Net force on box
Tension minus weight
Causes acceleration5.14 - Free-body diagrams
Free-body diagram: A
drawing of the external
forces exerted on an object.
Free-body diagrams are used to display multiple
forces acting on an object. In the drawing above, the
free body is a monkey, and the free-body diagram in
Concept 1 shows the forces acting upon the monkey:
the tension forces of the two ropes and the force of
gravity.
The diagram only shows the external forces acting on the monkey. There are other
forces present in this configuration, such as forces within the monkey, and forces that
the monkey exerts. Those forces are not shown; a free-body diagram shows just the
forces that act on a single object like the monkey.
Although we often draw force vectors where they are applied to an object, in free-body
diagrams it is useful to draw the vectors starting from a single point, typically the origin.
This allows the components of the vectors to be more easily analyzed. You see this in
Concept 1.
Free-body diagrams are useful in a variety of ways. They can be used to determine the
magnitudes of forces. For instance, if the mass of the monkey and the orientations of
the ropes are known, the tension in each rope can be determined.
When forces act along multiple dimensions, the forces and the resulting acceleration
need to be considered independently in each dimension. In the illustration, the monkey
is stationary, hanging from two ropes. Since there is no vertical acceleration, there is no
net force in the vertical dimension. This means the downward force of gravity on the
monkey must equal the upward pull of the ropes.
The two ropes also pull horizontally (along the x axis). Because the monkey is not
accelerating horizontally, these horizontal forces must balance as well. By considering
the forces acting in both the horizontal and vertical directions, the tensions of the ropes
can be determined.
In Example 1, one of the forces shown is friction, f. Friction acts to oppose motion when
two objects are in contact.A monkey hanging from two ropes.Free-body diagrams
Shows all external forces acting on body
Often drawn from the originDraw a free-body diagram of the
forces on the box.
(^98) Copyright 2007 Kinetic Books Co. Chapter 05