Transcript Force & Torque - netBlueprint.net

In this lesson, we will review some basic physics
terms and demonstrate them using Legos…
•
•
•
•
•
Force and Torque
Mechanical Advantage
Newton’s Laws
Work and Power
Friction
What is a “force?”
A force causes
something with
mass to move
(accelerate).
This can be summed
up with Newton’s 2nd
Law. F = m x a
(In fact, units of force are called “Newtons.”)
Question: What force
do we experience every
single day?
Answer: The force
of gravity.
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.
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).
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.
Where does all this “torque” come from?
Consider a pair of gears that are meshed together.
A torque
on this
axle...
F
t
r
…produces
a force at
the tooth.
The moment arm is the radius of the gear.
Remember: t = F x r
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...
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 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?
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
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?
…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...
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.
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.
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 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).
Answer: Horsepower and
Question: Can you
name 2 units for power? Watts.
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 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.
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.
• 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.