Welcome to "Physics on the Move"!
In this chapter, we are exploring one of our biggest Global Challenges: how to move people and things around safely. Whether you are walking to school, cycling, or driving a car, the laws of Physics are always at work. We will look at how fast things typically move, what happens when we need to stop in an emergency, and how cars are designed to keep us safe when things go wrong.
Don’t worry if some of the maths seems a bit fast at first—we will break it down step-by-step!
1. Typical Speeds and Accelerations
Before we can solve global transport problems, we need to know what "normal" looks like. In the exam, you might be asked to recall or estimate these values.
Typical Speeds (P8.1a)
You should know these average speeds for everyday travel:
- Walking: ~1.5 m/s
- Running: ~3.0 m/s
- Cycling: ~6.0 m/s
- Speed of Sound: ~330 m/s (This varies depending on the temperature of the air!)
- Wind speeds: Usually vary from 5 m/s to 20 m/s.
Typical Accelerations (P8.1b)
Acceleration is how quickly your speed changes.
- A family car accelerating normally: ~2 to 3 \(m/s^2\).
- An object falling freely (gravity): ~10 \(m/s^2\).
- A car involved in an emergency stop: ~6 to 10 \(m/s^2\).
Quick Review: Unit Conversions (P8.1c)
In Physics, we almost always use meters (m) and seconds (s). If a question gives you speed in km/h, you need to convert it!
The trick: To go from km/h to m/s, just divide by 3.6.
Key Takeaway:
Most human-powered movement is between 1.5 and 6 m/s. Anything much faster usually requires an engine!
2. Reaction Times (P8.1d)
Before a driver can even hit the brakes, their brain has to process what is happening. This is reaction time.
Did you know? The typical human reaction time is between 0.2s and 0.9s. It might sound fast, but at 70 mph, a car travels a long way in half a second!
Measuring Reaction Time
The most common way to measure this in class is the Ruler Drop Test:
- A friend holds a ruler vertically.
- You put your thumb and finger at the 0cm mark without touching it.
- They drop it without warning, and you catch it as fast as you can.
- The further the ruler falls before you catch it, the slower your reaction time.
What slows us down?
Your reaction time can be increased (made worse) by:
- Tiredness
- Distractions (like using a mobile phone)
- Alcohol or Drugs
3. Stopping Distance (P8.1e)
This is a vital concept for road safety. The Stopping Distance is the total distance a car travels from the moment the driver sees a hazard to the moment the car completely stops.
Memory Aid: Think of it as a two-part journey:
Stopping Distance = Thinking Distance + Braking Distance
Part 1: Thinking Distance
This is the distance the car travels while the driver is reacting.
Affected by: Speed and the driver's reaction time (tiredness, phones, etc.).
Part 2: Braking Distance
This is the distance the car travels after the brakes are applied.
Affected by:
- Speed: Faster cars take much longer to stop.
- Road conditions: Wet or icy roads have less friction.
- Vehicle condition: Worn brakes or "bald" tires.
Common Mistake:
Many students think alcohol affects braking distance. It doesn't! Alcohol affects the driver, so it only affects Thinking Distance. Icy roads affect the car, so they affect Braking Distance.
4. Estimating Stopping Distances (P8.1f - Higher Tier)
If you are taking the Higher Tier paper, you need to know how speed affects these distances.
- Thinking distance is directly proportional to speed. (Double the speed = Double the thinking distance).
- Braking distance increases with the square of the speed. (Double the speed = \(2^2\) = 4 times the braking distance!).
5. Forces and Safety (P8.1g, P8.1h, P8.1i)
When a car stops very suddenly (a large deceleration), it creates huge forces. These forces are what cause injuries in a crash.
The Danger of Large Decelerations
We use Newton’s Second Law: \(Force = mass \times acceleration\) (or deceleration).
If a car stops instantly, the deceleration is massive, which means the force hitting the passengers is massive too.
Safety Features
Car designers use Physics to save lives. Features like crumple zones, seatbelts, and airbags all have the same goal:
They increase the time it takes for you to stop.
Analogy: Imagine jumping off a wall. If you land with stiff legs on concrete, it hurts! If you land on a soft mattress, the mattress squashes, making your stop take longer and reducing the force on your legs. Crumple zones are the "mattress" for the car.
Estimating Forces (Higher Tier Only)
You might be asked to estimate the force in a typical road scenario.
Example: A 1000 kg car stopping from 15 m/s (about 30 mph) in 1 second.
1. Deceleration \(a = \frac{change in velocity}{time} = \frac{15}{1} = 15 m/s^2\)
2. Force \(F = m \times a = 1000 \times 15 = 15,000 N\)
Key Takeaway:
Increase the time, decrease the force. This is the golden rule of crash safety!
Quick Review Box
Stopping Distance = Thinking + Braking.
Thinking is about the Driver.
Braking is about the Car/Road.
Safety features work by increasing the time of a collision to reduce the force.
You've reached the end of the notes for Physics on the Move! Remember, the best way to master this is to practice using the formulas \(speed = \frac{distance}{time}\) and \(F = m \times a\) with real numbers. You've got this!