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# PH 121 6.1-6.3

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## Richard Datwyler

on 19 October 2013

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#### Transcript of PH 121 6.1-6.3

Brainstorm
I clickers
Research
Motion in 1-D
Notes
Kinematics
Vectors
Components
Important
Details
(cc) photo by Metro Centric on Flickr
(cc) photo by Franco Folini on Flickr
(cc) photo by jimmyharris on Flickr
Forces!!!
(cc) photo by Metro Centric on Flickr
NEWTON's
LAWS
1st Law
2nd Law
F=ma
if F = 0
then a = o
and v = ?
Static Equilibrium
Dynamic Equilibrium
gravity & weight
mass = Weight
NO! NO! NO!
Apparent Weight
Equilibrium
Weight
F=ma
Equilibrium
Sum of forces = Zero
Static
Velocity also = zero
NOT MOVING
Dynamic
Velocity is constant
IS MOVING
Note:
Only works on a particle model, just a dot,
we'll deal with rods and massive objects later
bird sits in the middle of a telephone wire
(5 ounce bird with a 1lb coconut, gripped by the husk)
The 1kg bird depresses the massless wire by 10 degrees, what is the tension of the wire?
Using F=ma
Two steps:
1. Find Acceleration
2. Use kinematics to find motion
Components, Components, Components
Lots of Algebra
Free body diagrams
Use acceleration in equations
Hints: Look for knowns (time, distance, velocities)
Repeat same steps
each time, always!!
500 g model rocket blasts off with 20.0 N of steady
thrust from the little engine, which lasts for 5 seconds.
How High does it go? (ignore weight loss, and air resistance)
Weight, Gravity, Mass
Newton's law of gravity
Yet we have said F = mg
G
Weight w = mg
ALWAYS
- An apparent weight!
Standing on a bathroom scale in an elevator,
Joe Bob, notices that he has a weight that is
75% his normal weight, just as the elevator begins its descent. How fast is it accelerating?
Examples
Take a closer
Look
SUM it all up!!
QUIZ
TIME
If an objects Net Force
is Zero then
A. Its not moving
B. It has constant velocity
C. Either / or Depends
In using Newton's Laws in
problems it is good to use:
A. FBD
B. Vector Components
C. Algebra
D. Kinematic equations
E. All the above, in order!!
If I wanted to have an apparent weight double my normal weight
what should my acceleration be?
A. 2 g
B. g
C. 0
D. -g
E. -2g
(g can change though)
"Could you please go over static vs dynamic equilibrium?"
" Can we go over the figure 6.1 with the man weighing himself in the accelerating elevator?"
"Is it appropriate to say "apparent weight" when talking about objects accelerating? "
" how do we measure mass?"
"Why is the cup "Accelerating" with the astronaut? from the pre-quiz questions?"
A student stands on a scale in an elevator that is accelerating at 2.5 m/s2. If the student has a mass of 78 kg, to the nearest newton what is the scale reading?
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