1
Sir Isaac Newton
Born:
4 Jan 1643 in Woolsthorpe, Lincolnshire, England
Died:
31 March 1727 in London, England
1
The key to understanding Newton's ideas about motion is
to investigate the description of "force" and "mass." The
main idea is that "forces cause accelerations," and, that
the amount of acceleration depends on the "mass" of the
object. Simply, a force is the reason why an object might
accelerate.
Newton’s First Law
Every object continues in its state of rest or of uniform speed in a straight line
unless it is compelled to change that state by a "net force" acting on the
object
.
•T
h
i
s
doesn't say
that every moving object has a "force" acting on it.
h
i
s
doesn't say
that a stationary object has no forces acting on it
•
Newton’s First law is really just a special case of Newton’s
Second Law (for
a=0)
2
Inertia
All objects resist changes in their state of motion.
Mass
(m) as a Measure of the Amount of Inertia.
3
Newton’s Second Law
The acceleration of an object is directly proportional to the net force acting on
it and inversely proportional to its mass. The direction of the acceleration is in
the direction of the applied net force.
y
y
x
x
ma
F
ma
F
=
=
∑
∑
,
a
m
F
r
r
=
∑
4
Example:
Newton's 2nd Law enables us to compare the results of the same force exerted on
objects of different mass.
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5
Example
:
To stop a 2000 kg car moving at 14m/s in 2.0s requires an average breaking
force,
F = ma = (2000kg)(14m/s)/(2s) = 14000 N.
Example
:
Two forces,
F
1
=45.0N and
F
2
=25.0N act on a 5.00kg block sitting on a table
as shown. What is the horizontal acceleration (magnitude and direction) of
the block?
6
F
1x
=
F
1
cos(65.0)
= 19.0 N
Σ
F
x
= m
a
x
Σ
F
x
= F
1x
F
2x
= 19.0 N  25.0 N = (5.00kg)a
x
a
x
= 1.2 m/s
2
BA
AB
F
F
r
r
−
=
Newton’s Third Law
Whenever one object exerts a force on a second object, the second object exerts an
equal and opposite force on the first.
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 Spring '10
 BalasubramaMaheswaran
 Physics, Force, Normal Force, tension force, net force

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