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Physics 1 honors

Chapter 8 – Energy Sections 8.1 to 8.4. Physics 1 honors. Work . Work is the amount of force that is applied over a given distance W=F x d Work has two components: Application of a force Movement of something by that force. Work.

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Physics 1 honors

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  1. Chapter 8 – Energy Sections 8.1 to 8.4 Physics 1 honors

  2. Work • Work is the amount of force that is applied over a given distance W=F x d • Work has two components: • Application of a force • Movement of something by that force

  3. Work • If we double the force necessary we double the work (if the distance is constant) • If we double the distance we move an object we double the work (if the force is constant)

  4. Work • If you lift a barbell from a rack to a position over your head, you are doing work. • However, holding the barbell in the position over your head does not involve doing work. • Although you are applying a force to hold the barbell over your head, you are no longer moving it, so there is no work being done.

  5. Work • Work generally falls into two categories: • Work done against a force • Sit-ups – you are working against the force of gravity on your upper body. • Tug of War – you are working against the force applied by the other team. • Work done to change the speed of an object • Pushing a shopping cart with your friend inside from a standing start. • Stopping your bicycle with the brakes.

  6. Work • The unit for work is the Newton-meter (N-m) • A N-m is also called a Joule (J) • 1 Joule of work is applying a force of 1 Newton for a distance of 1 meter.

  7. Power • Work has no time component. You do the same work if you carry a box up a set of stairs in 10 seconds or 10 minutes. • Why do you feel more fatigued when you carry the box up the stairs in 10 seconds. • The concept of power addresses this. Power is the rate at which work is done.

  8. Power • Power is the work done divided by the time it takes. P = W/t • When something is more powerful, it means it can do the same amount of work in less time.

  9. Power • Power is measured in Joules/second (J/s) which is also know as a watt (W). • One watt of power is done when one Joule of work is done for one second. • The unit of power is named in honor of James Watt who invented the steam engine.

  10. Power • In the U.S. we use both the watt and horsepower to measure power • The power of an engine is measured in horsepower • Electricity is measured in kilowatts • In the SI System both would be measured in kilowatts (kW) • 1 kilowatt = 0.75 horsepower

  11. Power • If you have a truck and you get a new lift gate which is twice as powerful; • How long will it take to lift a given load compared to the old lift gate? • How much more can it lift, in the same amount of time, compared to the old lift gate?

  12. Mechanical Energy • When a pile driver is lifted, it acquires the ability to do work on the pile when it is released and falls. • When you wind a watch or clock the spring inside acquires the ability to do work and keep the watch or clock running.

  13. Mechanical Energy • In both cases the object acquires something to allow it to do work: • The pile driver is pulled away from the pile – the attraction between the pile and the driver allows the driver to do work • Winding a watch or clock causes a spring to twist and tighten which allows the spring to do work • In both cases the “something” acquired is energy.

  14. Mechanical Energy • We will focus for now on two forms of mechanical energy: • Energy due to position (Potential Energy) • Energy due to movement (Kinetic Energy) • We can have energy as PE or KE or the sum of both

  15. Potential Energy • Energy stored in a object due to position is potential energy. • A stretched or compressed spring • A rock at the top of a hill • A student standing on a desk • A stretched rubber band

  16. Potential Energy • You can also have chemical potential energy, which is energy stored in the position of atoms in a compound, and their bonds. • When the positions and bonds of atoms are changed, the chemical potential energy can be used to do work • Energy of this type is stored in: • Fossil fuels • Batteries • Food

  17. Potential Energy • When work is required to elevate an object against gravity we have gravitational potential energy. • Water stored in the reservoir behind the Monksville Dam has graviational potential energy (GPE) which is released when the water flows over the dam and falls. (This is used to generate electricity in many places including the Hoover Dam) • A raised pile driver has gravitational potential energy, which is released when the driver falls.

  18. Potential Energy • The amount of gravitational potential energy an object has is equal to the work done to get the object in position. W = F x d

  19. Potential Energy • The upward force required to lift an object at a constant velocity is equal to the weight. F = mg GPE = m x g x h

  20. Potential Energy • How much work is done by moving a 100 N rock across a 10 m room? • What is the change in PE of the rock?

  21. Potential Energy • How much work is done if you lift the same rock 1 m? • What is the power expended if you lift the rock in 1 sec.? • What is the rock’s GPE?

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