Transcript U5 Net Force Model Notes

Unit 5
Unbalanced Force (Net Force)
Model
Aka: Newton’s Second Law (N2L)
First Law – No Un-balanced
Force…
• No change in motion
• No change in speed
• No change in direction
• MASS is a measure of the object’s
tendency to retain its current state of
motion.
Newton’s First Law
• If there is no “left over” force in a
particular direction, there will be no
change in velocity (speed or direction).
• But what if there IS a left over “net” force?
What will be its effect?
Combining these ideas…
F=ma
But, since more than one force
may be acting on an object…
The second law
Fnet = m a
A “force” is an interaction
between two objects.
Units
Metric
Unit
Force
Mass
Acceleration
Newton
(N)
kilogram
(kg)
m/s/s
Does this mean…
F = m
a
N = (kg) (m/s/s)
Does this look right?
Practice Problems
#1
Given: F=140N
Find: mass
Solution :
a=20 m/s/s
F=ma
140=m(20)
m=7 kg
Where did kg come from?
Algebra Triangle
Newtons 2nd Law Triangle
Net Force
Fnet Causes Acceleration
Weight – the Force of Gravity
It ALWAYS acts downward.
But gravity is different
• Gravity applies a
force proportional to
the mass
• More mass means
more force
• No other force seems
to behave in this way
The acceleration produced by
gravity does not depend on mass
• The ratio F/m remains
constant
• Its value is 10 m/s/s
at the surface of the
earth (9.8 m/s/s)
• It is true, but still very
amazing!
Fg = m g
This formula is still Newton’s
Second Law, just rewritten to be
specific for gravity.
How much does 1.00 kg weigh?
Fg  mg
Fg  1.00(10)
Fg  10 N
Look familiar?
What is the mass of an object that
weighs 575 N?
Fg  mg
575  m(10)
m  57.5kg
How much would the same 58.7 kg
object weigh on the moon
(g=1.6 m/s/s)?
Fg  mg
Fg  57.5(1.6)
Fg  92 N
Adding Easy Vectors
Important Lesson!!!
Newton’s Second Law
One Force Acting on One Object
Assignment
Worksheet 5a due tomorrow (only some
questions will be graded, not all of them)