Presentation on theme: "Teacher Professional Development Spring, 2002"— Presentation transcript:
1Teacher Professional Development Spring, 2002 The Basics of PhysicsTeacher Professional DevelopmentSpring, 2002
2Basic Physics TermsIn this lesson, we will review some basic physics terms anddemonstrate them using our Lego gearbox.Force and TorqueMechanical AdvantageNewton’s LawsWork and PowerFriction
3Gear Box Please make the following modifications to the gear box from the previous lesson.Place bushingshere and here.Remove the motor anddouble beam.Remove the rotational sensor.
4Gear Box Please make the following modifications to the gear box from the previous lesson.Add another1x16 beamhere.Add pulley wheels and gray pegs (for handles).
5Basic 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 = 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.
6Basic 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 withrotational motors and axles.
7Force and Torque How are force and torque related? moment arm. A force can create a torque by acting through a moment arm.…produces atorque 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).
8Gears 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 1That means the last axle rotates 75 times slower thanthe first axle. It also means the last axle has 75 timesthe torque as the first axle.
9Gears and Torque Try this experiment. Have one person turn this wheel.And have another persontry to hold on to this wheel.
10Gears and Torque Where does all this “torque” come from? Consider a pair of gears that are meshed together.FA torque on this axle...…produces a force at the tooth.trThe moment arm is the radius of the gear.Remember: t = F x r
11Gears and Torque The force from the small gear’s tooth pushes against the large gear’s tooth.F…and produces alarger torque on this axle.rtThe force acts through this larger moment arm...
12Gears and Torque t1 = F1 x r1 t2 = F2 x r2 Analyzing the forces... F1 = t1 / r1F2 = t2 / r2F1 = - F2t1 / r1 = -t2 / r2-t2 / t1 = r2 / r1F1 = -F2t2r2t1r1The ratio of torques is the ratio of the gear radii.This is the gear ratio!
13Gears and TorqueGears can increase the torque (and force) that they exerton something. This is known as mechanical advantage.torque increasesBUT, it comes at a price. Do you know what it is?
14Basic Terms - WorkWork (in physics) is defined as a force acting over a distance.W = F x dWork in terms of rotation is a torque acting over an angle.W = t x q
15Mechanical 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?
16Mechanical Advantage …but you don’t 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 don’t need as much force...
17Mechanical 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 dOr 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.
18Mechanical 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.
19Mechanical AdvantageWith 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
20PowerPower 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.
21PowerPower 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)...
22Friction = 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.
23Friction = 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.
24Summary - 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.