Newton’s 1st Law of MotionInertia

Ch 4.1-4.4

Objectives

1. Natural vs. violent motion

2. Galileo’s concept of motion

3. Newton’s 1st Law of motion

Pens and Cannonballs• What would you think if this pen started moving by itself?• We would look for a cause of its motion• Only living things move by themselves!• A cannonball remains at rest until a force is applied -->

expanding gasses drive ball out of cannon.• What keeps the ball moving when gasses no longer act on

it?• Inertia!

History: Aristotle & Galileo• Aristotle:

– Natural Motion• Straight up or down (boulder

falling, smoke rising)

– Violent motion• Imposed, external cause (ship

and the wind, cart with horse)

• Galileo:– Force is NOT necessary to

keep object going

– Force needed ONLY if friction present.

Force vs. Friction • Force:

– Any push or pull

• Friction:– Force that acts between materials

that touch as they move past each other

Examples of Friction

• Rub hands together

• Look at soles of shoes and compare to when they were new

• Any other examples?

Friction• In the ol’ Wild West, the doors slam open in the Slippy Surface

Saloon. There stands Sly Dingforce, the nasty varmint. “Set ‘em ‘up bartender!” snarls Sly, his hand slamming down on the bar.

• Nervously the bartender pours a slug of liquor and slides it down towards him. It stops about a yard away from his outstretched hand. The tinkling piano music stops.

• All is silent. The bartender croaks, “ Write on my headstone... too much friction!”

Let’s Try Galileo’s Test• Ball on downward slope

– Moves with earth’s gravity. What happens?

• Ball on upward slope– Moves against gravity. What happens?

• Ball on level plane– Maintains almost constant velocity– Only friction would bring it to rest– Ball wants to naturally keep on moving. Why?

• Inertia:– the property of a body to resist change

• Demo: – Pen vs. book—swing back and forth– Air track

Newton’s 1st Law:The Law of Inertia

• An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted on by an outside force

• What does this mean?• Things tend to keep on doing what they are doing• Objects continue to move (or not move) by

themselves

Tablecloth Demo

Clay and Soup

Applications of Newton’s 1st Law

• Pushing a “dead” car• Car turns left, you slide right• blood rushes from your head to your feet while

quickly stopping when riding on a descending elevator.

• the head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. Animation

• to dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and, thrusted downward at high speeds and then abruptly halted.

• headrests are placed in cars to prevent whiplash injuries during rear-end collisions.– Rear-ended!

Checkpoint Question

• If the force of gravity between the sun and planets suddenly disappeared, what type of path would the planets follow?– Straight lines at a constant speed

Let’s see for ourselves…

• Seatbelts/action figure demo

• Flying carseats. Not Good . ..

• Wear your seatbelt!

• Please wear your seatbelt...

• Washer’s and Clothespin Activity