Transcript Applied physics

The Basics of Physics
Basic Physics Terms
In this lesson, we will review some basic physics terms and
demonstrate them using our Lego gearbox.
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Force and Torque
Mechanical Advantage
Newton’s Laws
Work and Power
Friction
Basic Terms - Force
What is a “force?”
A force causes
something with mass to
move (accelerate).
This can be summed up
with Newton’s 2nd Law.
F=mxa
(In fact, units of force are called “Newtons.”)
Question: What force do we
experience every single day?
Answer: The force of
gravity.
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.
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).
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.
Gears and Torque
Where does all this “torque” come from?
Consider a pair of gears that are meshed together.
A torque
on this
axle...
F
t
r
The moment arm is the radius of the gear.
Remember: t = F x r
…produces
a force at
the tooth.
Gears and Torque
The force from the small gear’s tooth pushes against
the large gear’s tooth. This creates an equal (and opposite)
force in the large gear. This is Newton’s 3rd Law.
F
…and
produces a
larger torque
on this axle.
r
t
The force acts through this larger moment arm...
Gears and Torque
Analyzing the forces...
F1 = t1 / r1
F2 = t2 / r2
F1 = - F2
t1 / r1 = -t2 / r2
-t2 / t1 = r2 / r1
t1 = F1 x r1
t1
t2 = F2 x r2
F1 = -F2
r1
r2
t2
The ratio of torques is the ratio of the gear radii.
This is the gear ratio!
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?
Basic Terms - Work
Work (in physics) is defined
as a force acting over a
distance.
W=Fxd
Work in terms of rotation is a
torque acting over an angle.
W=txq
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?
Mechanical Advantage
…but you don’t
move very far.
If you hold the wrench
here, you need a lot of
force...
…but your hand
moves a long way.
If you hold the wrench
here, you don’t need as
much force...
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=Fxd
Or you can choose a little force but a lot of distance.
W=fxD
In many of our machines, we want to increase our
force, so we don’t mind going the extra distance.
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.
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
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.
Power
Power has the same trade-offs as work. A motor
produces the same amount of power.
So, you can
make a
robot that’s
fast, but
weak.
Or you can
make a
robot that’s
slow, but
strong.
The total power in must equal the total power out
(with an exception)...
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.
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.
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 robot’s 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.