Form and Function: Part 4 – External and Internal Forces Mr. Williams – Grade 7 Science

External and Internal Forces: There are two types of forces that designers need to consider when designing structures. They are external and internal forces. – External Forces – Forces that act on a structure from the outside – Internal Forces – Forces that act between two different parts of a structure If all possible forces are not considered, structures, particularly buildings can collapse.

External Forces The most obvious external force is gravity. On Earth, gravity always acts downward and is a non- contact force. Other external forces include: earthquakes, wind, the weight of people on floors, the weight of a building itself. Earthquakes can be disastrous

Application of Force When we need to move objects we apply an external force to them. How efficiently we are able to move the object depends on where we place our force and the direction of the force. These factors can be understood as the point of application and the plane of application. Point of application – the location on an object where an external force is applied of concentrated Plane of application – an imaginary flat surface through which an applied force passes.

Application of Force Cont’d… Point of application (Where the filing cabinet is being pushed) Plane of application (An imaginary flat surface that shows the direction of the external force

Application of force Cont’d… Remember earlier when we talked about magnitude and direction of forces?!? When pushing the filing cabinet in the previous slide we can change the amount of force we push with (magnitude) and the direction we push it. However, the point of application is important! If we put our point of application lower on the filing cabinet it will most likely slide across the floor. If we move the point of application higher it will most likely tip over.

Application of force Cont’d… Another important external force is the force that acts OPPOSITE to gravity. Think about it: Gravity is a force pulling you downward. So why are you not moving closer to the center of the Earth? The reason is because the magnitude of the force pushing downward (gravity) is equal to the force pushing upward (the chair or stool you are sitting on). That’s why you are able to sit still. So give your chair a big hug the next time you see it!

Internal Forces Internal forces can be broken down into four main categories Tension Compression Torsion Shearing

Tension Tension – An internal force that pulls the particles of an object apart. For example: pulling an elastic band apart is an external force, but it causes and internal force, pulling the elastic bands’ particles in opposite directions. Other examples include guitar strings, a trampoline, stretched rope, and suspension bridge cables.

Compression Compression – An internal force that presses or squeezes the particles of an object together. For example: The springs in a mattress compress together when you lie down on the bed. Objects, when compressed, usually return back to their original shape once the external force is removed. Other examples include kicking a soccer ball and stepping with your shoes.

Torsion Torsion – Internal twisting forces created in an object as a result of a twisting motion being applied to the object. For example: When a figure skater jumps and turns. Other examples include turning a doorknob or a washcloth being rinsed out. For torsion to occur, both ends of an object can turn in the same direction, or only one end may turn and the other stays stationary.

Shearing Shearing – Forces acting on an object as a result of pushes or pulls in opposite directions. Usually results in rips or tears in the object. For example: A strong wind being blown against trees. Other examples include scissors cutting paper in half. When shearing occurs the object is usually torn apart, bent or cut.

What Internal Forces are Acting Below?

Forces Review The forces that act on a structure from the outside Tension Compression External Forces Shearing Torsion Internal Forces Internal twisting of the object from twisting forces Pulling the particles of an object apart Pressing the particles of an object together Ripping or tearing of an object when forces pull or push in opposite directions