3.4 Mechanics and materials

Question 1

What is friction?

Answer 1

• Resistance to motion between and object and a surface or an object moving through a fluid
• Friction is a force that acts in the opposite direction to the movement

Question 2

What is lift? What causes it?

Answer 2

• An upward force which acts on objects travelling in a fluid
• Caused by the object creating a change in direction of fluid flow and acts perpendicular to the direction of fluid flow

Question 3

What does Hooke’s law state?

Answer 3

• Extension is directly proportional to the force applied up to the limit of proportionality

Question 4

What is the breaking stress?

Answer 4

Minimum stress needed to break a material

Question 5

Area under a force - extension graph?

Answer 5

Work done to deform the material

Question 6

stress? strain?

Answer 6

NO. Tensile stress and tensile strain

Question 7

Why are loading and unloading lines parallel on force-extension graph for plastically deformed materials?

Answer 7

• Plastic deformation has produced permanent extension
• There has been re-alignment of bonds in materials hence the intercept is non-zero
• But the stiffness constant (k) hasn’t changed, as the forces between the atoms is the same when loading and unloading

Question 8

Highest point on a stress-strain graph?

Answer 8

Ultimate tensile stress

Question 9

What is meant by ductile?

Answer 9

Can undergo a large amount of plastic deformation before fracturing

Question 10

3 ways of describing the centre of mass

Answer 10

1. the point through which the line of action of a force has no torque
2. where the mass of the body can be considered to be concentrated
3. where the weight can be considered to act

Question 11

What is meant when a material is described as brittle?

Answer 11

• It doesn’t deform plastically
• Instead, it breaks when the stress reaches a certain value

Question 12

2 features of cars that make them safer in crashes

Answer 12

• Crumple zones deform plastically in a crash using the car’s kinetic energy so less is transferred to the passenger’s
• Seat belts stretch to convert the passenger’s kinetic energy into elastic strain energy

Question 13

What is a moment?

Answer 13

A turning force: the product of a force and the perpendicular distance from the point to the line of action of the force

Question 14

What is meant by terminal velocity?

Answer 14

When the forces acting on a falling object are balanced, the resultant force is zero, so acceleration becomes zero, and object moves at maximum velocity

Question 15

What is Young’s modulus?

Answer 15

tensile stress / tensile strain

Question 16

How is the work done to stretch or compress a material elastically stored?

Answer 16

Elastic strain energy

Question 17

Why isn’t all work done stored as elastic strain energy when a stretch is plastic?

Answer 17

Work is done to move the atoms apart, so energy is dissipated as heat, instead of stored as strain energy

Question 18

Explain when a triangle of forces can be used

Answer 18

• When there are 3 forces acting on an object, and the object is in equilibrium
• so the forces sum to zero
• hence the vectors can be joined to form a closed triangle

Question 19

What is tensile stress?

Answer 19

Force exerted per unit cross-sectional area

Question 20

What is the yield point?

Answer 20

The stress at which a large amount of plastic deformation takes place with a constant or reduced load

Question 21

Area between loading and unloading lines

Answer 21

Work done to permanently deform the wire