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Unit Two: Dynamics
Section 2: Newton’s First Law of
Inertia
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, 2 cards, penny or coin
 Procedure: 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?
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?
 5. If you put two cards on top and try to flick one card
only, what happens? Why?

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.
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. 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.

https://www.youtube.com/watch?v=sabH4bJs
xWA

Explain how inertia/Newton’s 1st Law works in
each situation.
Check Your Learning
1. 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?
Page 210: Q1, 2, 3, 4, 5
Page 211: Q18
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?

Remember when we looked at the Frames of
Reference video (the guy who poured pop “upside
down”)? That video showed that we view things
differently when we have different frames of
reference (ie, what we see depends on where we
are viewing from).
Inertial Frame of Reference
(copy)

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 (copy)

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.
Inertial: not moving (no acceleration)
You are standing in an elevator that is just starting to
move.
Non-inertial: Elevator is speed up (accelerating)
You are standing in an elevator going down at a
constant speed.
Inertial: constant speed so no acceleration
Yesterday’s HW:
Page 210
 Q1, 2, 3, 4, 5


Page 211
18
Today’s HW:




Page 211
Q13a
Page 212
Q29