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

Use the knees, not the back! For example, when you lift a stack of books, your arms apply a force upward and the books move upward. Because the force and distance are in the same direction, your arms have done work on the books For example, when you lift a stack of books, your arms apply a force upward and the books move upward. Because the force and distance are in the same direction, your arms have done work on the books

Tiring, yes. Work, no. When you carry books while walking, you might think that your arms are doing work. However, in this case, the force exerted by your arms does no work on the books. The direction of the force and the direction of the movement of the object are different.

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 used 2.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 done 2.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

Section Review Pg 91