Form and Function:Part 3 – Weight and Mass, Load and Classifying StructuresMr. Williams – Grade 7 Science

Distinguishing Mass and Weight

• We often use the terms “mass” and “weight” interchangeably. For example you may ask someone “How much do you weigh?”. In Science however, mass and weight are two very distinct things!

Distinguishing Mass and Weight…Cont’d

• Mass – Is the quantity of matter in an object. Mass is often measured using a balance scale. Mass is often measured with the units gram (g) or kilogram (kg)

• Weight – Is the force of Earth’s gravity acting on an object. It is measured in Newtons (N).

• An objects mass ALWAYS stays the same no matter where you are in the universe. Its weight however can change depending on its location.– For example: A person would have a weight of 900N on Earth, but may only

weight 200N on the moon because of the moons smaller force of gravity– Can you imagine how much fun it would be to play basketball on the moon!– Would you be able to jump higher or less high on the moon?

Loads

• When building structures we have to consider all of the forces that will act on it.

• Load – the force that is acting on a structure.• Loads are classified into two main areas: Static

and Dynamic Loads. • Static loads are loads resulting from the force

of gravity and are always acting downward on the structure.

• Dynamic Loads however, are caused by other forces that act on the structure that are not a result from the forces of gravity.• For Example: Wind blowing against the side of a

bridge or rushing water pushing against the side of a wall.

Loads and their breakdown:

Load forces that act on a structure other than gravity

The weight of the structure itself

The weight of an object that the

structure supports

Dynamic Loads

Live LoadDead Load

Static Loads

Loads Cont’d…

• Lets look more closely at the definitions of static loads:

• Dead Load – The weight of the structure itself

• For example: A desk is made up of metal bars, wood for the top and different screws, bolts and other small connectors. The total weight of all these items is the dead load.

• Live Load – Is the weight of the objects that a structure supports.

• For Example: On a desk there are many books, pencil cases, erasers…etc. These are live loads that the desk is supporting.

Classifying Structures

• Why are some structures similar and others different?

• We are able to group structures together based on their function, but we can also group structures together based on their form.

• We can classify the form of structures into 4 distinct categories:

• Solid• Frame• Shell• Combination

Classifying Structures

• Solid Structures - An object that uses solid construction to support loads.

• For example: Dams, power-poles, marble statues.

• A well made solid structure can last for a very long time as they are often made of strong materials with a large mass.

Classifying Structures

• Frame Structures – A network or skeleton of parts that support a load.

• For example: A goalie net, a spider-web or a human skeleton.

• A frame structure can be very strong if their parts work together to support each other and resist the forces acting on them. A single part of a frame structure can not support loads by itself

Classifying Structures

• Shell Structure – Is a hollow, curved shape that provides high strength and stability

• For example: A bicycle helmet, an egg, a pop can.

• A shell structure is usually very light, but provides a lot of rigidity and strength.

Classifying Structures

A combination structure is made up of more than one type of structure. Ex. Shell and frame, or solid and frame.

Most structures in the world are considered combination structures. In fact, the human body is an excellent example.

How many different types of structures can you classify within the human body?