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PH 121 5.3-5.4

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by

Richard Datwyler

on 16 May 2018

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Transcript of PH 121 5.3-5.4

Forces
Identifying forces

What can I say, you see them you draw them

so lets try some.
An elevator going down, and increasing its speed as it goes down.
Rocket launched up, don't neglect air resistance.
Jet plane on runway before take off, don't neglect air resistance
Car braking going down a hill of 20 degrees.
Spring cannon pushing a ball down the barrel of the cannon.
Person throws a rock straight down off a bridge, the rock has just been thrown
Now a virtual experiment:
Or not.
Force is proportional to acceleration.

the text goes into pulling rubber bands
and noting the acceleration
and the force
bigger the force, bigger the acceleration
They are proportional.
Mass
The proportionality of force and acceleration is mass

If I graphed that virtual experiment
Acceleration vs Force
the slope is 1/mass
Mass is an intrinsic property of a substance.
It is how much matter there is.

Better it is how much an object resists being moved.
This is the idea of inertia.
Inertia is defined as:
The amount of force needed to accelerate and object.
Mass, What is it?
Mass is not weight, it is inertia
it is a resistance of motion for an applied force
This ties in immediately to Newton's 2nd law.




F = ma
Before we dive into F=ma
let us talk about kinematics and dynamics
this law relates the two.

The dynamics is what causes the motion

The kinematics is the resultant motion.
When something is moving it doesn't have multiple
accelerations. It has just one acceleration.

It can have multiple forces, but it will only have
one net or resultant force.
This distinction can help us as we add up all the forces.

If we find the direction of the one acceleration it will make solving the problems easier.
So let us try a few.
What is the direction of the acceleration in these examples:
Box pushed across a flat surface.
Car parked on a hill?
Bicycle picking up speed as it goes down a
hill.
Car going the same speed around a turn.
A cannon ball flying through the air (with no air resistance.)
A skydiver falling, and eventually reaching terminal velocity.
And finally to the point,
a pendulum.
O
V
V

t
V
x
y
V
t
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x
O
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"Why is friction described as a force in the opposite direction?"
"Can you go over how to identify the type of force?"
"can you go over the idea of inertia please?"
"Is there a way to make sure we found all the forces applied to the object?"
"I don't understand the concept of acceleration increasing at a constant rate when a force is applied to it. Could you explain it for me?"
"Are we going to have to draw lots of pictures in this unit?"
"Can you explain how to draw a proper motion diagram?"
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