Lesson: Fairly Fundamental Facts about Forces and StructuresContributed by: K-12 Outreach Office, Worcester Polytechnic Institute
Educational Standards :
Learning Objectives (Return to Contents)
After this lesson, students should be able to:
Introduction/Motivation (Return to Contents)
Everyone knows from experience that a force is a pushing or a pulling action that moves, or tries to move, an object. Engineers design structures, such as buildings, dams, planes and bicycle frames, to hold up weight and withstand forces that are placed on them. An engineer's job is to first determine the loads or external forces that are acting on a structure. Whenever external forces are applied to a structure, internal stresses (internal forces) develop inside the materials that resist the outside forces and fight to hold the structure together. Once engineers know what loads will be acting on a structure, they calculate the resulting internal stresses, and design each structural member (piece of the structure) so it is strong enough to carry the loads without breaking (or even coming close to breaking).
Lesson Background & Concepts for Teachers (Return to Contents)
The five types of loads that can act on a structure are tension, compression, shear, bending and torsion.
(For example, pulling on two pieces of wood that have been glued together - the glue joint is "being subjected to a shear loading").
A Moment of a Force
Before you can understand the last two types of loads, you need to understand the idea of a moment of a force. A moment is a "turning force" caused by a force acting on an object at some distance from a fixed point. Consider the diving board sketch in Figure 5. The heavier the person, and the farther s/he walks out on the board, the greater the "turning force," which acts on the concrete foundation.
The force (F) produces a moment or "turning force" (M) that tries to rotate the diving board around a fixed point (A). In this case, the moment bends the diving board.
The stronger the force, and the greater the distance at which it acts, the larger the moment or "turning force" it will produce.
A moment or "turning force" (M) is calculated by multiplying a force (F) by its moment arm (d). The moment arm is the distance at which the force is applied, taken from the fixed point:
(As long as the force acting on the object is perpendicular to the object)
If you have a force measured in Newtons multiplied by a distance in meters, then your units for the moment are N-m, read "Newton-meters." If your force is measured in pounds and you multiply it by a distance given in inches, then your units will be lb-in., read "pound-inches." The units for moments can be any force unit multiplied by any distance unit.
Associated Activities (Return to Contents)
Assessment (Return to Contents)
Questions: Evaluate students' understanding of the material, individually or as a group, using the Investigating Questions provided in the associated activity.
Problem 1: Calculate the moment resulting when a person weighing 150 lbf stands at the end of a 120 in. diving board (use the moment equation: M = F x d) (Answer: 18,000 lbf-in.)
Problem 2: If 1 N = 0.2248 lbf and 1m = 3.28 ft, convert the units in the previous problem to obtain a solution in Nm (Answr: 18,000 lbf-in. x 1 N / 0.2248 lbf x 1 ft / 12 in. x 1 m / 3.28 ft = . 203 Nm)
ContributorsDouglas Prime, Tufts University, Center for Engineering Educational Outreach
Copyright© 2013 by Regents of the University of Colorado; original © 2005 Worcester Polytechnic Institute
Supporting Program (Return to Contents)K-12 Outreach Office, Worcester Polytechnic Institute
Last Modified: September 1, 2014