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: • The amount of force being used • 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: • The amount of work being done • 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

NOTES 11 – Energy, Work, & Power