Work, Power, and Energy

Mrs SedlockPrinciples of Chemistry and Physics

Review

• Newton’s Laws were used to predict and describe an object’s motion

• In this unit we will discuss motion in terms of energy and work

Work

• Work - when force acts on an object and causes displacement of the object – Force– Displacement– Cause • In order for work to be done there must be a force that

causes a displacement

Examples of Work

Examples of work

A teacher applies a force to a wall and becomes exhausted. A book falls off a table and free falls to the ground. A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. (Careful! This is a very difficult question that will be discussed in more detail later.) A rocket accelerates through space.

Work

• Any part of a force that does not act in the direction of motion does NO WORK in an object

Negative Work

• Sometimes force acts in the opposite direction of the displacement to prevent motion– Ex: • car skidding to a stop• Baseball player sliding into home plate

Calculating Work

• Work = force x displacement

• Unit of work = J (Joules)• 1 J = 1 N*m

• Ws work problems

Power

Power

• Power – is the rate of doing work– Doing work at a faster rate requires more power– to increase power, increase the amount of work

done in a given amount of time – Or do the same amount of work in less time

Power

• The snow blower can do more work in less time – so it has more power than the person shoveling

Calculating Power

Power = work time

Units Work is in Joules (J)Time is in seconds (s)Power is in watts (W) which is 1 Joule /second

Ex: a 40-watt lightbulb requires 40 Joules each second that it is lit

Calculating Power

• You exert a vertical force of 72 Newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?

• (hint: remember that work = force x displacement )

• 36 Watts

Horsepower

• One horsepower (hp) = 746 Watts

• slapshot physics

Work and Machines

drones

Machine

• Machines make work easier to do – Change the size of the force – Or the direction of the force– Or distance over which a force acts

• Increase force• Each rotation applies a small force over a large

distance, but each rotation lifts the car a short distance

• If a machine increases the distance over which you exert a force, then it decreases the amount of force you need to exert

• Increasing distance• The oars act as a machine to push the boat

through the water • Pulling the oar short distance near the boat

translates to a large distance in the water – but you increase the force needed

• A machine that decreases the distance through which you exert a force increases the amount of force

• Change of direction• Some machines change the direction of the

applied force• Pulling back on the handle of the oar causes

its other end to move the opposite direction

• Machines can change the direction of the force

Work Output

• The force that is exerted by a machine is called the output force

• The distance the output force is exerted through is the output distance

• work output = output force x output distance•

Work Input and Work Output

• Because of friction, the work done by a machine is always less than the work done on the machine

Work input and work output

• The force you exert on a machine is called the input force

• the distance the input force acts through is called the input distance

• The work done in this process is called the work input

• Work input = input force x input distance

Work Input

• For the oar- – the input force is the force exerted on the handle – The input distance is the distance the handle

moves– The work input is the work you do to move the

handle • You can increase the work input by increasing

the input distance, the input force, or both

• The force the oar on the water causes an equal and opposite reaction force to be exerted by the water on the oar – this reaction force propels the boat through the water

• The only way to increase the work work output is to increase the amount of work you put into the machine

Mechanical Advantage and Efficiency

Mechanical Advantage

• Mechanical advantage of a machine is the number of times that the machine increases an input force

Actual Mechanical Advantage

• A loading ramp is a machine used to move heavy items into a truck– The longer the ramp, the less force is needed to

lift a refrigerator into the truck

Actual Mechanical Advantage (AMA)

• AMA = output force • input force

Actual Mechanical Advantage

– If the ramp has a rough surface it will have less mechanical advantage than a ramp with a smooth surface • It takes a greater force to overcome the friction

Ideal Mechanical Advantage(IMA)

• Ideal mechanical advantage of a machine is the mechanical advantage in the absence of friction– Because friction is always present, the actual

mechanical advantage of a machine is always less than the ideal mechanical advantage

Ideal Mechanical Advantage(IMA)

IMA = input distance output distance

Ideal Mechanical Advantage(IMA)

• A woman drives her car up onto wheel ramps to perform some repairs. If she drives a distance of 1.8 meters along the ramp to raise the car 0.3 meter, what is the ideal mechanical advantage of the wheel ramps?

• IMA = input distance = 1.8 m = 6 output distance 0.3 m

Efficiency

• Efficiency – the percentage of work input that becomes work output- usually expressed as a percentage

• The efficiency of ANY machine is always less than 100%

Efficiency

Efficiency = work output x 100% work input

Quiz Review

• What is the unit for force?• What is the unit for power?• What is the unit for distance/displacement?• What is the unit for time?• What is the unit for work?

Quiz review

• You must exert a force of 4.5 newtons on a book to slide it across a table. If you do 2.7 Joules of work in the process, how far have you moved the book?

Quiz Review

• A catcher picks up a baseball from the ground. If the unbalanced force on the ball is 7.25 x 10 -2 Joules of work is done to lift the ball, how far does the catcher lift the ball?

Quiz Review

• A machine has a work output of 8 joules and requires 10 joules of work input to operate. What is the machine’s efficiency?

Quiz review

• What is the output distance of a machine with an input distance of 3.0 cm and an ideal mechanical advantage of 12?

Simple Machines

6 Types of Simple Machines

• Lever• Wheel and axle• Inclined plane• Wedge• Screw• Pulley

Simple machines

• Many mechanical devices are combinations of the six types of simple machines

Lever

• Lever- a rigid bar that is free to move around a fixed point

- the fixed point is known as the fulcrum

Lever

• There are 3 classes of levers based on the locations of – input force, – output force, – and the fulcrum

Lever

• First Class lever– Fulcrum of a first class lever is always between the

input force and the output force – Mechanical advantage depends on location of the

fulcrum

Lever• Second class lever- output force is between

the input force and the fulcrum• Input distance is larger than output distance ,

which means you need less force • The mechanical advantage of a second class

lever is always greater than 1

Lever

• Third class lever – input force is between the fulcrum and the output force

• Input distance is smaller than output distance • Mechanical advantage is less than 1

Levers

• Levers

• Paul Rabil

Lever

Wheel and Axle

• Wheel and axle is a simple machine that consists of two disks or cylinders, each with a different radius– The outer disk is the wheel and the inner disk is

the axle– The input force can be applied to the wheel or the

axle

Wheel and Axle

• To calculate the mechanical advantage of the wheel and axle

• Can have a mechanical advantage less than or greater than 1

Mechanical advantage = radius of input radius of output

Wheel and Axle

Inclined Plane

• If the input distance is greater than the output distance, the input FORCE is DECREASED

Inclined plane

• Inclined plane- a slanted surface along which a force moves an object to a different elevation

• The ideal mechanical advantage of an inclined plane is the distance along the inclined plane divided by its change in height

• IMA = distance of inclined plane change in height

Teacher demo

Wedges and Screws

• Wedges- v-shaped object that has inclined planes on the sides sloped toward each other

• The sloping sides push the wood a small distance apart

• Mechanical advantage is greater than 1

Wedges and Screws

• Screw- an inclined plane wrapped around a cylinder– Screw that have threads closer together have a

greater ideal mechanical advantage

Pulleys

• Pulley is a simple machine that consists of a rope that fits into a groove in a wheel– Pulleys produce an output force that is different in

size, direction, or both from the input force

Pulleys

• The ideal mechanical advantage (IMA) of a pulley system is equal to the number of rope sections supporting the load being lifted– Three types of pulleys• Fixed Pulley• Movable pulley• Pulley system

Pulleys

• Fixed Pulley– Wheel attached in a fixed location – Changes direction of the exerted force– IMA is 1 because the rope will lift the load the

exact distance you pull the rope

Pulleys

• Movable Pulley– Is attached to the object being moved – Reduce the input force

Pulleys

• Pulley system– Mechanical advantage depends on how the

pulleys are arranged– Each segment of the rope exerts a force equal to

the force you exert on the rope– Pulleys

Compound Machines

• Combination of two or more simple machines that operate together– The output force of one of the simple machines

becomes the input force for another– Ex: • Clocks• Bicycles

Compound Machines

Simple machines

• Bill Nye Simple MAchines

Energy

Energy

• Energy is the ability to do work – Energy is transferred by a force moving an object

over a specific distance– Sooooo– Work is a transfer of energy– Both are measured in Joules

Types of Energy

• Kinetic energy–Energy of motion

• Potential energy–Energy of position, stored energy

Kinetic Energy

• The kinetic energy (KE) of an object depends on its mass (in kg) and speed (velocity v in meters per second)

KE=½ mv2

if you double the mass, the KE is doubled

if you double the speed, the KE is quadroupled!

Practice problem

• A 70 kg man is walking at a speed of 2.0 m/s. What is his kinetic energy?

• KE = ½ mv2 m= 70 kg v = 2.0 m/s• KE = ½(70 kg) (2.0 m/s)2

• KE = 140 J

Potential Energy

• Potential energy is the energy with the potential to do work– Two common forms• Gravitational• Elastic

Gravitational Potential Energy

• Potential energy that depends on an objects height is called gravitational potential energy

Gravitational Potential Energy

• An object’s gravitational potential energy depends on its mass (m), height (h), and acceleration due to gravity (g)

• Potential Energy (PE) = mgh

Elastic Potential Energy

• Potential Energy of an object that is stretched or compressed is known as elastic potential energy – Something is considered to be elastic if it springs

back to its original shape– Rubber band- energy you add is stored as

potential energy

Forms of Energy

• Mechanical energy• Thermal energy• Chemical energy• Electrical energy• Electromagnetic energy• Nuclear energy

Mechanical Energy

• Energy associated with motion

Thermal Energy

• All particles of matter are in constant motion so they have kinetic energy

• The total of the potential and kinetic energy of all microscopic particles make up its thermal energy

Thermal Energy

Chemical Energy

• Energy stored in chemical bonds– When bonds are broken, energy is released that

can do work

Chemical Energy

Electrical Energy

• The energy associated with electrical charges– Electric charges can exert a forces that do work

Electrical energy

Electromagnetic Energy

• Form of energy that travels as a wave

Nuclear Energy

• Energy stored in atomic nuclei is nuclear energy

Nuclear Energy

How a Nuclear Power Plant Produces Electricity

Energy

• Bill Nye Energy