Physics on the move

Welcome to Physics on the Move!

In this chapter, we are going to look at how things move in our everyday world. We see things moving all the time—cars driving, people running, and even the wind blowing. But how do we measure these movements, and more importantly, how do we stay safe when things move very fast?

This topic is part of the Global Challenges section because road safety and transport are huge issues for people all over the planet. Don't worry if physics usually feels a bit "maths-heavy"—we will break everything down step-by-step!

1. Typical Speeds in Everyday Life

To understand movement, we first need a "feel" for how fast things usually go. In your exam, you might be asked to recall these typical speeds.

Common Everyday Speeds:

• Walking: ~1.5 \(m/s\)
• Running: ~3 \(m/s\)
• Cycling: ~6 \(m/s\)
• Cars in a built-up area: ~13 \(m/s\) (about 30 mph)
• Aeroplanes: ~250 \(m/s\)
• Speed of Sound in air: ~330 \(m/s\)
• Wind speed: ~5–20 \(m/s\) (depending on how breezy it is!)

Quick Review: The Speed Formula

Before moving on, remember the basic formula you learned in earlier years:
\( \text{speed (m/s)} = \frac{\text{distance (m)}}{\text{time (s)}} \)
Memory Trick: Remember the "Distance, Speed, Time" triangle!

Did you know?

The speed of sound can change depending on the temperature of the air, but for your GCSE, 330 \(m/s\) is the magic number to remember!

2. Acceleration in the Real World

Acceleration is just a fancy word for how quickly something speeds up or slows down. In physics, we measure it in meters per second squared (\(m/s^2\)).

For most "everyday" objects like family cars, acceleration is usually around 2 to 5 \(m/s^2\). However, if you drop an object, gravity pulls it down with an acceleration of 10 \(m/s^2\). This is known as acceleration in free fall.

Common Mistake to Avoid: Students often confuse speed and acceleration.
Speed is how fast you are going right now.
Acceleration is how much your speed changes every second.

Key Takeaway: Most things you see daily accelerate at less than 10 \(m/s^2\). Anything higher than that usually involves a very powerful engine or a serious collision!

3. Human Reaction Time

Before a driver can hit the brakes, their brain has to see the danger and tell their foot to move. This delay is called reaction time.

A typical human reaction time is between 0.2s and 0.25s. It might sound fast, but at high speeds, a car travels a long way in a quarter of a second!

How do we measure it? (The Ruler Drop Test)

You can test this with a friend!
1. Your friend holds a ruler vertically.
2. You put your fingers at the bottom (at the 0cm mark) without touching it.
3. They drop the ruler without warning, and you catch it as fast as you can.
4. The further the ruler falls before you catch it, the slower your reaction time is.

Encouraging Note: Don't worry if your reaction time is a bit slower today—factors like tiredness, distractions, or even being ill can change it!

4. Stopping Distance: The Big Equation

This is one of the most important parts of the chapter. When a car needs to stop, the total distance it travels is made of two parts:

Stopping Distance = Thinking Distance + Braking Distance

Part A: Thinking Distance

This is the distance the car travels while the driver is reacting.
Factors that increase thinking distance:

• Tiredness (Brain works slower)
• Alcohol or Drugs
• Distractions (Using a mobile phone)
• High Speed (The faster you go, the further you travel during your reaction time)

Part B: Braking Distance

This is the distance the car travels after the brakes are applied.
Factors that increase braking distance:

• Icy or Wet roads (Less friction/grip)
• Worn-out tires or brakes
• Heavier car (More mass takes more force to stop)
• High Speed (This has a massive effect on braking distance!)

Quick Review Box:

Thinking is about the Driver.
Braking is about the Car and Road.
Speed affects BOTH.

5. The Dangers of Large Decelerations

When a car stops very suddenly (like in a crash), it undergoes a large deceleration. This is dangerous because of the huge forces involved.

Physics tells us that \( \text{Force} = \text{mass} \times \text{acceleration} \).
If the deceleration (negative acceleration) is very large, the Force on the passengers will be very large, which can cause serious injury to organs and bones.

Safety Features: Saving Lives with Time

Modern cars have safety features like crumple zones, airbags, and seatbelts. They all work using the same bit of physics:
They increase the time it takes for the person to stop.

If you increase the time a crash takes, you decrease the deceleration. If the deceleration is lower, the force on your body is lower.

Analogy: Think of jumping onto a fluffy mattress vs. jumping onto a concrete floor. The mattress "gives" and increases the time you take to stop, so it hurts much less!

Key Takeaway: Large decelerations create large forces. Safety features work by stretching out the time of the impact to keep the force small.

6. Summary and Final Tips

- Memorize the typical speeds (Walking 1.5, Running 3, Cycling 6).
- Remember that Stopping Distance is a two-part story (Thinking + Braking).
- If you are asked why a car safety feature works, the answer is almost always: "It increases the time taken to stop, which reduces the force."

You've got this! Just remember to keep the driver factors and the car factors separate in your mind, and the rest will fall into place.