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# PH 121 5.5-5.7

Newton's Laws
by

## Richard Datwyler

on 16 May 2018

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#### Transcript of PH 121 5.5-5.7

Newton's Laws of Motion
1 & 2
Newton's First Law of Motion
Catch phrase:
objects in motion stay in motion,
objects as rest stay at rest
Stated:
Every object continues in its state of rest, or
uniform velocity in a straight line, as long as
no net force acts on it.
The initial thought of the day (2400 years ago)
was that objects natural state was at rest. That it took
a force to keep them moving.

I ask can you refute this,
or see why people would believe this?

Can you think of an object that doesn't seem to be
'naturally' at rest?
Amazingly only ~400 years ago this idea was
changed. Galileo said, no, things can be in just
as 'natural' a state if they have constant velocity.

Think of pushing a piece of metal, shaped like a
hocky puck, with the same initial force on:
cement
roller skating rink
Ice
lubricated metal
Super conductor
Whats the difference?
Galileo worked a lot in celestial physics.
not as much friction,
Thus he could see that constant velocity was a
natural state of motion too.
Netwon's First law is a restate or iteration of
Galileo's work, and he give him the credit.
Newton's First law is also known as
'the law of inertia'
What is inertia?
Say we are sitting in a vehicle, and somhow
'magically' I am pulled towards the cute
opposite gender counterpart next to me.

What happened?

Did some mysterious force pull me towards this, lovely individual?
Inertial Reference frames
An inertial reference frame is one where Newton's laws are valid. There must be no acceleration, or you could say they must have a contant velocity.
Mass is the measure of inertia.

The more mass something has
the more force is required to give it
some motion.

SI units : Kilogram , kg.
Is mass the same as weight?
Newton's Second Law

Catch phrase:
F=ma
Stated: The acceleration of an object is directly
proportional to the net force acting on it, and
inversely proportional to its mass.
Nicer statement:
Are these the same?
Mathematically , Physically?
A Force is an action that can accelerate an object

Its SI units are Netwons , N

The base unit of a N comes from this 2nd law of motion

F =ma [N] = [kg][m/s ]

2
What net force would be required to bring a 2000 kg truck from rest up to 25 m/s in a time of 50 seconds?
Notice it is a vector equation
Thus it can be broken into components.

Also notice the sum, this means we have to
ADD all the different vectors together.
This is most effectly done in their individual components.

Free-Body Diagrams
THIS IS KEY
Key parts to FBD

A. Particle
B. Coordinate system
C. All forces, note net force not on particle
" Can you explain how an object can be in equilibrium and still be moving?"
"Can we go over free-body diagrams?"
"What is this equilibrium deal they speak of?"
"Why is Newton's second law mentioned before the first law?"
"Can you go over inertial reference frame concept in relation to passengers in a braking car? pg. 129
" could you discuss more about inertial reference frame and what is its purpose?"
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