The%20Basics%20of%20Physics

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The Basics of Physics
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Basic Physics Terms

• In this lesson, we will review some basic physics
  terms and
• demonstrate them using our Lego gearbox.
• Force and Torque
• Mechanical Advantage
• Newtons Laws
• Work and Power
• Friction

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Gear Box
Please make the following modifications to the
gear box from the previous lesson.
Place bushings here and here.
Remove the motor and double beam.
Remove the rotational sensor.
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Gear Box
Please make the following modifications to the
gear box from the previous lesson.
Add another 1x16 beam here.
Add pulley wheels and gray pegs (for handles).
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Basic Terms - Force
What is a force?
A force causes something with mass to move
(accelerate).
This can be summed up with Newtons 2nd Law.
F m x a
(In fact, units of force are called Newtons.)
Answer The force of gravity.
Question What force do we experience every
single day?
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Basic Terms - Torque
What is torque?
Torque can be thought of as rotational force.
Torque causes something with mass to rotate.
This motor produces a torque.
We are interested in torque because we deal
with rotational motors and axles.
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Force and Torque
How are force and torque related?
moment arm.
A force can create a torque by acting through a
moment arm.
produces a torque here.
A force here...
The relationship is t F x r. r is the length
of the moment arm (in this case, the length of
the wrench).
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Gears and Torque
Up to now, we have been talking about how gears
change speed. But they can also change torque.
Question What is the gear ratio of this gear box?
Answer 75 to 1
That means the last axle rotates 75 times slower
than the first axle. It also means the last axle
has 75 times the torque as the first axle.
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Gears and Torque
Try this experiment.
Have one person turn this wheel.
And have another person try to hold on to this
wheel.
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Gears and Torque
Where does all this torque come from?
Consider a pair of gears that are meshed together.
F
A torque on this axle...
produces a force at the tooth.
t
r
The moment arm is the radius of the
gear. Remember t F x r
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Gears and Torque
The force from the small gears tooth pushes
against the large gears tooth. This creates an
equal (and opposite) force in the large gear.
This is Newtons 3rd Law.
F
and produces a larger torque on this axle.
r
t
The force acts through this larger moment arm...
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Gears and Torque
t1 F1 x r1
t2 F2 x r2
Analyzing the forces...

• F1 t1 / r1
• F2 t2 / r2
• F1 - F2
• t1 / r1 -t2 / r2
• -t2 / t1 r2 / r1

The ratio of torques is the ratio of the gear
radii. This is the gear ratio!
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Gears and Torque
Gears can increase the torque (and force) that
they exert on something. This is known as
mechanical advantage.
torque increases
BUT, it comes at a price. Do you know what it is?
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Basic Terms - Work
Work (in physics) is defined as a force acting
over a distance. W F x d
Work in terms of rotation is a torque acting over
an angle. W t x q
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Mechanical Advantage
Understanding the 2 components of Work is the key
to understanding mechanical advantage.
Question Where would you hold the wrench for it
to be most effective?
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Mechanical Advantage
but you dont move very far.
but your hand moves a long way.
If you hold the wrench here, you need a lot of
force...
If you hold the wrench here, you dont need as
much force...
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Mechanical Advantage
It takes the same amount of work to turn the bolt.
You can opt for a lot of force and little
distance. W F x d
Or you can choose a little force but a lot of
distance. W f x D
In many of our machines, we want to increase our
force, so we dont mind going the extra distance.
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Mechanical Advantage
Question Which ramp would you prefer to use to
move a heavy weight to the top of the box?
Answer This ramp requires less force, but you
have to move the weight a longer distance.
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Mechanical Advantage
With our gear box, you were able to create a
large torque here...
but you had to turn this handle many
times. Remember W t x q
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Power
Power is the rate at which work is done. It can
be thought of as work per second. Power Work /
sec.
Like work, power has 2 components, force and
speed. P F x v (v stands for velocity).
Question Can you name 2 units for power?
Answer Horsepower and Watts.
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Power
Power has the same trade-offs as work. A motor
produces the same amount of power.
So, you can make a robot thats fast, but weak.
Or you can make a robot thats slow, but strong.
The total power in must equal the total power out
(with an exception)...
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Friction Bad
Friction is caused by two surfaces rubbing
together.
Friction in our gear box causes a loss in the
input power. It is lost in the form of heat and
sound energy.
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Friction Good
But friction is also what makes our robot move.
The turning wheel produces a frictional force
against the ground, which causes the robot to
move.
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Summary - Key Points

• A force causes something with mass to move.
• Torque is rotational force.
• Work has 2 components, force and distance.
• Mechanical advantage lets you increase one
  component of work at the expense of the other.
• Gears are one way to get mechanical advantage.
  They can increase the robots torque.
• Power is the product of force and speed.
• A robot can either be slow and strong, or fast
  and weak.
• Friction causes both losses in power, but also
  makes robots move on the ground.