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

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.

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).

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).

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.

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.

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.

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.

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.

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.

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.

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

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.