Topic 5

Question 1

What is a lever?

Answer 1

Rigid bars that can rotate around a fixed point called a pivot or fulcrum.

Question 2

What does a lever do?

Answer 2

Reduce the force or effort needed to carry out a task, such as hitting a baseball,
opening a bottle, pulling a nail and cutting paper.

Question 3

How do structures fail?

Answer 3

• External forces can cause internal forces to a structure
• Each type of internal force can cause certain types of damage

Question 4

What is shear failure?

Answer 4

■ Materials almost always have microscopic
cracks or weaknesses
■ When one section is compressed it
may cause these cracks to enlarge or
break apart
■ One section may shear
■ Often seen in soil or unstable ground

Question 5

What is bend/buckle failure?

Answer 5

■ Compression forces cause the material to bend on the inside
of the curve and pull or snap on the outside

Question 6

What is torsion failure?

Answer 6

■ Brittle structures like dry spaghetti
and plastic cutlery often shear when
twisted
■ Very flexible structures shear less
easily
■ They will twist into tangles and knots
though
■ Structure is not broken it has lost it’s
shape, a form of failure

Question 7

How can we use shear, tension, and buckle stress to our advantage?

Answer 7

Shear: ■ Shear pins are used in situations
where rotational motion may cause
damage to a machine. Instead of sending the force of the spinning propeller through the motor and damaging it, the force dissipates by the shear pin breaking
Buckle:■ Car bumpers and sheet metal can be designed to buckle in a collision
■ As the metal deforms it absorbs some of the energy
■ Car may be damaged; passengers are much less likely to be seriously injured
Tension: ■ Spinning wheels twist cotton or wool
fibres tightly together
■ Fibres lock together
■ The twisted yarn is must stronger than
a bundle of straight fibres
■ If fibres are twisted too much they
tangle and shorten
■ Controlled twisting creates braids,
ropes and cables

Question 8

What is metal fatigue?

Answer 8

■ Metals weaken when they are bent and
twisted over and over again
■ Can be explained by the particle model
■ Bending and twisting changes the
arrangement of particles
■ Where particles move apart the forces
holding them together weaken
■ Small cracks develop
■ Eventually the structure may fail under a
small stress it could withstand when
new