- a push or a pull, just what we usually mean.
- Units Newtons. 1 N = 1 kg m s
^{-2}. I.e. 1 N will give a mass of 1 kg an acceleration of 1 m s^{-2}. 1N is a gentle push. - Takes about 10N (9.8N) to hold up 1kg.
- Takes about 9N to hold up text book.
- Takes about 700N to lift up a 70kg student(that is about 164lbs).
- NOTE: you MASS 70kg, you WEIGH 700N. Weight is a force.
- (How do you tell the difference?)
- Newton’ s First Law says that unless there is a force then a body will stay still or move at constant velocity for ever.
- Forces needed to make air move and give us sound. We will need to consider where they come from!

- •Pressure: Force/area; 1Pa = 1N/m-2 or 1Nm-2.
- •The air around us exerts ~100,000Pa or about same as supporting 15lbs of weight on each square inch of your body!
- •You can actually suck nearly that much but only blow about 1/6 of that.
- •Lying on floor pressure about 900Pa.
- On bed of nails each nail has about 1N!
- •In sound, pressure changes about 0.1Pa.

- Springs obey Hooke's Law (Robert Hooke, 1635-1703).
- F=-k x
- F is force that spring exerts
- x is the extension, stretch from rest
- k is Spring Constant-stiffness
- - sign means force opposes motion
- if you stretch (x>0) it tries to shrink back (F<0)
- If you compress it tries to expand.
- Plot of force against extension is a straight line so call this a Linear Force
- Spring Constant is slope of line.

- Amplitude is max excursion.
- Period is time to repeat = 1/Frequency.
- Thus Frequency=1/Period
- Units are Hertz: 1 Hertz is 1 period per second
- Phase is when wave goes through zero.

- Types of Energy
- Kinetic Energy, Potential Energy, Heat
- Electrical & Chemical (related to PE), Sound (bit of both)
- Heat is really a kind of localized kinetic energy but common enough to treat on its own.

- Energy of movement
- Name from a Greek word for motion
- Energy a body has simply because of its motion.
- Kinetic Energy =½ m v
^{2} - The heavier it is the more energy it has.
- The faster it goes the more energy it has.
- Energy grows MUCH faster with speed than with mass!

- Energy that an object has because of its position or state. There but not apparent.
- E.g. Gravitational P.E. = m g h.
- E.g. Spring P.E. = k x
^{2} - k is the spring constant
- x is the extension, the amount of stretch
- the stronger the spring the more energy it has
- the further you stretch it the more energy you store.

- K.E. ½ m v
^{2}greatest as mass goes through equilibrium where it is going fastest. - K.E. minimum at ends where mass is still.
- P.E. ½ k x
^{2}greatest at ends where spring is stretched most. - P.E. minimum at center, where spring is un-stretched from equilib.
- So total K.E. + P.E. = Constant.
- Total energy = Max. P.E. ∝ Amplitude2.

- Kinetic energy is in the movements of the air.
- Air is the mass and the velocity is the air moving from high to low pressure regions.
- Potential energy is stored in the springiness of the air.
- Air in the most compressed and most expanded regions has the most P.E.
- As the wave moves the energy is carried along at the speed of sound.