Forces behind forces
Chemical structures,
which are essentially
combinations of
electromagnetic
attractions and
repulsions, give solid
objects their intrinsic
“springiness” and
“stickiness.”
They can push, adhere
and drag.
11/7/14
Oregon State University PH 211, Class #18
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Common Forces
With the exception of gravity, most forces that we commonly
encounter appear* to result from contact between two bodies:
Pushing perpendicular to a surface
(called a normal force)
Pushing parallel to a surface
(called a friction force)
Pulling via contact (called a tension force)
Pulling via gravity (called the gravitational force) is
one common force that doesn’t require such
“contact.” It is most clearly “force at a distance.”
*Actually, all forces act at a distance; “contact” has no meaning at the atomic/subatomic
level.
11/7/14
Oregon State University PH 211, Class #18
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Normal Force
A normal force is a push on an object by a surface—a push
directed perpendicular to that surface. (“Normal” means
“perpendicular.”)
Notice: The surface exerts the force. Normal forces often
arise when objects are stacked or pushed together—
“reaction” forces.
Caution: A normal force is not always up-or-down—and it
is not always equal in magnitude to an object’s weight.
11/7/14
Oregon State University PH 211, Class #18
3
An unknown force is pushing horizontally on Block A (10.0
kg). Block B (15.0 kg), in turn, is being pushed by block A.
Both blocks are sliding together across a frictionless
horizontal surface in the positive x-direction with an
acceleration of 3.00 m/s2.
Find the normal force FBA.
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1.
45.0 N
2.
–45.0 N
3.
30.0 N
4.
–30.0 N
5.
None of the above.
Oregon State University PH 211, Class #18
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Static Friction Force
A friction force is a “drag” force on an object by a
surface—directed parallel to that surface. There are three
kinds of friction forces: static, kinetic and rolling.
Static friction is a force that prevents a stationary object
from beginning to move (slide) along a surface in response
to another force. Thus, the magnitude of the static friction
force, FS, will vary (depending on the force it is reacting
to)—up to a maximum, FSmax. This maximum is different
for each object and surface and is proportional to the normal
force exerted on the object by the surface: FSmax = SFN.
The coefficient of static friction, S, is a unitless number
(often less than 1) that describes how the maximum static
friction force compares to the normal force.
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Oregon State University PH 211, Class #18
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Example: A 40-kg block sits at rest on a horizontal surface.
The coefficient of static friction between the surface and the
block is 0.25. What is the magnitude of the frictional force
acting on the block?
11/7/14
1.
0N
2.
10 N
3.
98 N
4.
392 N
5.
None of the above.
Oregon State University PH 211, Class #18
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Kinetic Friction
When any object is already sliding along any surface, there
is a certain amount of “drag” force—resistance to that
motion. This is kinetic friction force (and it’s usually less
than the maximum static friction force; thus it’s harder to
get an object started in a sliding motion than it is to
maintain that motion.) Notice: The direction of the friction
force always opposes the surfaces’ sliding motions.
For most reasonable sliding speeds, the magnitude of the
kinetic friction force does not depend on that speed. It is
proportional only to the normal force exerted by the surface
on the object: FK = KFN The coefficient of kinetic friction,
K, is a unitless number (often less than 1) that describes
how the kinetic friction force compares to the normal force.
11/7/14
Oregon State University PH 211, Class #18
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11/7/14
Oregon State University PH 211, Class #18
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