The document explains key concepts of mass and weight, defining mass as the amount of matter in an object and weight as the force of gravity acting on it. It outlines how to calculate weight using the formula fg = mass * g, and discusses the types of forces acting on structures, including external and internal forces, as well as various internal force types such as tension, compression, shear, torsion, and bending. Additionally, it notes the strengths of different materials when faced with these forces during structure design.

1. Science 7 – Unit D:
Structures and Forces
Topic 3-4: Mass and Forces;
Forces, Loads, and Stresses.

2. What is Mass?
 Mass: amount of matter in an object. The
mass of an object never changes unless
you remove part of the object.
 The standard unit for mass is the
kilogram (kg). Smaller masses are
measured in grams (g) or milligrams (mg)
 Note: When you are trying to lose weight,
you are actually talking about losing mass.

3. What is Weight?
 Weight: is the amount of force you are
pulled with by the Earth. Note that your
weight can change depending on where
you are.
 You can be weightless while in space, but
you can never be massless.
 While we talk about weight being
measured in pounds, in actuality should
be measured in newtons (N).

4. How can we calculate weight?
 To find your weight on Earth, you must
use the following formula:
 Fg = mass * g
 where g=9.8 N/kg. This is the gravity of
the Earth. On a larger planet its g will be
larger, and on the moon it will be smaller
 Fg stands for force of gravity and means
the same thing as weight.
 Remember that the unit for weight (and
all forces) is the newton (N).

5. How is a kilogram determined?
 A solid block of
platinum metal of
mass of 1 kg is kept in
special conditions in a
vault in Paris.
 Every few years it
must be replaced
because all matter
gives off particles in
the form of radiation.

6. How was Weight and gravity discovered?
 In 1688, Sir Isaac
Newton wondered
why the moon orbited
the Earth. Seeing an
apple fall to the
ground, he figured out
that the same force
that kept the moon
orbiting pulled.
 Newton then figured
that this force is tied
to all matter in the
universe.

7. Force Meter
 Force Meter: device
that measures the
amount of force is
being used or the
weight of an object in
newtons. It usually
uses springs.

8. Force Diagram
 Force Diagram:
simple picture that
uses arrows to show
the strength and
direction of one or
more forces. A longer
arrow means a
stronger force, and
the direction the
arrow is pointing
indicates the force’s
direction.

9. Types of Forces acting on Structures
 There are two main types of forces acting on
structures: External and Internal.
 1. External Forces: are stresses that act on a
structure from outside the structure. Eg. winds,
heavy snow, earthquakes, etc.
 2. Internal Forces: are forces or stresses within
a structure’s materials or joints. Internal forces
usually come from external forces.
 Deformation: the changing of the shape or size
of a structure as a result of internal forces.

10. Types of Internal Forces
 1. Tension Force: stretching force: a
material is being pulled apart.
 Tensile Strength: measures the largest
tension force the material can take before
failing.
 2. Compression Force: crushing force: a
material is squished.
 Compressive Strength: measures the
largest compression force the material can
take before failing.

11. Types of Internal Forces Cont’d
 3. Shear Force:
Sliding or tearing
force when different
parts of a material or
component is
pulled/pushed in
different directions at
the same time.

12. Internal Forces Cont’d
 4. Torsion Force:
twisting force. The
ends of a component
are twisted in opposite
directions. When you
wring out a towel you
are applying a torsion
force.
 Torsion is shown in
the second-from-the-
right figure.
 5. Bending is a
combination of shear
and torsion

13. Resisting Stress – Which Materials to
Use?
 Concrete has a large compressive strength
and therefore is an excellent material if
you’re worried about compression forces.
However, it does not have a high tensile
strength.
 Most metals have a large tensile strength,
but a weaker compressive strength.
 You must be aware of all internal and
external forces your structure will face
when designing it.