Download Force Diagrams

Force Diagrams
And Types of Forces
Review
• Force = push or pull.
• Measured in Newtons.
– 1 lb = 4.45 N
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Fnet = ma
a = Fnet / m
Big force = big acceleration
Big mass = small acceleration
An object’s motion is constant UNLESS a force
is applied.
– Constant velocity = NO NET FORCE!
Types of Forces
• Two general classes of force:
– Contact forces
– Action-at-a-distance forces
• Some contact forces
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friction, Ff
tension, Ft
normal force, FN
drag (air resistance), Fd
applied force, Fa
spring force, Fs
• Some action-at-a-distance forces
– gravitational force (weight), Fg or Fw
– electric force, FE
– magnetic force, FM
Weight
• Weight is the force exerted on an object by
gravity.
• Weight = mass x gravity
– Fw = mg
– Always oriented downward
• On Earth, g = 9.81 m/s2
– Weight changes based on location.
Weight of a 50-kg Mass on
Selected Bodies
Location
g (m/s2)
x Earth
Weight (N)
Weight (lb)
Sun
266
27.1
13,300
2980
Mercury
3.73
0.380
185
41.6
Venus
8.90
0.907
445
99.9
Earth
9.81
1.00
491
110.
Moon
1.62
0.165
81.4
18.2
Mars
3.70
0.377
185
41.5
Jupiter
(cloudtops)
23.3
2.38
1160
261
Saturn
(cloudtops)
10.4
1.06
522
117
Pluto
0.651
0.0664
32.8
7.30
Normal Force
• Normal force – exerted by a surface
against an object resting on that surface.
– Always perpendicular to surface.
– “Normal” means “perpendicular”.
FN
FN
Fw
Fw
Normal Force
• On a horiz. surface, FN = Fw
• On a slanted surface, FN = Fw cos = mg cos
–  = angle of incline
• What is the weight and normal force for a box sitting on a
15º incline if the box’s mass is 150. kg?
– Fw = mg
• Fw = (150. kg)(9.81 m/s2)
• Fw = 1470 N
– FN = Fw cos
• FN = (1470 N)(cos15º)
• FN = 1420 N
Weight vs. Normal Force
FN

Fw = mg
Force Diagram
• AKA free-body diagram.
• Shows all forces acting on an object.
– If you can’t draw, don’t worry!
– Usu. represent objects as boxes or circles for
simplicity.
• Forces drawn as vector arrows.
– Bigger arrows for bigger forces.
– Only draw forces that are known or requested.
– Other forces may exist but aren’t important to the
problem.
• Label force arrows.
Draw a Force Diagram
• A box is resting on a horizontal surface.
• A box is sliding at constant speed across a
horizontal surface.
• A box is hanging from a cable.
• A box is hanging from two cables that form an
angle of 45º.
• A box is being pulled along a horizontal plane by
a rope that forms an angle of 45º with the floor.
• A box is sliding down a frictionless ramp.
• A box is sliding down a ramp with friction.