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# PH 121 2.3-2.4

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## Richard Datwyler

on 28 April 2015

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#### Transcript of PH 121 2.3-2.4

Chapter 2
2.3-2.4

Motion!!!
Now we're moving
Derivation:

What does it mean to derive something?
The good:

This is how we learn.
start at something we know,
get somewhere new!!

It's MATH
The ugly:

Not always intuitive.
Can get hairy.
Example what can we do with this? (acceleration)

Let us change them, but to what?

What would be a good change for the 1's? Why?
Now this equation takes the form of:
Constant Acceleration:

any examples?

All of what we will do with motion
will have constant acceleration
(rare exceptions will be stated)
From the definition of acceleration
we have DERIVED an equation for Velocity.
So what is the difference between them
and when would you use them?
Following the same flavor with acceleration, we can change velocity to:
What can we do with this average velocity?
What is an average?

what is the average of 2 numbers?
say 0 and 10?
We can do the same for the average of two velocities.

****Caution when does this not work?****

We are ok as long as we have a constant acceleration.
Now what, what's going on, why are we doing this?

Deriving quantities from other base quantities is all
about relationships. We need to take thing we know
and express new concepts in terms of those known ones.

The math can get messy, but we are still ok here.

Recall we had an equation for x but there was an average
velocity in it, and we found a different equation with
average velocity.
Did we do it, is this right?

Can we do anything else?
Yup, one more substitution.
one more!!!!!
Collect the three:
To use these equations I need:

A. Initial position
B. Initial velocity
C. Acceleration
D. Time
E. Some / All : Depends
To solve for 2 (final position) directly
I only need:
1. initial position 2. final position
3. inital velocity 4. final velocity
5. acceleration 6. time

A. 1, 3, & 5
B. 1, 3, & 4
C. 1, 3, 4, & 5
1

2

3
To solve for final position
without time (assuming you
have the other variables):

A. Equation 1
B. Equation 2
C. Equation 3
To solve for final Velocity without distance (assuming you have other variables):

A. Equation 1
B. Equation 2
C. Equation 3
What is the overall assumption
about Acceleration in all of these?

A. it is averaged
B. it is zero
C. it is constant
Here is an example straight from the text:
How long does it take a car to cross a 30.0 m intersection
after the light turns green if the car accelerates from rest at a constant 2.00 m/s^2?a
what values are defined here?
(what are we looking for)
What basic equations have these variables?
Solve, number, units, ballpark
x = 30.0 m
a = 2.00 m/s^2
t = ?
x = 30.0 m
a = 2.00 m/s^2
t = ?
Technically and specifically we
don't have inital position or velocity
rest = initial velocity = zero
cross = start at one end finish other
initial position = zero
A car slows down uniformly
from a speed of 21.0 m/s to
rest in 6.00 s. How far did it
travel in that time?

what is defined?

A. 1, 2, 3
B. 2, 4, 6
C. 1, 3, 5
D. 1, 2, 4
E. 2, 3, 4

Can we solve this with
just one equation?

A. YES B. NO

1

2

3

what are we looking for?

A. 1 B. 3 C. 5

So we have:
x = 30.0 m
a = 2.00 m/s^2
t = ?
tips:

Draw, define, describe
Basic equations
Solve, numbers, units,ballpark

3D be SNUB
Define: list what you know, or is
defined in the problem.
Also list what you don't.
"Where in reality would you be able to find something with constant velocity and acceleration?"
"Can you explain displacement or area under a curve and why it is important."
"Are there other graphs like the position vs. time or acceleration vs. time graph where the area under the curve gives us new information?"
"How do you know what set of kinematic equations are we supposed to use for a given problem?"
"What would be the integral of an acceleration vs. time graph?"
"Can you explain finding position from velocity equations?"
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