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Mechanics
Newton’s Laws
of Motion
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Law
1:
A body
will continue in a state of rest
or uniform motion unless acted on by an external
force.
This is
often called the law of inertia.
Law
2:
The net
(or resultant) force acting on a body is equal to the
product of its mass and
its acceleration.
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Force =
mass
× acceleration |
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SF
= ma |
Law
3:
If body
A exerts a force on body B, then body B exerts an equal
but opposite force on body A. |
The symbol
means "the sum of" so, if we have two forces
and
,
acting on a body, then
means
.
So, remember,
represents a
vector addition.
For example:
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The
sum of these two forces is found by vector
addition. |
If the two forces act along the
same line then the arithmetic becomes a little easier.
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In
this case, SF
is equal to
+10 – 4 =
6 N (to the right) |
Using Newton’s
Second Law of Motion
An object of
mass m = 20kg rests on a horizontal surface. Calculate the
magnitude of the force exerted by the surface on the object
in the following situations.
a) Stationary
or moving with
constant velocity
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F
= ma
also,
F =
R
+ mg
but, in this
case a = 0
therefore, R
+ mg = 0
and so R =
-mg
R =
196
N |
b) Accelerating upwards
at 2 ms-2
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F
= ma
also,
F
= R
+ mg
therefore,
R + mg = ma
and so R = m(a
– g)
R = 236
N |
c) Accelerating downwards
at 9·8 ms-2
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F
= ma
also,
F
= R
+ mg
therefore,
R
+ mg = ma
and so R = m(a
– g)
R =
0
So, in this case, the body would
appear to be
weightless. |
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© David
Hoult 2008 |