Basic Physics TermsIn this lesson, we will review some basic physics terms anddemonstrate them using our Lego gearbox.

• Force and Torque• Mechanical Advantage• Newton’s Laws• Work and Power• Friction

Basic Terms - ForceWhat 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

Question: What force do we experience every single day?

Answer: The force of gravity.

(In fact, units of force are called “Newtons.”)

Work is only done by a force on anobject if the force causes the objectto move in the direction of the force.

Objects that are at rest mayhave many forces acting on them,

but no work is doneif there is no movement.

Basic Terms - WorkWork (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

WorkThe scientific definition of work is

… The transfer of energy through

motion (distance). Work (like energy) is measured in

Joules (J) Formula: W = F * d

Mechanical AdvantageUnderstanding the 2 components of Work is the key tounderstanding mechanical advantage.Question: Where would you hold the wrench for it to be most effective?

Two factors have to be considered when deciding if work is being done….

Was a force applied?

Was there a change in distance?

The SI unit of poweris the Watt,

named in honor ofJames Watt.

One Watt, W, of poweris the power achieved

when 1.0 J of work is done or1.0 J of energy is transferred

in a time of 1.0 s.

Work done against gravity

W = mgh

height object raised (m)

gravity (m/sec2)

work (joules)

mass (g)

In which case would you do more work?

a. lifting 1 book over your head

a. holding 2 books at shoulder level

a. holding a stack of books and walking them to the counter

b. lifting 2 books over your head

b. lifting 2 books over your head

b. pushing a stack of books over to the counter

When doing work problems, what units must be used for . . .Work (W)

Force (F)

Distance (d)

Joule (J)

Newton (N)

meter (M)

The Basics of Machines

Simple Machines“a device that is used to manipulate the

amountand/or direction of force when work is

done”A common misconception is that

machines are used to do a task with less work than would be needed to do the

task without the machine. They do not! In fact (mainly because of friction), you actually do more work with a machine than without it (for the same task).

The major benefit of a machine is thatthe work can be done with less applied

force, but at the expense of the distance through which the force must be applied.

Basic Terms - TorqueWhat is “torque?”

Torque can be thought of as rotational force.

Torque causes something with mass to rotate.

We are interested in “torque” because we deal withrotational motors and axles.

This motor produces a torque.

Force and TorqueHow are force and torque related?

A force here...

…produces atorque here.

moment arm.A force can create a torque by acting through a moment arm.

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 TorqueUp to now, we have been talking about how gears changespeed. 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 TorqueWhere does all this “torque” come from?

Consider a pair of gears that are meshed together.

A torque on this axle...

…produces a force at the tooth.

Ft

The moment arm is the radius of the gear.Remember: t = F x r

r

Gears and TorqueThe force from the small gear’s tooth pushes againstthe large gear’s tooth. This creates an equal (and opposite)force in the large gear. This is Newton’s 3rd Law.

…and produces alarger torque on this axle.

F

t

The force acts through this larger moment arm...

r

Gears and Torque

F1 = -F2

t2

r2

t1

r1

Analyzing the forces... t1 = F1 x r1 t2 = F2 x r2

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!

Gears and TorqueGears can increase the torque (and force) that they exerton something. This is known as mechanical advantage.

BUT, it comes at a price. Do you know what it is?

torque increases

Mechanical Advantage

If you hold the wrench here, you need a lot of force...

If you hold the wrench here, you don’t need as much force...

…but you don’t move very far. …but your hand

moves a long way.

Mechanical AdvantageIt 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 DIn many of our machines, we want to increase our force, so we don’t mind going the extra distance.

Mechanical AdvantageQuestion: 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

Mechanical advantage (MA) is the ratio of output force to input force for a machine. Foutput

MA = Finput

A force of 200 newtons is applied to a machine in order to lift a 1,000-newton load. What is the mechanical advantage of the machine?

MA = 1000 N / 200 N = 5 note no label just five

PROBLEM

PowerPower 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.

PowerPower 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 = BadFriction 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 = GoodBut friction is also what makes our robot move.

The turning wheel produces a frictional force against the ground, which causes the robot to move.

WHERE ARE WE?• 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.

WedgeInclined Plane

Screw

Wheel and Axle

Lever

Pulley

Have you ever tried to unscrew a nut, bolt, or screw from something with your bare hands and discovered that it was just too tight to loosen even if you had a good grip?

You got the proper tool, such as a screw driveror wrench, and unscrewed it!

The wrench and screw driver are examples of a wheel and axle, where the screw or bolt is the axle and the handle is the wheel. The tool makes the job easier by changing the amount of the force you exert.

AxleWheel

All of the simple machines can be used for thousands of jobs

from lifting a 500-pound weight to making a boat go. The

reason why these machines are so special is because they make

difficult tasks much easier.

Fulcrum

Lever

Load

Click Here to Learn More

3 classes of levers

Determined by position of load, fulcrum, and effort

1st Class Lever

The claw on a hammer is a lever. We call this kind of lever a first-class lever.

2nd Class Lever

A bottle opener is a second-class lever, which means the fulcrum is at the end of the lever and the load is in the middle.

3rd Class Lever

A third-class lever has its fulcrum at one end and the load at the other end, with the work you do in the middle. It's how a fishing pole works. You lift just a short distance at the handle, but the end of the pole pops up several feet

A screw is an inclined plane wrapped around a post. Click on the light bulb to learn more.

Two inclined planes make a wedge.

Axle

Wheel

Wheel

Rope This box is easier to lift because of the pulley.