Friction Blocks
static friction - dynamic friction - forces in equilibrium - components of forces
What it shows:
There are actually three (or more) demonstrations under the wing of this title: (1) A block or box is
placed on an adjustable inclined plane which can be raised until the block slides. The tangent of the
angle then gives the coefficient of friction between the surfaces. (2) The difference between
dynamic and static friction may be demonstrated. A large dial spring balance is used to determine the
forces required to drag a box on the floor. (3) Rectangular wooden blocks can be pulled across the
lecture table with a dial scale. The block may be turned on different edges to show the independence
of frictional forces on area. Similar blocks with various types of surfaces can be used to show the
effects of different kinds of contact planes.
How it works:
As indicated above, these demonstrations are all variations on the same theme. We have large floor
models, lecture bench-top versions, and the Klinger™ Blackboard Mechanics set to choose from.
Choose the most appropriate for your needs, whether they be qualitative or quantitative or something
else. Here is a quick overview.
(1) The adjustable inclined plane is a large wooden floor model measuring 240 cm long and 34 cm wide.
It can be set at any angle between 0 and 21°. The surface is covered with Formica™
and is quite smooth. Large wood blocks or cardboard boxes (filled with stuff) can be used as the sliding
objects.
(2) Here we typically use a very large (up to a meter3) cardboard box with one or two
lead bricks placed on the bottom for weight. A 100 N dial spring-scale
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is attached to the box via some rope and the box is dragged across the floor by pulling on the scale.
The audience notes the different scale readings before and after the box begins to slide.
(3) On a smaller scale to (2) above, small wood blocks (around 10×10×30 cm) can be
dragged on the Formica™-covered lecture bench-top using a 20 N scale.
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Additionally, by flipping the block over onto one of its edges, one can demonstrate that the frictional force
between two sliding surfaces is independent of the area of contact. Further, by stacking identical blocks
on top of the sliding block, the spring-scale indicates that the frictional force is a function of the normal
force applied by the sliding object to the plane surface. And finally, to demonstrate the dependence of
the coefficient of friction (both static and dynamic) on the nature of the sliding surfaces, sandpaper and
rubber have been glued onto some of the surfaces.
Comments:
A bit of a drag. Rating *
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Sargent-Welch cat. no. S-3775-30K
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Sargent-Welch cat. no. 4091