NOTES 11 – Energy, Work, & Power

NOTES 11 – Energy, Work, & Power

What is energy & why do we need it?

• Energy – the ability to do work• Work – moving an object by exerting a

force• Force – a push or a pull• Whenever force is used to move an

object, energy is required– EX. When you walk, your muscles pull on

your bones in order to move your body. This requires your muscles to use energy.

What is energy & why do we need it?

• Without energy, there could be no motion– Atoms and molecules could not move– Stars could not shine– Planets could not orbit around stars– Animals could not walk, run, swim, or fly– The wind could not blow– Messages could not be sent from your

brain to your body

Is work being done when you hold a book over your head?• No• But it did take work to get

it there• Whenever work is being

done, energy is being used• The energy was

transferred from your body to the book

How much work is being done?• We can measure the amount of work being

done to move an object• We need to know 2 things in order to measure

work:1. The amount of force being used2. The distance of the movement

• We can calculate work using the following formula:

Work = Force x Distance (W = F x d)

Calculating Work – Units of Measurement

• When calculating work, use the following units:– Force is measured in newtons (N)– Distance is measure in meters (m)– Work is measured in joules (J)

• 1 joule equals 1 newton multiplied by 1 meter• You do about 1 joule of work when you pick

up an apple and put it on a desk

Calculating Work – Sample Problem

• If you lift a 3N book 2 meters off the floor, how much work did you do?

• W = F x d• W = 3N x 2m• W = 6J• You did 6 joules of work

Question

• When you carry a heavy bag of groceries from your car to your kitchen, what does most of the work, your arms or your legs? Explain why.

• Your legs, because they move you and the groceries from the car to the kitchen. Your arms only lift and hold the groceries.

• The legs apply more force over a greater distance so they do more work

Energy Is Also Measured In Joules

• Since energy is required to do work, it is measured using the same unit (joules)

• The amount of energy required to do work is ALWAYS greater than or equal to the amount of work being done

• EX. If you do 6J worth of work to lift a book, you need at least 6J of energy to do it

Doing Work Gives Energy to Objects• When work is done on an object:– Energy is transferred from the object doing the

work to the object having work done on it– The object doing the work loses energy– The object having work done on it gains energy

• EX. A student pushing a desk across the floor is doing work on the desk – energy is transferred from the student to the desk and the student loses energy

Why do we get tired after doing work?

• We give up our energy to all the objects we touch and move around

• In any energy transfer in our body, some energy is changed to heat and transferred to our environment – the energy is LOST from our body

• Staying alive requires a lot of energy– Heart beating, brain sending messages, cells moving

substances in and out, muscle contractions, etc.

• As our body’s energy gets low, we get tired

What is power?• Power – the rate at which work is done• More power means…– More work is done in the same amount of time – The same amount of work is done in less time

• EX. A person that is a more powerful runner is faster and can run farther in the same amount of time as a less powerful runner (more work in equal time)

• EX. A car with a more powerful engine can accelerate to 60mph faster than a car with a less powerful engine (equal work in less time)

Calculating Power

• Power is measure in watts (W)• We need to know 2 things in order to measure

power:1. The amount of work being done2. The amount of time it takes to do the work

• We can calculate power using the following formula: Power = Work / Time (P = W / t)

• 1 watt equals 1 joule divided by 1 second

Calculating Power – Sample Problem 1

• If an engine does 100,000 joules of work in 10 seconds, how much power did it use?

• P = W / t• P = 100,000J / 10s• P = 10,000W• The engine used 10,000 watts of power

Calculating Power – Sample Problem 2

• Because work equals force multiplied by distance, another way to write the power formula is:

Power = (Force x Distance) / Time

P = (F x d) / t

• P = W / t is the same as P = (F x d) / t

Calculating Power – Sample Problem 2

• If an engine exerts 3500 newtons of force to move a car 50 meters in 10 seconds, how much power did it use?

• P = (F x d) / t• P = (3500N x 50m) / 10s• P = 175,000J / 10s• P = 17,500W• The engine used 17,500 watts of power

A Watt Measures Work Done and Energy Used In an Amount of Time• A Watt equals 1 Joule per second• The more watts, the more work is done each

second• Joules also measure energy, so a watt also

measures energy use per second• EX. A 100W light bulb uses 100J of energy

each second that it is on