Determining Net Force
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Transcript Determining Net Force
Determining Net Force
What is Net Force?
Net Force is the
vector sum of all of
the forces on an
object.
In other words, it’s
the “total” of all
forces.
So, can’t we just add them up?
Its not that simple. You must use the
free-body diagram of the forces to
decide what type of math to do to total
up the forces.
Same Direction…
Only if the forces are in
the same direction can
you add.
Since Fa and Ft are
both upward, we can
add them.
Fa + Ft = 2 + 4 = 6N
Fa = 2N
Ft = 4N
Fg = 6N
Opposite direction….
If forces are in the
opposite direction, you
must subtract them
from each other.
Since Ft and Fg are
opposite, we subtract.
Ft – Fg = 4 –6 = -2N.
The negative sign
indicates a net force
that is downward.
Ft = 4N
Fg = 6N
Let’s sum up the forces in both
directions.
Fa + Ft= 9 + 7 =
16N, upward.
Fa = 9N
Ft = 7N
Fg = 20 N,
downward.
Fg= 20N
Simplified…..
16N upward and 20N
downward.
Since these are
opposites we subtract.
16 – 20 = -4N
downward.
Remember the minus
sign just tells us that the
net force is downward.
Once you simplify,
draw the new FBD.
Fnet = 4N, downward
What about left and right?
The same rules still apply.
Same direction = add
Opposite direction = subtract
What happens if you have forces
that are horizontal and vertical?
Simplify in each plane
first using the rules for
same and opposite
directions.
Then, draw the new
FBD. You should have
one force in each plane.
Notice that the vectors
are PERPENDICULAR
now.
What happens if you have forces
that are horizontal and vertical?
Once you have
perpendicular vectors, you
must continue to simplify.
a
How? See the triangle?
The hypotenuse is the
Fnet!
c
Now
since we have the
lengths of both sides we
just have to use
Pythagoreans Theorem!
b
Using Pythagorean’s Theorem
a2 + b2 = c 2
We
want to find c (net
force)
42 + 3 2 = c 2
16 + 9 = c2
25
= c2
c = 5N (up and right)
4N
3N
Finding Fnet in Summary…
Simplify in both planes:
– Same direction = add
– Opposite direction = subtract
Draw the simplified FBD then use
Pythagoreans theorem IF the new
vectors are PERPENDICULAR.
Classwork/Homework….
Get started on Forces: Worksheet 2.
This is due tomorrow!