Section 2: Newton’s First Law of Inertia Unit Two: Dynamics Section 2: Newton’s First Law of Inertia

Intro Video on Newton’s Laws https://www.youtube.com/watch?v=iHeMqK3XZbc First 6 minutes

Newton’s First Law of Motion - Newton’s Law of Inertia An object at rest or in uniform motion (ie, constant velocity) will remain at rest or in uniform motion unless acted on by an external force. Section 5.1 in text (pages 154 to 159) Reworded: An object at rest will remain at rest until a force is applied. An object moving at a constant velocity will continue to move at a constant velocity if no force is applied (ie, no acceleration).

Inertia the natural tendency of an object to remain in its current state of motion (either moving or at rest)

Where did this come from? Galileo performed many experiments and speculated that if a perfectly smooth object were on a perfectly smooth horizontal surface it would travel forever in a straight line. Newton developed this idea.

Newton’s First Law Summary Also called Newton’s Law of Inertia An object at rest or in uniform motion (ie, constant velocity) will remain at this constant velocity unless acted on by an external force. Inertia: the natural tendency of an object to remain in its current state of motion (either moving or at rest)

Quick Experiment Materials – cup, card, penny or coin What to do: Set up the card on top of the cup and the penny on the card in the middle. Flick the card. What happens to the card? The penny? Why? Law of Inertia (Newton’s 1st Law) says the penny wants to stay the way it is (still) so it will fall straight down.

Questions 1. To which object was a force applied by the flick and which object was not acted upon by the flick? 2. Why did the penny fall into the cup and not fly off with the card? 3. What force held the penny in place while the card was flicked out? What force brought the penny down into the cup? 4. Would the penny move in the same way if sandpaper was used instead of the card?

Summary of experiment The inertia of every object resists the change in motion. In this case, the inertia of the penny held it in place while the card was flicked out from under it. The force acting on the card was not applied to the penny. After the card was moved from under the coin, gravity supplied the force to bring the penny down into the cup. If a force had been applied to both the card and the penny, then both would have moved and the penny would not have fallen into the cup.

https://www.youtube.com/watch?v=iHeMqK3XZbc After 6 minutes to 8.5 minutes

Check Your Learning 1. Why does a package on the seat of a bus slide backward when the bus accelerates quickly from rest? Why does it slide forward when the driver applies the brakes? Use as many physics terms as possible and describe in detail.

The bus is initially at rest, as is the package The bus is initially at rest, as is the package. In the absence of any force, the natural state of the package is to remain at rest. When the bus pulls forward, the package remains at rest because of its inertia (until the back of the seat applies a forward force to make it move with the bus). From the point of view of someone on the bus, it appears that the package is moving backward; however, someone watching from outside the bus would see the bus move forward and the package trying to stay in its original position. Once the package is moving with the bus, its inertia has now changed. It now has a natural tendency to be moving forward with a constant speed. When the bus slows down, the package continues to move forward with the same constant speed that it had until some force stops it.

Check Your Learning A. A physics book is motionless on the top of a table. If you give it a hard push, it slides across the table and slowly comes to a stop. Use Newton’s first law of motion to answer the following questions: A) Why does the book remain motionless before the force is applied? B) Why does the book move when the hand pushes on it? C) Why does the book eventually come to a stop? Draw a FBD as well to help. D) Under what conditions would the book remain in motion at a constant speed?

Newton’s First Law Examples If an apple is sitting on Mrs. Evans’ desk, it will remain there until the desk is removed (so gravity acts on it) or someone lifts it up (applied force). If a car is driving along a straight road at 100km/h, it will continue to do so (given the car still has gas!) until the brakes are applied (applied force), there is a turn or the road surface changes (more or less friction).

Frames of Reference Imagine you are driving in a car. Does it feel like you have moved? If you are watching from the road, how does your frame of reference change?

Inertial Frame of Reference A frame of reference that is at rest or moving at a constant velocity. Example: You moving in a car on cruise control. Example: You sitting at your desk. Newton’s Laws of Motion are valid here!

Non-inertial Frame of Reference An accelerating frame of reference Example: When you suddenly stop in a car. Example: When you are speeding up and passing a car. Newton’s Laws of Motion do not apply! Read pages 156-158

What type of frame of reference??? You are standing in an elevator waiting for it to go up 10 flights. You are standing in an elevator that is just starting to move. You are standing in an elevator going down at a constant speed.

Page 159 Q1, Q3, Q5 – last night’s homework Page 210 Q1, 2, 3, 4, 5 Q13a, 18 Page 212 Q29 10 minutes total