8 Lesson 2: WORK AND ENERGY Student B did double the work of student AStudent C did double the work of student AStudent D did four times the work of student AStudent B did half the work of student DStudent C did the same work as student B
9 W = F x d The bigger the force, the bigger the work The longer the distance, the bigger the workSo, mathematically…Work = Force x distanceW = F x d
14 1. A road vehicle with two wheels is a… motorbike2. What’s the name of the distance between the ground and something above it?Height3. If you …….. something, you elevate it.lift4. It is a thick cord or wire.Rope5. A ……..……. is a series of actions which are executed in order to achieve a result.process6. A …………… is any cause that can change the movement or the shape of an object.force7. When you move something making a force, you are doing…work8. To use force in order to move something towards you.To pull9. To use force to make something move away from you.To push
15 W = F x d W F d Remember… Work Unit to measure work: JOULE (J) Force Unit to measure force: NEWTON (N)dDistanceUnit to measure distance: METRE (m)
20 What’s a machine?A m a c h i n e i a n y d e v i c et h a t u e e n e r g y t op e r f o r m o m e a c t i v i t y
21 Sentence 1Usually cars have four wheels but motorbikes and bicycles have just two.Sentence 2To get high speed in ski jumping, skiers slide down a ramp or inclined plane.
22 Sentence 3Screws can be used to join pieces but also to make moving devices like a car’s jack.Sentence 4Taking water from a well is much easier if you have a rope and a pulley.
23 Sentence 5You can cut logs with a saw or you can split them with a wedge and a sledgehammer.Sentence 6A wooden pallet is easier to break if you have a crowbar to use as a lever.
24 1. What is a simple machine? A simple machine is a device that changes the direction or the magnitude of a force.2. Why are simple machines useful?Simple machines are useful because they make work easier.3. How does a simple machine work?A simple machine works by using a single applied force to do work against a load.4. How many moving parts has a simple machine got?A simple machine has got few or no moving parts.5. Why is learning the basics of simple machines important?Learning the basics of simple machines is fundamental to understanding more intricate mechanisms.6. What’s the relationship between simple machines and more complicated machines?Simple machines can be thought of as building blocks for more complicated machines.
25 Simple machines 7. Which are the two basic simple machines? The two basic simple machines are the inclined plane and the lever.8. What devices are variants on the inclined plane?The screw and the wedge are variants on the inclined plane.9. What devices are variants on the lever?The wheel and axle and the pulley are variants on the lever.Simple machinesInclined planeLeverWheel and axleScrewWedgePulley
28 a)Conservation of energy principle:b)Friction:c)Frictionless system:d)Ideal machine:e)Actual machine:f)Efficiency:Energy can neither be created nor destroyed; it can only be transformed from one state to another.Force that makes the relative motion between two objects more difficult .Ideal system with no friction forces within it.Theoretical machine in which there is no loss of energy (e.g., because of the friction).Machine in which there is loss of energy (i.e., real machine)Is the ratio of energy used by a machine to the useful work the machine has done.
30 mm mb L F mb = 280 kg F mm = 70 kg L FRAGILE Mass of the man. Unit to measure mass: KILOGRAM (kg)mbMass of the object we want to move.Unit to measure mass: KILOGRAM (kg)LLoad we want to move. It’s the weight of the object. It can also be called resistance or output force.Unit to measure force: NEWTON (N)FForce we have to apply to move the object. It can also be called effort or input force.Unit to measure force: NEWTON (N)
31 The load L is the weight of the box, so… FRAGILEmb = 280 kgFmm = 70 kgL The load L is the weight of the box, so… If the man hang from the bar to move the box, his weight will be the force F:
32 FRAGILEmb = 280 kgmm = 70 kgMA = 4F = 686,7 NL = 1692 N The man can lift 1692 N (280 kg) applying a force of 686,7 N (70 kg). The force done is a quarter of the force we would need to lift the box without the bar. The machine (in this case, a lever) multiplied the force of the man by 4. This number is called mechanical advantage (MA).
33 = 4 Mechanical advantage (MA) is… … the factor by which a machine multiplies the force applied to it. … the force amplifying effectiveness of a simple machine. … the ratio of load to effort. … the ratio of the force exerted by a machine (the output) to the force exerted to the machine (the input).In the example:= 4
34 Winput > Woutput Winput = Woutput FRAGILEmb = 280 kgL = 1692 NF = 686,7 Nmm = 70 kgMA = 41. Do simple machines multiply energy?No, simple machines do not multiplying energy but force.2. What’s the relationship between the input energy and the output work of an actual machine? Why?The input energy of an actual machine is always bigger than its output work. The reason is that in actual machines there is always a loss of energy due to friction. The efficiency is not 100%.Winput > Woutput3. What’s the relationship between the input energy and the output work of an ideal machine? Why?The input energy of an ideal machine is equal to its output work. The reason is that in ideal machines there is no loss of energy due to friction. The efficiency is 100%.Winput = Woutput
35 4. What principle related to energy do simple machines violate? Why? None because no energy is created by simple machines. They just multiply force. The conservation of energy principle cannot be violated.Winput = Woutput5. What’s the cost of making less force with a simple machine to get the same energy?To get the same energy with less force you have to apply the force over a longer distance.In the example, to move the box a short distance (db), the man had to move the other part of the lever a long distance (dm).dbdm
36 5. How can we get the mechanical advantage formula? By applying the conservation of energy principle.The ratio of distances is equal to the ratio of forces. We call that ratio mechanical advantage.