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C. Some of the following are correct descriptions of types of forces that could
be added on as new branches of the classification tree. Others are not really
types of forces, and still others are not force phenomena at all. In each case,
decide what’s going on, and if appropriate, figure out how you would incorpo-
rate them into the tree.
sticky force........makes tape stick to things
opposite force...the force that Newton’s third law says relates to every force
you make
flowing force......the force that water carries with it as it flows out of a hose
surface tension..lets insects walk on water
horizontal force.a force that is horizontal
motor force........the force that a motor makes on the thing it is turning
canceled force...a force that is being canceled out by some other force
5.3Analysis of Forces
Newton’s first and second laws deal with the total of all the forces
exerted on a specific object, so it is very important to be able to figure out
what forces there are. Once you have focused your attention on one object
and listed the forces on it, it is also helpful to describe all the corresponding
forces that must exist according to Newton’s third law. We refer to this as
“analyzing the forces” in which the object participates.
Example
A barge is being pulled along a canal by teams of horses on the shores. Analyze all the forces in which the
barge participates.
force acting on bargeforce related to it by Newton’s third law
ropes’ forward normal forces on bargebarge’s backward normal force on ropes
water’s backward fluid friction force on bargebarge’s forward fluid friction force on water
planet earth’s downward gravitational force on bargebarge’s upward gravitational force on earth
water’s upward “floating” force on bargebarge’s downward “floating” force on water
Here I’ve used the word “floating” force as an example of a sensible invented term for a type of force not
classified on the tree in the previous section. A more formal technical term would be “hydrostatic force.”
Note how the pairs of forces are all structured as “A’s force on B, B’s force on A”: ropes on barge and barge
on ropes; water on barge and barge on water. Because all the forces in the left column are forces acting on
the barge, all the forces in the right column are forces being exerted by the barge, which is why each entry in
the column begins with “barge.”
Often you may be unsure whether you have forgotten one of the forces.
Here are three strategies for checking your list:
(1)See what physical result would come from the forces you’ve found so
far. Suppose, for instance, that you’d forgotten the “floating” force on the
barge in the example above. Looking at the forces you’d found, you would
have found that there was a downward gravitational force on the barge
which was not canceled by any upward force. The barge isn’t supposed to
sink, so you know you need to find a fourth, upward force.
(2) Another technique for finding missing forces is simply to go
through the list of all the common types of forces and see if any of them
apply.
(3) Make a drawing of the object, and draw a dashed boundary line
around it that separates it from its environment. Look for points on the
boundary where other objects come in contact with your object. This
strategy guarantees that you’ll find every contact force that acts on the
object, although it won’t help you to find non-contact forces.
Section 5.3Analysis of Forces