General Physics
By Nick Brennan, Sam Mills, and Matt
Dunster
FRC 11 - Mount Olive Robotics Team
Objectives
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Review for most; Introduction for some
Earn a greater understanding of applicable physics concepts
Recognize these concepts are used all through robot design
Learn to apply relevant physics topics to FRC
Have fun
Physics Topics
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Trigonometry
Kinematics
Projectile Motion
Acceleration
Force
Types of Acting Forces
Circular Motion
Torque
Friction
Center of Mass
Moment of Inertia
Power
Trigonometry
= the branch of mathematics that deals with the relations between the sides
and angles of plane or spherical triangles
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Used extensively in the process of designing the physical robot
Triangular structures are much stronger than any polygon and are
frequently used as frames and supports
Removing too much material will begin to weaken the
structure
Kinematics
PROS
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CONS
Simple equations which require
any 4 of 5 select variables
in order to be solved
Used for horizontal, projectile, and
rotational motion
Variables
Δx
v
v0
a
t
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Must know the values for at least
4 variables
Restricted to basic, ideal
scenarios
Acceleration must be constant
EQUATIONS
v = v0 + a*t
Δx = v0*t + ½ a*t2
Δx = ½ (v0+ v)*t
v2 = v02 + 2*a*Δx
Projectile Motion
= the motion an object experiences when the only force acting on it is gravity
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Critical for games where game pieces are launched
Allows for accurate, consistent shots
Need to account for inconsistent variables
o Deformation in resilience would play a large role in how an object's
projectile motion changes over time, making shots inaccurate
Uses simple equations which require values for Δy, v0, v, a, and t
Important knowledge for designing, constructing, and programming the
end effector
Acceleration
= the change in velocity during an interval of time
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Drivetrain
Determines the ideal:
o gearbox
o combination of motors
The acceleration for FRC robots
needs to be very quick in order to
reach top speeds immediately.
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End Effector
Critical to establish the speed of
flywheels for shooting games
Must be driver friendly (not too
fast/slow)
Deceleration of arm may be
necessary to minimize
stress/strain
Force
= any influence that causes an object to undergo a change in velocity or
direction
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F = Mass * Acceleration
There are different types of forces and each one affects the robot
differently
Each force is considered a load and loads cause stress points across the
robots frame and components
Power to lift or push an object
Particularly useful to calculate for hanging games
Types of Acting Forces
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Static Load: Forces on a still object (weight of a robot)
Dynamic Load: Forces in motion (force of robot's moving arm on the robot)
Forces Continued
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Tension: When the ends of an object are pulled away from its center
Compression: When the ends of an object are pushed towards its center
Shear: Applied parallel or tangential to a face of a material
Torsion: Occurs when an object, such as a bar with a circular or square
cross section, is twisted
Circular Motion
= rotation along a circular path
ac = vt2/r = r*w2
Fc = m *vt2/r = m*r*w2
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Relevant for all wheels on the robot
Speed along the outside edge of the wheel can be found by multiplying
RPMs by circumference. (this is known as tangential speed)
This has an impact on wheel size in that the radius and distance travelled
are directly proportional
THIS DOES NOT
MEAN BIGGER
WHEELS ARE
ALWAYS BETTER
Torque
= the tendency of a force to rotate an object about an axis, fulcrum, or pivot
𝜏 =𝐹∗𝑟
Torque is directly proportional to the radius
Thus, the greater the radius, the larger the torque is necessary to produce
the same force.
Greater torque translates to greater pushing strength
While large wheels may seem appealing, small wheels may be more
advantageous
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Friction
= the force which resists motion
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All objects undergo friction, and friction generates HEAT
Different types of wheels will behave differently with the carpet
Some mechanisms will require grease or another lubricant to reduce
friction and heat
If friction between some objects is too high, they could bind together (i.e.
gears) This causes inefficiencies.
In some applications, increased friction could be beneficial such as wheel
contact with the carpet, but more doesn't always mean better
Center of Mass
= A point representing the mean position of the matter in a body or system
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Keeping the center of mass nearest the center or the body, will not only
help balance, but other things like reducing the turning radius and ensuring
the weight is evenly distributed
Keeping the center of mass low will minimize the risk of tipping over
Other Examples
2013 Hanging
2012 Balancing
2009 Back heavy
Moment of Inertia
= a property of rotating bodies; the resistance to a change in angular velocity
about an axis of rotation
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Determines how quickly the wheel accelerates and how easily it maintains
its angular velocity
Must consider for all wheels & rollers
Hollow cylinders have greater moments than solid cylinders (i.e. better
suited for different purposes)
Power
= The rate at which energy is transferred, used, or transformed
Power = Force * Distance / Time
OR
Power = Torque * Rotational Velocity
FRC definition - how fast you can move something
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Many different motors can be used to accomplish the same task, but
power determines how quickly any particular motor can perform the task
Motors can be geared together and their power adds together.
All motors can lift the same amount (assuming 100% power transfer
efficiencies) - they just do it at different rates
Efficiency in a system is never 100% (friction, heat, etc)
Power Examples
Greater power is needed when creating mechanisms which need to reach top
speed quickly: Flywheels, drivetrains, elevators, arms
When motors are paired together properly, the speed of an action is increased.
118's 2007 swerve - 4 CIM + 2 Fischer
Price
Questions?
Sources & Links
Power - http://www.instructables.com/id/Understanding-Motor-and-GearboxDesign/?ALLSTEPS
https://www.khanacademy.org/science/physics
http://mort11.org/
Definitions of all topics - www.wikipedia.org