Transcript Slide 1 - Fort Bend ISD

Forces and Free Body
Diagrams
Common Forces
• Gravity- attractive force between two
objects that have mass.
• AKA Weight
• To calculate Weight:
– Force of Gravity (Weight) = m x a (due to
gravity)
• Example: 70 kg man
– Weight= 70 kg x 9.8 m/s/s = 686 Newtons
Common Forces
• Normal Force- support force that is
exerted on an object that is in contact with
another object
• Normal force is always perpendicular to
surface of stable object.
• Example: book laying on table
Common Forces
• Friction- force exerted by a surface as an
object slides across it.
• Usually opposes the motion of object; i.e.
in opposite direction of motion.
Common Forces
• Drag force or Force of air resistancefriction due to an object moving through air
• Opposite direction of motion
Common Forces
• Tension- force transmitted through a
string, rope, cable, or wire when it is pulled
tight by forces acting on opposite ends.
• Directed along the length of the wire.
Common Forces
• Applied force- force that is applied to an
object by a person or another object.
– Examples: kicking a soccer ball, lifting a box,
pulling a rope.
Free-body diagrams
Free-body
diagrams are
used to show the
relative
magnitude and
direction of all
forces acting on
an object.
Problem 1
A book is at rest on a table top. Diagram the
forces acting on the book.
Problem 1
Problem 2
An egg is free-falling from a nest in a tree.
Neglect air resistance. Draw a free-body
diagram showing the forces involved.
Problem 2
Problem 3
A flying squirrel is gliding from a tree to the
ground at constant velocity. Consider air
resistance. A free body diagram for this
situation looks like…
Problem 3
.
Problem 4
A rightward force is applied to a book in
order to move it across a desk. Consider
frictional forces. Neglect air resistance.
Construct a free-body diagram.
Note the applied force arrow pointing
to the right. Notice how friction
force points in the opposite
direction. Finally, there is still
gravity and normal forces involved.
Problem 5
A skydiver is descending with a constant
velocity. Consider air resistance. Draw a
free-body diagram.
Problem 6
A man drags a sled across loosely packed
snow with a rightward acceleration. Draw
a free-body diagram.
Problem 7
A football is moving upwards toward its peak
after having been booted by the punter.
Draw a free-body diagram. Ignore air
resistance.
The force of
gravity is the
only force
described. It is
not a windy
day (no air
resistance).
Problem 8
A car runs out of gas and is coasting down a
hill.
Even though the
car is coasting
down the hill,
there is still the
dragging friction
of the road (left
pointing arrow)
as well as gravity
and normal
forces.
Net Force
Now let’s take a look at what happens when
unbalanced forces do not become
completely balanced (or cancelled) by
other individual forces.
An unbalanced forces exists when the
vertical and horizontal forces do not cancel
each other out.
Example 1
Notice the upward
force of 1200
Newtons (N) is
more than gravity
(800 N). The net
force is 400 N up.
Example 2
Notice that while the normal force and gravitation
forces are balanced (each are 50 N) the force of
friction results in unbalanced force on the
horizontal axis. The net force is 20 N left.
Unbalanced Forces
• Unbalanced forces cause an object to
accelerate (change direction, change
speed, or both)
Review: Forces are Balanced
Object at Rest
V = zero m/s
a = 0 m/s2
Stay at Rest
Objects in Motion
V ≠ zero m/s
a = 0 m/s2
Stay in Motion
(same speed
and direction
Balanced or Unbalanced?
Calculate Net Force: