# 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