Chapter 7 Energy

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Chapter 7 Energy

Work

• • • • Work = force * distance Work = mass * acceleration * distance. Units are Joules (J) In a equilibrium situation (where nothing is moving) work is zero.

Power

• • • • Power = work done / time Units are J/s or watt Power is a measure of who fast work is done. A lot of work done over a long time makes for low power. A little bit of work done in a very short time makes for a lot of power.

Mechanical Energy

• • • Where do we acquire the ability to do work? Attracting bodies, molecular compression, rearrangement of electrical charges, ect. Mechanical Energy can be classified as: – Kinetic Energy – Potential Energy

Potential Energy (PE)

• • • • • Energy in a stored state. Units of Joules (J) This stored energy could be chemical, gravitational, stretched or compressed springs. Gravitational Potential Energy = weight*height PE = mgh

Kinetic Energy (KE)

• • • • Energy in motion Units of Joules (J) KE is dependant on mass, but is more dependant on velocity. KE = 0.5 * mass * speed 2

Work-Energy Theorem

• • • • Work = D KE Work is not a form of Energy, but a means by which it is transferred from one system to another. This means the net work, which is based on the net force. Applies to accelerating and decelerating objects.

Conservation of Energy

• Energy cannot be created or destroyed. It can only be changed from one form to another, but the total energy is the same.

Machines

• • • Is a device for multiplying forces. – Levers – Pulleys Work in = Work out All machines that multiply force due so at the expense of distance.

Efficiency

• • In the real world machines cannot use 100% of the energy put into them Efficiency = Useful energy / total energy input

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