 1C: Objects and systems have properties of inertial mass and gravitational mass that are experimentally verified to be the same. Inertial mass is the property of an object or a system that determines how its motion changes when it interacts with other objects or systems.

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