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```					What Causes
Motion?
(Newton’s Laws)

IDS Physics:
Unit 02
Some Initial Questions ….

Why do trains always have the right of way at
Why does a baseball hurt more than a foam ball if
Why did some “hypothetical kid” get in trouble for
messing with his sister?
Inertia

Inertia is the property of matter that resists any
changes in its state of motion.
The inertia of an object is proportional to the
amount of matter in the object
Matter, and thus inertia, is measured by the
mass.
In the metric (SI) system, we use kg to measure
the amount of matter.
In the imperial (English) system, the unit of matter
is the slug.
Isaac Newton

Sir Isaac Newton (1642-1727),
mathematician and physicist,
was one of the foremost
scientific intellects of all time.
Developed the calculus, theories
of matter, motion, force, light
and color, heating and cooling
and many other ideas.
Formulated the Three Laws of
Motion
Newton’s First Law
All objects in motion will remain in motion, in a
straight line, unless an external force acts on
them.
All objects at rest will remain at rest unless an
external force acts on them

Demo: The Dirty Tablecloth
Law of Inertia

Newton’s First Law is also known as the Law of
Inertia.
“change in its state of motion” = acceleration
If there is no external force, there can be no
acceleration.
Objects will continue their current motion
forever, unless something interacts with it.
Examples: toilet paper and tailgates
Law of Inertia

Example: carrying a
cup of tea
Any change in the
state of motion of the
tray will spill the tea
The Effect of Forces

What if there is a force?
• the object will change its state of motion
(accelerate)
What does the acceleration depend on?
Lab Connection

Mass vs. Acceleration for a Cart pulled by a 20 g
falling weight
Acceleration ( m/s/s)

0.08

0.06

0.04

0.02

0
0     200       400       600       800        1000
Mass ( in grams)
Lab Connection
In lab, we found the
following relationships:

1
a                         aF
m
Combined Result of Lab

If we combine the results of our lab,          F
we get the relationship                     a
m

With the right units, we can write:            F
a
m

The acceleration of an object is
directly proportional to the force on it
and inversely proportional to its inertia
(mass)
So, what’s a Force

Any interaction between two physical objects can
be considered a force.
Types of Forces

Force at a distance (field forces)
•Gravity
•Magnetism
•Electricity
Contact forces
•Tension
•Applied Forces
•Friction
Units for Force
Rearrange our equation

F  ma
 F    m a 
  kg   m/s 

2

The Newton
We will define a unit for
force called the Newton

kg  m
1 N 1 2
s
What’s a Newton?

Which of these will exert a downwards force of 1
• Kiwi
• Banana
• Cantaloupe
• Watermelon
A handy Conversion

Imperial Unit of Force = the pound
SI Unit of Force = Newton

1.000 lbs = 4.448 N
Ballpark: 1 N is about ¼ of a
pound
A problem

But do all forces cause objects to accelerate?

Only net, external forces cause accelerations.
How do we find the net force?
FBD’s
Free Body Diagrams are used to draw the forces on an
object.
All forces are vectors
• They have a magnitude
• And a direction
We will represent them with arrows.

5N
2N
Net Forces
The net force on an object is the vector sum of all of the
forces acting on it.
The net force is also known at the resulting force.
The summation notation SF will be used to denote the
net force
SF = F1 + F2 + F3 …
In one dimension, vector addition simply means that we
need to account for the sign of the vector, which
represents it’s direction.
Finding SF

A force of 50 N pulls on a
wagon to the left.
A force of 35 N pulls on a
wagon to the right.
Make a Free Body
Diagram and find the net
force.
Newton’s 2nd Law

We can now revise our statement of this law:

SFext  ma
Example Problem

A typical diesel locomotive (the WDM-3A) has a
mass of 112,000 kg and exerts a forward force of
305,000 N. If this engine is coupled to a load that
is 20 times it’s own mass, what will be the
acceleration of the train?
Example Problem (2)

The engine exerts a force of 12,000 N forward on
a 500 kg. boat. The boat accelerates at 2.0 m/s2.
What is the backwards force of the water on the
boat?
Example Problem (3)

A rope exerts a force of 300. N on sled, which
accelerates forward at 1.5 m/s2. There is also
present a friction force of 50.0 N, which acts
opposite the tension. What is the mass of the
sled?
Gravity: A Special Force

Gravity exerts a force on
each of us. This force is
called our weight.
This force causes every
mass to accelerate at a
rate of -9.807 m/s2.
Weight

We can calculate
this force with
Newton’s Second
Fgravity  ma
Law:

Fg  mg
Example (3)

Find the weight, which is the force that gravity
exerts on an object on earth that has 60. kg of
matter.
Falling Objects

Why do they fall at the same rate if the force is
twice as big on the 1 kg weight?
The bigger mass has a greater inertia, so the
greater force meets a greater resistance to
changes in the state of motion.
Similar Problem: Why does a truck not accelerate
faster than a car if it has a greater force pushing it
forwards?
Newton’s 3rd Law

Every interaction involves two objects. The forces
on these two objects are equal in magnitude and
opposite in direction
“Every action has an equal and opposite reaction”
is an equivalent but archaic statement of the 3rd
law.
Forces come in pairs

Identify the force pairs:
• A hammer exerts a force on a nail
• Earth pulls downwards on a person
• A car pushes backwards on the road
Example
A bug collides with the window of a
speeding bus.

The force of the bus on the bug is the same
as the force of the bug on the bus.
The effects of the force are not equal,
however.

What is the last thing that goes through a
bug’s mind as he collides with a car
windshield?
Game

Time to play the third law game ….

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