Download Everyday Forces

Everyday Forces
• Weight
• Normal Force
• Friction Force
Weight is Force of gravity acting
on mass.
Mass is amount of matter.
Weight = Fg = mg.
g = -9.81 m/s2 on Earth.
The direction of weight is straight
toward the center of Earth .
Inertial and Gravitational
Mass
What is the difference between the two?
Nothing!!
Inertial mass - defined by F = ma.
When a force F is applied to an object, it
accelerates proportionally, the constant of
proportion is the mass of that object.
a = F/m.
Gravitational mass has to do
with force of gravity pulling on
an object.
Gravitational mass is measured by
comparing the Fg of an unknown mass
to the Fg of a known mass. This is done
with a balance scale.
Physically, no difference between gravitational and
inertial mass.
Einstein used the fact that gravitational and inertial mass were equal to begin his Theory of General Relativity
in which he postulated that gravitational mass was the same as inertial mass.
Normal Force Fn.
Perpendicular to
surfaces in
contact.
Fn
How does Normal Force Arise?
How does a wall know to push back
harder when I push with increasing
force?
Objects on a surface are repelled by
electromagnetism.
Outer electrons in the object are electrically
repelled by the electrons that at the surface.
The electrons offer a stronger and stronger
repulsive force the closer and closer the object is
moved to the surface.
The more massive, the greater gravity pulls them
onto a surface, & the greater the surface repels the
object.
The object will be at rest only if the surface
can exert an equal & opposite force to
sustain it, otherwise the object crashes
through the surface.
Friction Between Surfaces
Friction Force Ff
Acts in direction opposite motion (or
attempted motion). Which way is the box
below moving?
Left
Friction depends on the nature of
the materials in contact and the
smoothness of their surfaces.
A closer look at friction
Magnified view
of surfaces
At the microscopic level even two smooth surfaces
look bumpy  this is what produces friction
. Friction is independent of surface area.
. Ff is directly proportional the normal
force. Ff = mFn. Where m is a constant.
The ratio of Ff,
to Fn, is
the coefficient of friction, m.
m=
Ff
Fn
m has
no units.
Look at reference table
File Cabinet Demo
m
• m depends on two surfaces in contact
• the type of sliding motion – starting or
moving.
• Does not depend on any other factor.
• It is a constant.
Types of Friction
• Static Friction use mst – Present for objects at
rest. Need to overcome to start objects
moving.
• Kinetic Friction or sliding use mk – Present
for objects in motion. Need to overcome to
keep objects in motion. Always less than
static friction.
Static friction
If we push on an object and it doesn’t move then
the force we exert is less than the max Ff.
push, P
friction, f
This is the static friction force at work.
If I push a little harder, the block may still not
move  the Ff can have any value up
to some maximum.
Kinetic friction
• Keep increasing the Fap, at some point the
block moves  this occurs when the push P
exceeds the maximum static friction force.
• When the block is moving it experiences a
smaller friction force called the kinetic Ff .
• It takes more force to get something moving
than to keep it moving.
• m for kinetic always less than static.
1. A 10-kg rubber box is at rest on a dry horizontal concrete
surface. Find:
•. a. Sketch the free body diagram.
• a.
•
b. the weight of the box.
• b. 98.1-N.
•
c. the normal force on the box.
•
c. 98.1-N
•
d. the force needed to start the box
•
d. 88-N.
•
e. 66.7-N.
moving.
•
e. the force to keep the box moving a
constant velocity.
2: A 60-kg sled is being dragged on a
horizontal snow surface by a force of 20-N
at a constant velocity.
• Sketch the free body diagram.
• What is the friction force on the sled?
3. A 1-kg plastic toy chest is dragged across a
horizontal wooden floor at constant velocity by an
applied force of 5-N.
• What is m, the coefficient of friction?
• Would that be static or kinetic friction?
Fs
Fs  Fk
Max
Fk
Fs  Fk
Static
Region
Kinetic Region
Force Causing the Object to Move
Constant Velocity with applied
force.
Ff
F ap
More weight (Fg) = Higher Fn.
Ff = mFn increases.
On a flat surface, as Fg increases Ff must
increase.
Push down on a book as you slide it across a
table – Fn increases so Ff increases too.
The coefficient of friction, m cannot change, it is
a constant.
Friction Problems 1 wksht
• Hwk Txt Read 141 – 145
• Do pg 145 #2,3 & pg 149 #4, 6
on separate sheet.
Friction Problems Inclines.
Inclined Planes
Fn is perpendicular to surface.
Where is friction force if box is at
rest?
Friction opposes motion or
attempted motion.
What is causing box to want to
slide??
What holds the box in place?
Friction
For a box at rest or constant
velocity motion, Ff = Wll.