Work and Energy What is work?

Work Work: when a force causes change in the position or direction of an object – The object will move in the direction of the force – Work = force x distance – W = F x d – Measured in Nm also known as Joules – 1 Nm = 1 J = 1 kg m 2 /s 2 – You may apply a lot of force to try and move and object, but if the object does not move, then you have not done any work –in the physics sense. (Although it may feel like you have done work, unless it moves, you haven’t done work)

The object must move or change direction to have had work done to it. – Is holding a book above your head doing work? Ex 1: A crane uses an average force of 5200 N to lift a girder 25m. How much work does the crane do on the girder? Ex 2: While rowing in a race, John uses his arms to exert a force of 165 N per stroke while pulling the oar m. How much work does he do in 30 strokes?

Power Running up a flight of stairs does not require more work than walking up slowly does, but it is more exhausting. The amount of time it takes to do work is an important factor when considering work and machines. The quantity that measures work in relation to time is POWER Power = work P = W time t

How are power and work related, if time is constant? How are power and time related, if work is constant? Power is measured in Watts (W) (or hp- horsepower) A watt is the amount of power required to do 1 J of work in 1 second.

Problems: W = F x d P = W t 1.It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process? 2.Anna walks up the stairs on her way to class. She weighs 565 N and the stairs go up 3.25 m vertically. Calculate the power output if she climbs the stairs in 12.6 seconds. 3.What is Anna climbs the stairs in 10.5 seconds, what would be her power output?

Machines and Mechanical Advantage Which is easier, lifting a car yourself or using a jack? Machines can be used to take advantage of the fact that force and distance are inversely proportional. – So increasing one will decrease another – The longer the distance, the less force needed to do the same work Machines do not increase the quantity of work that one can do

Why is it easier to push a box up a ramp to a truck, rather than lift it up to a truck? – Because you are increasing the distance, thus lowering the force needed to dot he same work Machines help us to do work by redistributing the work that we put into them. – Machines can change the direction of an input force – Machines can increase or decrease the force by changing the distance

Some machines amplify force and some amplify distance and thus speed. A baseball bat is a machine that increases speed by increasing the distance

Simple Machines The most basic machines are called simple machines There are six simple machines Other machines are just combination of the six simple machines Two families of simple machines – The lever family and the inclined place family

The lever family Simple lever, pulley, wheel and axle are the three types of simple machines in the lever family Simple lever: Like a hammer pulling out a nail – All levers have a rigid arm that turns around a point called a fulcrum (the pivot point) – Force is transferred from one part of the arm to another – First Class lever: the fulcrum is center, input at one end and output at the other. They either multiply force or increase distance (hammer). The fulcrum is between the effort and the load

– Second Class lever: the fulcrum is at one end and input force is at the other so as to multiply force. The output force is in the middle (wheelbarrow). The load is located between the fulcrum and the effort. The pivot is at one end and the strength is in the middle..the effort is at the opposite end. – Third class lever: the fulcrum in at one end and the input force is in the middle. The output force is at one end. They always increase distance. (the human arm) The effort is between the fulcrum and the load. The pivot is at one end and the strength is at the other….the effort is in the middle

Pulleys: are modified levers – The point in the middle of the pulley is like the fulcrum….it is the pivot point – Pulleys are like 1 st class levers because the “pivot” point is in the center….between the input and the out…between the effort and the load/strength. – Pulleys can be added together to amplify the advantage The lever family

Wheel and Axel: lever or pulley attached to a shaft – Like a steering wheel When the wheel is turned, the axel also turns. When a small force is applied to turn the wheel, the force is multiplied to become a large output force applied to the steering column, which turns the front wheels of the car. – Screwdriver and cranks The lever family

The Inclined Plane Family An inclined plane : ramp – Changes magnitude and direction of force – Pushing a box up a ramp requires less force than lifting it directly. – The work is spread over a greater distance A wedge: modified inclined plane – Turns downwards force into two forces directed out to the sides, like a nail A screw: an inclined plane wrapped around a cylinder – Jar lids and spiral stair cases are examples – Gentle slopes of the threads of a screw make it easier because it requires less force.

Inclined Plane Family

Compound Machines Many of the devices that we use are a combination of more than one simple machine Car Jack: uses a lever and a screw Bicycle: uses a variety of simple machines

What type of simple machines are these? Scissors: Hammer: Boat Oar: Can opener: Flag Pole: Bottle opener: Door Nob: Axe: Jar: Tweezers:

Types of Simple Machines

Energy and Work When you use stretch a sling shot, you do work and you transfer energy to the elastic band. The elastic band then does work on the rock by transferring energy. Energy can not be created or destroyed Energy can be transferred Energy can be defined as the ability to do work, so both use Joules as the unit

Energy can be present but undetected. It may only get noticed when it is actually transferred. Potential energy (PE): energy of position or energy that is stored and unused – Elastic PE: energy stored in stretch or compressed elastic material – Gravitational PE: any two objects separated by a distance (like an apple falling from a tree- the greater the height, the greater the PE)

Kinetic energy (KE): energy of motion or energy that is used – KE depends on the mass and speed of the object – The faster an object is going the more KE it has – KE = ½ mv 2 – The units are joules (J) – KE depends on speed more than mass which is why it is squared. This is why a car crash at high speeds is so much more dangerous than at lower speeds despite the mass being the same.

Increasing temperature will increase movement and thus increase Kinetic Energy Also, the more Kinetic Energy you have the higher the temperature. KE b/c Temp

Problems 1.Calculate the Kinetic energy of a 1500 kg car moving at 29 m/s. 2.Calculate the kinetic energy of 2000 kg car moving at 13 km/hr. 3.A 35 kg child has 190 J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill?

Other Types of Energy Mechanical: the amount of work an object could do based off of the object’s potential and kinetic energy. Chemical Energy: the energy from chemical reaction. Electrical Energy: results from a flow of charged particles through conductive materials – Moving electrons can cause light or magnetic fields

Flow of Energy People get energy from living things – We eat sugars and fats and carbs to get energy Living things get energy from the Sun – This energy travels through electromagnetic radiation known as ultraviolet and visible light – Photosynthesis: when plants use energy from sunlight and convert it to chemical energy, which is stored as sugars The Sun gets energy from nuclear reactions. – Nuclear reactions are a form of potential energy – Fusion: when two nuclei are combined or fused to form a heavier nucleus – Fission: when a heavy nucleus is split into two lighter nuclei

Energy can be transferred But not created or destroyed! If total energy cannot be changed then when KE goes up, PE goes down. KE and PE are inversely related TE = KE + PE This is how a tennis ball can bouce – If you drop a tennis ball it will bounce up to the height it was dropped (in a perfect world) – If you throw the ball downward, the KE will be transferred into elastic PE as it compresses and then back to KE as it bounces back up to you. – Mechanical energy can turn into sound energy or heat energy, thus a bouncing ball will not return to its original height. – Friction and air resistance can also be a source of transferred energy

Transfer of energy can result in loss of work: Because of friction and other factors, only some of the work done by a machine is applied to the task at hand. Some may be “lost” or transferred to some other form of energy. There is a difference between the total work and “useful” work Can something be in perpetual motion? Why?