9/25 2nd Law

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9/25 2nd Law

• Text Chapter 4 This is a long chapter-watch
  the notation
• HW 9/25 Snow Drift due Friday 9/27
• Lab Manuals are in the bookstore
• Tomorrow we tackle the 3rd Law
• Exam II on Thursday 10/17

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What do you think now?
All objects come to rest unless there is a force
on it.
To keep an object moving at constant speed,
there must be a force on it in the direction it
is moving.
These ideas fall short and, in fact, contain the
principal misconceptions you must overcome!
Newtons 2nd Law seems so simple but you will
resist what it says. The problem (I believe) is
that the concept of acceleration is still
entwined with velocity.
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Newtons Second Law
This is it. This predicts everything! The rest
is learning how to use it and to trust what it
predicts even when we know better. (You will
likely not believe what this says about the
tension in the strings in your HW.)
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Newtons Second Law
For 2-D problems we break down forces into the
net force in x and the net force in y.
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Free Body Diagram Example
FBD of book on table What are the forces? Are
there any contact or non-contact forces?
But how do we know how long to draw them?
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Free Body Diagram Example
If the acceleration is zero the net force is zero
also.
Need both vectors to be the same length so Fnet
0.
NT,B
Book
WE,B
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Free Body Diagram Example
If the acceleration is zero the net force is zero
also.
Need both vectors to be the same length so Fnet
0.
Keep an eraser handy and be willing to adjust the
length of the forces as you apply the 2nd law.
NT,B
Book
WE,B
8
Problem Solving

• Draw a free body diagram
• Do you know acceleration?
• Draw the force arrows so that the net force is
  consistent with the acceleration.
• Label all force arrows by type, cause, and object
  it is applied to like this.

NA,B - Normal force by object A on object B
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Example
Hint This is much like your homework assigned
today.
Elevator moving down and slowing down.
The magnitude of the elevators acceleration is
2m/s2. Let g 10N/kg.
Find the normal force that the floor puts up on
the block
But first find the direction of acceleration
Draw FBD
Adjust the arrow length(s)
Apply 2nd law
N - W ma
5kg
N - 50 5(2) 10
N 60N
a
mg 5(10) 50N
Fnet
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Example
The magnitude of the elevators acceleration is
2m/s2. Let g 10N/kg.
Note the danger in calling g an acceleration!!!!
g tells us the Weight force on an object and is
not an acceleration.
But first find the direction of acceleration
5kg
a
mg 5(10) 50N
Fnet