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# Newton’s first law of motion: Inertia

## If moving, objects require a force to stop moving

A quick summary.

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

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!)

• ## Animations showing Newton’s Law of Inertia

• ### The Truck and Ladder

• The Elephant and The Feather – Free Fall

## 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.

## 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.