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# PH 121 8.1-8.3

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## Richard Datwyler

on 30 May 2018

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#### Transcript of PH 121 8.1-8.3

Chapter 8
Dynamics II:
motion in a plane
Amazingly enough, we have been doing this for some time now.

When we write

We somehow knew it was a vector.
And we solved for the components separately and put them into vector kinematic equations.
That is the crux of section one of this chapter
Now we get some helpful material

Often we work in x,y,z coordinates

It can be easier for rotational motion to instead
be in r,t,z coordinates
tangential
Uniform Circular motion
B. tangential velocity
It is amazing that with these new
coordiantes, UCM problems make
more sense.
Now on to a little bit of new material.
Dynamics of UCM

now that we have reminded ourselves of the
acceleration of going in a circle,
we can get dynamics by multilplying by mass.
This says, that if I want to move in a circle I must have
some force or combination of forces that point in the
If I do, I will move in a circular path of constant speed.

The real question here, is what is that force?
Some examples:

Sling (think David and Goliath)
Turning corner in car
Banked corner
roller coaster
Tether ball
Moon
Electron?
...

What force is causing it to turn?
Do two turns on board
In UCM using an rtz - coordinate system which motion variables are non-zero?
A. v and v
B. a and a
C. a and v
D. a and v
True / False: In circular Dynamics for an object to move in UCM there can't be any force pointing away from the center of a circle
A. True
B. False

For a car turning around a banked corner too quickly for the banking angle, which forces point towards the center of the circle?
A. All of the Normal force
B. All of the Frictional force
C. All of the Normal and part of the Frictional
D. All of the Frictional and part of the Normal
E. Part of the Normal and part of the Frictional
27
.5 kg rocket falls off 40-m high wall with initial horizontal speed of .5 m/s and engine fires immediately with 20 N thrust, horizontally.
A. Where does it land
B. Describe trajectory
39 10 g steel ball spins in 12 cm diameter steel tube with speed of 150 rpm, will it fall or stay same height
Normal
gravity
friction
F
F
F

CQ: 2, 5
Pr: 1, 22, 23, 26 and 5, 9, 33, 35, 43

Linear Circular