Oscilloscopes

- A signal generator, such as a keyboard, generates an electrical signal that we can transmit to a speaker to create a sound wave.
- A microphone transforms sound waves into an electrical signal, which an oscilloscope can display on its screen.
- The oscilloscope plots a graph of amplitude (voltage of the electrical signal) on the y-axis vs. time on the x-axis.

→ Like drawing a graph.
→ Time period of one whole wave is shown.

EXAMPLES
- The time period allows you to calculate frequency.

- By attaching a signal generator to a speaker, you can generate sound waves of a specific frequency.
- You can use two microphones and an oscilloscope to determine the wavelength of the sound waves generated.
- The oscilloscope displays the detected waves at each microphone as separate waves.
- Place both microphones next to the speaker at first, then slowly move one away until the two waves align on the display but are exactly one wavelength apart.
- Measure the distance between microphones to find exactly on display find λ.
- Use v = fλ to find the speed of sound in air.

Hearing

- Range of human hearing is 20Hz to 20 000 Hz.
- Any frequency above that range is called ultrasound.

- (Air) particles vibrate and hit the eardrum which vibrates. These vibrations cause the bones of the ear to vibrate. Those vibrations travel to the cochlea and produce electrical signals which travel in the nerve.

- Repeated exposure to loud noises and age can damage hearing.

Sound

- Sound waves are longitudinal waves produced when molecules vibrate.
- Sound needs a medium to travel through; it can’t travel in a vacuum or space. 
- Sound waves produced in an area of high pressure occur at a rarefaction and low pressure at a compression.
- Rarefaction and low pressure occur at a compression point in an area of high pressure.
- Sound travels at different speeds in different materials. In solids, sound travels faster than in air as the vibrating particles are closer together, so the energy travels along the solid faster.
- Sound waves can be reflected/refracted like all other waves.
- Speed of sound in air: 330 m/s - 340 m/s
- Sound travels through materials by making the particles in them vibrate. The materials pass on energy from particle to particle using these vibrations.
- The hotter the material, the faster the particles vibrate. Sound travels faster in hot air compared to cold air as the particles vibrate faster and have more kinetic energy.
- Sound waves will be reflected by hard materials and absorbed by softer ones like carpets.

Experiments

How can we use echoes to determine the speed of sound?

1) Two people plus the side of a building are needed.
→ Stand 100 m away from the building and hit 2 wooden blocks together.
→ The building reflects the sound waves back to you.
→ Clap again when you hear the echo.
→ The other person measures the total time for 10 intervals between claps; this is the amount of time required. Divide this by 20. (10, then 2).
→ Divide the distance (100 m) by the time taken to get the speed of sound in air.

2) Have one person stand 100 m away from the other person/people. 
→ One person will have 2 wooden blocks, which they will hit together.
→ The other person/people will have a stopwatch and time from the time they see the blocks hit together till the point they hear the blocks clap. This is the time taken.
→ You can divide the distance by the time to find the speed of sound in air.

- Inaccuracies: The time interval between seeing the blocks and hearing them is difficult to measure accurately due to its short duration.
- How can we make it more accurate? Increase the distance, perhaps to more than 1 km instead of 100 m. Ensure that multiple individuals measure the time interval repeatedly. This makes it easier to spot anomalies and have accurate results. 
- The temperature of the air matters, as sound travels slower in colder air than it would in hotter air.

Frequency, Pitch, Amplitude and Volume

- FREQUENCY: The number of full wavelengths per second in vibrations, expressed in hertz (Hz).

- AMPLITUDE: Maximum displacement of an oscillation from the equilibrium position. 

- In areas of low pressure, sound waves occur at rarefactions, and in areas of high pressure, they occur at compressions.

EXAMPLES:

- The higher the pitch, the higher the frequency.


- The louder the sound, the higher the amplitude.

1) Sound waves have the same loudness but differ in pitch.

2) Identical pitch but different loudness. 

Force and Friction

- Force: Push or pull 

Types of forces:

- Gravity (weight): Acts straight downwards. 
- Lift refers to the force exerted by an aeroplane wing.
- Thrust, such as in the nose of an engine or rocket, is used to speed up something.
- Tension: e.g., pull in rope.
- Drag/Air resistance/friction slows things down. 
- The reaction force acts both away from and perpendicular to a surface.
- The direction of the electrostatic force between two charged objects depends on the type of charge.

Friction:

- If there is no force propelling an object along, it will always slow down and eventually stop due to friction.
- To maintain a constant speed, objects require a driving force to mitigate friction.

Friction occurs in three main ways:

1) Static friction: The friction that occurs between gripping solid surfaces.
2) Lubricant reduces solid surfaces sliding past each other.
3) Resistance/Drag: From fluids 
→ Higher drag = Lower top speed. 
→ Reduce by streamlining.

- Friction always increases as speed increases.

Momentum

Property that moving objects have:
→ More momentum = Harder to stop
→ More mass/velocity = More momentum

Momentum (kgm/s) = Mass (kg) x Velocity (m/s) 
p = m x v 

Momentum is a vector quantity.

- Law of conservation of momentum: In a closed system, the total momentum before an event = total momentum after an event.
- Closed system = No external forces are acting on the object in the system. 

E.g.

Momentum Calculations

P total = (m₁v₁) + (m₂v₂)

Examples:
1) Van - 2500 kg at 10 m/s - collides with stationary car - 1500 kg. Find velocity after collision. 

P (before) = Car (1500 x 0) + (1500 x 0) = 25,000 kgm/s 
P (after) = 2500 kgm/s ∴ p before = p after → Conservation 

P = mv 
25000 = (1500 + 2500)v 
25000 = 4000v
v = 6.25 m/s

Force and Momentum change

- Force is the rate of change in momentum.
- Slowing down = Negative force
- Speeding up = Positive force.

- When a force acts on a moving or able-to-move object, momentum will change because:
1) A resultant force will induce acceleration,
2) resulting in a change in velocity.
3) Momentum is the product of mass and velocity, so
4) if the velocity changes, so does its momentum.

Soft objects, such as crash mats, serve as safety features for the following reasons:
1) Softer objects take longer to stop or lose momentum, which
2) reduces the rate of change of momentum positively,
3) reduces the force on the object.

Example:

1) Football travels at 10 m/s when it hits the goal net, taking 0.02 s to stop. Football = 450g. What is the impact force? 

v = Final → Initial → 0 - 10 = -10 m/s (opposite direction) 
m = 0.45 kg
t = 0.02

2) Why are items less likely to break when wrapped in cardboard or bubble wrap? (3)
- Bubble wrap lengthens the time it takes for an item to fall and hit the ground.
- This decreases the overall rate of momentum change.
- Bubble wrap reduces the impact of the force. 

Moments

- A force can cause an object to rotate around a pivot.

- Moment = Turning effect of a force
- A longer-handled spanner makes it easier to turn the bolt. ∴ of moments → You apply a smaller force over a greater distance.

- Pivot = Point where an object can rotate around. 

Principles of Moments

- Anticlockwise and clockwise moments must be equal = applies to balanced (e.g., stationary) objects.
- If an object is balanced, then the total anticlockwise moment about a pivot = the total clockwise moment about that pivot. 

Example Q: A seesaw isn’t turning. Clockwise force = 8N + distance = 0.4m. Anti-clockwise force = ?, distance = 0.5 m.

Since seesaw isn’t turning, it’s balanced.
∴ M₁ = M₂
∴ 0.5x = 0.4 x 8 → 0.5x = 3.2Nm
3.2 ÷ 0.5 = 6.4N

Levers

- Levers can be used to transmit the rotational effect of a force from one side of a pivot to another. 

- The longer the lever, the less effort required.
- Levers and gears can be used to increase the moment by allowing us to apply the force further from the pivot.
→ Less force is required to get the same moment.
→ Lever acts as a force multiplier.

→ If moments are unbalanced, the object will turn one way.

Centre of Mass

- An object will balance when its centre of mass is within its base.

Examples:

- Easy to find the centre of mass for flat, symmetrical objects. 
→ Along axes of symmetry - draw at least 2 lines of symmetry. 

Finding the centre of mass for irregular objects

1) Put a hole in one corner of the shape and suspend it from the clamp stand rod.
2) Use a plumb line from the same point and wait till they stop moving.
3) Draw a line across the plumb line.
4) Repeat but suspend the shape from a different pivot point.
5) The centre of gravity is where lines cross/meet. 

- A freely suspended object will swing until the center of gravity is directly under the suspension point.

Supporting Forces

If Tb = Pivot
→ Tb = Clockwise moment
= Tb = 6 x 600 = 3600
= Ta = 3600 ÷ 9 = 400N

200 + 400 = 600N → weight of load