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Newton’s First Law of Motion States: an
object at rest remains at rest, and an
object in motion continues in motion with
constant velocity (constant speed in a
straight line) unless it experiences a net
external force.
The net external force is the total force
resulting from a combination of external
forces on an object.
Find the net external force:
Fnet= 400N up
Find the net external force
Fnet= 200N down
Find the net external force
Fnet= 20N left
Find the net external force
Fnet=0
Find the net external force for each
A= 50 N
B= 200N
C=1100 N
D= 20 N
E= 300N
F= x
G= 50N
H= x
Joe ponder, from North Carolina, once used
his teeth to lift a pumpkin with a mass of
275 kg. Suppose Ponder has a mass of 75
kg, and he stands with each foot on a
platform and lifts the pumpkin with an
attached rope. If he holds the pumpkin
above the ground between the platforms,
what is the force exerted on his feet?
Fnet= (9.81)(275+75)
Fnet= 3433.5 up
The net force exerted by a woodpecker’s head when it
strikes the bark of a tree can be as large as
4.90 N, assuming that the bird’s head has a mass of
50.0 g. Assume that two different muscles pull the
woodpecker’s head forward and downward, exerting
a net force of 4.90 N. If the forces exerted by the
muscles are at right angles to each other and the
muscle that pulls the woodpecker’s head downward
exerts a force of 1.70 N, what is the force exerted by
the other muscle? Be sure to draw a free-diagram
showing the forces acting on the woodpecker’s head.
Fnet²=(Fx)² +(Fy)²
4.90N²= Fx² +(1.70N)²
Fx= 4.59 N