Mechanics

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 (see vocabulary section for more information). 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. The second area is motion (see vocabulary section for more information). Mechanical devices usually create either some kind of movement within the machine itself or an action applied to an object outside the machine.

There are many physics properties that have to be taken into account when designing and building a robot. One is center of mass. This is not the center of the height, width and length of the robot, but instead is the balance point. 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 touch, they ‘grab’ eachother. In robotics, you may 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 applies a force on the structure, causing strain. Think of how a swing in motion, pulls the support bars back and forth as the swing moves. Stress or strain can be made and used on purpose. For example, if you stretch a rubber band, it will snap back into shape. This tension can be used to create a motion. Springs use compression to do the same thing. This is called elastic potential energy.

To learn more about forces, inertia and other physics principles use links below: The Physics Department-Mechanics, Forces at Zona Land Education Website
Rice University (Houston TX) Basic Mechanics page for Elec 201 class

A machine is made up of parts designed to transfer a force from one place to another, maybe changing the direction or making the force bigger or smaller or delaying when it is applied. All machines use the same two basic simple machine families in their parts. See the test below for more information regarding simple machines.

Simple Machines Test

First match the definitions to the two main families of simple machines, then look at the sketches and select the correct name for that simple machine.

Definition Family name Your Answers Key
  1. A bar that pivots over a fulcrum (a non moving point), when force is applied at one end, it creates a force in the opposite direction at the other end. length of bar and position of fulcrum can increase or decrease force. A teeter-totter is an example of this simple machine.
  2. A flat surface that is at an angle. Lessens force needed to lift an object over distance. A ramp is an example of this simple machine.
  1. Inclined Plane
  2. Lever and Fulcrum
Definitions Machines Your Answers Key
wedge.jpg1. Two inclined planes back to back. Changes direction of applied force by 90 degrees.cam.jpg2. Off center wheel, Oval wheel or wheel with inclined planes along edge. Changes rotation motion to back and forth motion

screw.jpg3. An inclined plane wrapped around a cylinder. Changes rotation motion to back and forth motion, also uses friction to hold in place.

wheel.jpg4. A circular object that pivots around a fixed point making a continous lever, object may be attached to the axle or pivot freely for different uses.

crank.jpg5. A lever attached to a wheel or axle. Changes a push/pull to a rotation force.pulley.jpg6. A wheel with a grooved edge to hold a belt or rope. Transfers motion over distances. Keeps rotation direction same but can increase or decrease speed by different sizes.

gear.jpg7. A toothed wheel. Transfers motion and can change rotation direction, may increase or decrease speed by using different sizes.

Bonus: Can you name the simple machine that combines elements from both families?

  1. Cam
  2. Crank
  3. Gear
  4. Pulley
  5. Screw
  6. Wedge
  7. Wheel & Axle

Use links below to learn more about simple machines: Franklin Institute Resources for Science Learning
Rice University (Houston TX) Basic Mechanics page for Elec 201 class

Vocabulary

Definitions Vocabulary words Your answers Key
Structure
  1. The organization and arrangement of parts into one unit.
  2. The size or amount of something.
  3. The forces that will be put on structure, may be “dead” meaning unchanging or static or “live” meaning intermittent or changing.
  4. A push or pull or other influence that causes a change in movement, shape, etc. of an object or group of objects.
  5. A pushing force that shortens or compacts an object.
  6. What happens when an applied force causes a change in an object.
  7. A change in shape, size or temperature of an object due to stress.
  8. Changing, not stable or the same at all times
  9. The action of spreading stress over a larger area or mass.
  10. shows direction and strength of a force.
  11. A pulling force that lengthens or stretches an object.
  12. Stable, steady, unchanging
  13. A change in position caused by stress.
  14. The action of moving stress from a weak point to a stronger one via a connection.
  15. 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.
  1. Center of Mass
  2. Compressions
  3. Deformation
  4. Displacement
  5. Dissipation
  6. Dynamic
  7. Force
  8. Load
  9. Magnitude
  10. Static
  11. Stress
  12. Structure
  13. Tension
  14. Transfer
  15. Vector
Motion
  1. Measurement of size or amount.
  2. A force applied at a distance from a pivot or fixed point.
  3. Stored energy based on the position or shape of an object.
  4. Energy created by speed and movement of an object.
  5. A push or pull or other influence that causes a change in movement, shape, etc. of an object or group of objects.
  6. The resistance to movement when one object rubs or slides along another.
  7. Power or the ability to do work
  8. Measurement of speed
  9. Energy created by the effect of gravity on an object.
  10. Where an object at rest will stay at rest and an object in motion will stay in motion until a force is exerted on the object.
  11. Applying a force to move an object over a distance.
  12. Energy created by tension or compression of an object that has the ability to return to its original shape when released.
  13. The quantity of matter in an object.
  1. Elastic energy
  2. Energy
  3. Force
  4. Friction
  5. Gravitational energy
  6. Inertia
  7. Kinetic energy
  8. Mass
  9. Magnitude
  10. Potential energy
  11. Torque
  12. Velocity
  13. Work

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 forces and motion:   wheelie    gears     catapult

Here is a couple of links to games about simple machines:   Ed Heads Simple Machines (good for younger kids)
Boston Museum of Science Inventors toolbox(good for older kids)

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