Lesson 22: Net Force
The
net force
is the vector sum of all the forces acting on an
object.
●
If the forces are parallel we can just add them
together as positive and negative forces.
●
If the forces are at an angle we have to add them as
components of vectors.
Example 1
: A car is stuck in a snow drift. Niels and Katrien attach two ropes to the vehicle and try to
pull it out by pulling in the same direction. Niels pulls with a force of 75N while Katrien pulls with a
force of
68N. There is a force due to friction of 40N acting on the car.
Sketch
a free body diagram of
the situation and
determine
the net force acting on the car.
Sketch a topdown diagram, remembering to show the friction acting against the two people
pulling.
●
Notice how Niels and Katrien and both pulling in
the same direction, parallel to each other. Since
we've shown it to the right, we will call these two
forces positive.
●
Friction is acting against them, in exactly the
opposite direction. We will call this force negative.
To determine the net force, we will need to write out a
formula.
●
We won't find this on the data sheet, since every net force
question can be based on different forces.
●
Net force will be equal to all the forces from the free body
diagram added together.
F
NET
=
F
an
+
F
ak
+
F
f
= 75 + 68 + 40
Fnet = 103 N
The net force is 103 N acting to the right.
Example 2
: As Niels and Katrien pull the car, they notice a patch of ice on the road directly in front of
them. To keep on pulling without slipping on the ice, they must begin to pull at an angle as they walk
around the ice. Niels still pulls with a 75N at [E15
O
N] and Katrien pulls with 68N at [E20
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 Fall '08
 Staff
 Force, Friction, net force, Katrien

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