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Everyday Forces
•Weight- Fg
•Normal Force-Fn
•Friction-Ff
Normal Force-Fn
• Force exerted by one object on
another in a direction
perpendicular to the surface of
contact.
The normal force
opposes gravity,
typically.
Fn
Fn=mg
Fg
Fg=mg
Force of Friction
• The amount of frictional force between two
objects is proportional to the normal force.
Normal force
Friction Force
Applied Force
Force of gravity
• The amount of force required to move an
object starting from rest is usually greater
than the force required to keep it moving at
constant velocity once it is started.
• Friction is also dependent on the type of
surface (tile vs. carpet), and the weight of
the object (heavier objects, more friction).
Coefficient of Friction
• MU or  is the coefficient of friction
• = Force required to slide object on surface at constant speed
normal force (or weight)
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Since we are at constant speed, Ff=Fapplied
= Sliding Force (Ff)/Fn
No unit, just a ratio
Should be between 0 and 1.
Coefficient of Friction
Calculations
• = Force required to slide object on surface at constant speed
normal force (or weight)
• = Ff/Fn
• Example: Find the coefficient of friction that is present
when a 600. N ice skater uses 30.0 N of force to push
off with each of his strides to maintain a constant
velocity.
• Answer: =30.0 N / 600. N = 0.0500
Coefficient of Friction
Calculations
• = Force required to slide object on surface at constant speed
normal force (or weight)
• = Ff/Fn
• Example: A 90. kg crate can be pushed along the floor
at a constant speed with a force of 78 N. What is the
coefficient of friction between the two?
= Ff/Fn= 78 N/ (90 kg)(9.8m/s2)= .088
Coefficient of Friction
Calculations
• = Force required to slide object on surface at constant speed
normal force (or weight)
• = Ff/Fn
• Example: How much force is needed to keep a 88.0 kg
sofa moving across a floor at a constant speed if the
coefficient of friction between the two surfaces is 0.220?
0.220=F/(88.0kg)(9.8m/s2)
F=190 N