MECHANICS

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Definition

The behavior of physical objects in relation to eachother, particularly when a force is applied. Mechanics is applied physics.

Robotics Application

How to make physical objects interact, is the basis of the FIRST challenges. The challenge may include lifting, tossing or kicking an object, such as a ball. It may include moving the robot over or under challenges. The team uses mechanical devices to apply the properties of physics to successfully achieve these goals.

In robotics, mechanics primarilly deals with two main applications. First is structure. This is the framework upon which all other parts are attached. Deciding a robot's structure has to consider the physical factors of the materials, such as weight, strength, flexibility, etc. and how any movement will cause strain on those materials (both incidentally and on purpose).

The second area is motion. Many mechanical devices allow or create motion when a force is applied. For example a wheel and axle, is designed to roll and move itself and anything attached to it, a hinge is a device designed to allow a change in position of the objects attached to it, there are also gears, levers and pulleys, etc.

There are many physics properties that have to be taken into account when designing and building a robot. One is center of mass, the center point of the mass of an object. This does not have to be the center of the height, width and length of the robot and usually isn't. In simplest terms it is a function of height and weight. For example, you can lower the center of mass in two objects of the same height, by putting heavier parts nearer the bottom.

Another property is friction. When parts move next to eachother, they 'grab' eachother. This can create heat and slows down motion. In robotics, you often need to both increase and decrease friction. For instance, you add grease or bearings to a wheel and axle to decrease friction and allow quicker movement, but you may need to add friction to the surface of a wheel to provide grip or traction to manuever over and around obstacles.

Also, when parts move, it can put a force on the structure, causing stress or strain. Think of how a swing in motion, pulls the support bars back and forth, which is why swing frames are often triangles. Stress can create tension. Tension can be useful by creating a controlled force. For example, if you stretch a rubber band and let it go, it will snap back into shape and create a force that can create motion. Springs do the same thing.

Vocabulary

Axle

A shaft that supports a wheel, pulley or lever so it can spin around it.

Mass

The quantity of matter in an object. Often expressed as a weight or gravitational pull. Changes with the speed of an object.

Center of mass

This is the central point in an object where if it could be suspended, it would pivot, making mass equal above and below, right and left, front and back, etc.

Force

A push or pull or other influence that causes a change in movement, shape, etc. of an object or group of objects.

Friction

The resistance to movement when one object rubs or slides along another.

Fulcrum

An object that supports a lever, creating a pivot point.

Gear

A wheel like object with teeth (parts that stick out from an edge) that mesh with another gear to transfer motion.

Hinge

A joint between two parts that allows a bend or partial rotation of one or both parts.

Lever

A bar that pivots at a specific point when force is applied at one end, it creates a force at the other.

Motion

To change position.

Pivot

To rotate about a specific point.

Pulley

A special wheel and axle. It has a groove in the edge that can hold a rope or wire allowing a lever action.

Stress

A force that produces a strain on an object causing it to change shape.

Structure

The organization and arrangement of parts into one unit.

Tension

Stretching or straining an elastic object.

Activities

If you wish to experiment with mechanics you can download these instruction sheets:

about structure:   bridge challenge 1   bridge challenge 2
about center of gravity:   balancing jet
about friction:   friction test
about motion:   wheelie    gears    catapult

Also here is a link to a game about the interactions of physics and mechanics:   Armadillo Run