Workshop 5 Web Highlights
When Katie places the pencil between her hands and presses them
together, equal forces are placed on both her palms. If we had used
an unsharpened pencil she would probably say they felt about the
same. She doesn't feel the forces but rather senses the pressure
on the nerve endings in her hand. The point of the pencil has an
area about fifty times smaller than that of the eraser end. Since
pressure is the force divided by the area over which the force acts,
Katie feels fifty times the pressure and some pain on the palm that
is pushed against the point of the pencil.
We have seen the effect of pressure before in the series when we
dropped a raw egg on sand and hard surfaces. Besides the big difference
in the time to bring the egg to rest, when the egg hit the sand,
the force was spread over a larger area reducing the pressure on
the shell. When we check the air pressure in our tires, we measure
the pressure in pounds per square inch (psi). Scientists measure
pressure in the unit Pascal (P). A Pascal is equal to a force of
one Newton acting on a surface of one square meter.
We showed a ball rolling along the very flat surface going part
way up and back down the wooden ramp and continuing back on the
blue surface. The purpose of this demonstration was to show that
the ball moves in a straight line unless a force acts to change
its motion. It does move in a straight line before it reaches and
after it leaves the ramp.
The demonstration also shows something else. There is considerably
more friction on the ramp than on the flat blue surface. You can
even hear the difference in the sound the ball makes as the ball
rolls along. Objects will move in straight lines at constant speed
unless a force acts to change their motion. The force acting down
the ramp changes the direction of the ball and the frictional force
slows the ball down slightly so that it moves more slowly after
it leaves the ramp. This is why the spacing of our dots is smaller
at the end of the ball's journey.
We looked at the forces acting on the coffee filter dropped through
air and the marble falling through the dishwashing liquid and saw
that when the forces were balanced they fell at a constant speed.
They did have to accelerate first to arrive at that final constant
When the objects start to fall gravity acts on them. Frictional
force of the fluid also acts on the falling objects, in the opposite
direction, slowly at first but increasing with the speed of the
objects until the forces are equal. The final speed that falling
objects reach is called terminal velocity.
The fluid friction depends on several things including the shape
of the object, its frontal area and the properties of the fluid
it is moving through. If the fluid flows smoothly over the object
as in the case of the marble the force is directly related to the
speed but in the case of the filter or a parachute that disturb
the air the force increased more quickly.