Science 7 - Unit #4 - TOPIC 5 – HOW STRUCTURES FAIL
1. What is a lever? What is a fulcrum? P. 315
A lever is a device that can change the amount of force needed to move an object. A fulcrum is
the pivot point of a lever.
Examples: shovel, teeter totter, a crow bar
2. Describe three ways materials can fail, and identify the type of internal
forces that causes each kind of failure. P. 315-316
a)SHEAR: compression and shearing. One section of land may slide over another section
along a weakness/crack.
Example: Rock slide or soil sliding underneath a building.
b) BEND OR BUCKLE: compression and bending. Compressive forces cause material to
bend.
Example: squishing a piece of paper.
c)TORSION: torsion and shearing. Often materials will shear when they are twisted.
Example: twisting dry spaghetti and plastic cutlery
4. Which type of material failure occurs when you: WHY?
a) Leave a trail of footprints on the carpet?
BEND OR BUCKLE. Carpet is compressing/bending and it leaves the material deformed.
b) Sprain your ankle in a soccer game?
TORSION. Your ankle twists when you sprain it.
c) Accidentally hit a baseball through a window?
**BEND OR BUCKLE. The baseball compresses the glass and causes it to break.
d) Crinkle a new $5 bill as you stuff it into your pocket?
BEND OR BUCKLE. The $5 bill gets squished as you put into your pocket.
e) Twist the lid of a partly opened tin can back and forth until it breaks off?
TORSION. Twisting the lid back and forth causes it to break off.
5. What is metal fatigue and how can it affect a structure? P.318
Metal Fatigue is when a metal object weakens after being bent or twisted many times. It affects
a structure by weakening certain materials and may potentially cause the structure to fail.
Example: twisting a tab on a pop can back and forth until it breaks off.