Physics (9203) | Safety in public transport

Welcome to Safety in Public Transport! (Physics 9203)

Hi everyone! In this chapter, we are going to connect the concepts of Forces and Motion that we’ve already studied to something really important: keeping people safe when they travel.
This isn't just about rules; it's about understanding how physics principles—like inertia and momentum—are used to design vehicles that protect us. Don't worry if this seems tricky at first; we’ll break down these big ideas using simple, relatable examples!

Section 1: The Role of Inertia – Why You Keep Moving

When you are sitting on a bus or train, you are travelling at the same speed as the vehicle. If the vehicle suddenly changes its speed or direction, your body tries to keep doing what it was doing before. This resistance to change is called Inertia.

What Happens During Sudden Stops?

Newton’s First Law of Motion, sometimes called the Law of Inertia, states that an object will remain in its state of rest or uniform motion unless acted upon by an external resultant force.

Imagine you are standing in a stationary train (at rest). If the train suddenly starts moving, your feet move forward with the train, but your upper body wants to stay at rest. You feel like you are being pushed backward.

Now, the crucial safety scenario: the vehicle is moving fast, and the driver applies the brakes suddenly (a quick deceleration).

• Your seat belt (or the friction from your seat) stops your lower body.
• However, your head and upper body are still moving forward at the original speed of the vehicle due to inertia.
• If there is nothing to stop you, you will continue moving forward until you hit the dashboard, the seat in front, or the floor.

Analogy: Think about riding a skateboard. If you hit a small stone, the skateboard stops, but your body flies forward because of inertia!

Key Takeaway (Inertia)

Safety features like seatbelts are needed because inertia causes passengers to keep moving forward when a vehicle suddenly stops or slows down.

Section 2: Forces, Deceleration, and Impact Time

In a crash or sudden stop, the vehicle experiences a rapid deceleration. We need to understand how this deceleration relates to the force experienced by the passengers.

The Force of Impact (F = ma)

We know from Newton’s Second Law that Force is related to mass and acceleration:

\[F = ma\]

If a vehicle (and the passengers inside) has a certain mass (\(m\)), the only way to reduce the force (\(F\)) exerted on the passengers during a crash is to reduce the deceleration (\(a\)).

But wait—if the vehicle must stop from 50 km/h to 0 km/h, the total change in speed is fixed. How can we reduce the force?

The Secret: The Momentum-Time Relationship

A better way to understand crash physics is through momentum. Momentum (\(p\)) is the product of mass and velocity (\(p=mv\)). In a crash, the passenger's momentum must change from a high value (moving fast) to zero (stopped).

The force exerted on an object is equal to the rate of change of its momentum:

\[F = \frac{\text{Change in Momentum}}{\text{Time taken for the change}}\]

Or using symbols:

\[F = \frac{\Delta p}{\Delta t}\]

Since the change in momentum (\(\Delta p\)) is fixed (you must stop!), the only variable we can control to reduce the force (\(F\)) is the time taken for the impact (\(\Delta t\)).

• If the time taken for the change in momentum is very short (a rigid crash), the force experienced is huge.
• If the time taken for the change in momentum is made longer (a "soft" crash), the force experienced is smaller.

Memory Aid: Longer Time (\(\Delta t\)) = Less Force (\(F\)).

Analogy: Catching a hard ball. If you stop the ball instantly (short time), it hurts your hand (large force). If you move your hand backward as you catch it (longer time), it hurts much less (smaller force).

Section 3: Designing Vehicles for Safety (Increasing Impact Time)

Here we look at the specific features in public transport (and cars) designed to increase the time of impact and reduce the forces experienced by passengers.

1. Crumple Zones

Crumple zones are areas at the front and rear of a vehicle that are designed to deform (crush) progressively during a collision.

• How they work: Instead of the vehicle stopping instantly, the crumpling process takes a small amount of extra time.
• Physics Link: This deformation increases the impact time (\(\Delta t\)), which means the deceleration is spread out over a slightly longer period. This reduces the maximum force (\(F\)) on the passenger compartment.
• Did you know? The rigid passenger cell remains mostly intact, protecting the occupants, while the exterior zones absorb the energy by crushing.

2. Seat Belts

Seat belts are essential safety features that serve two main physics functions:

A. Preventing Forward Movement (Inertia Control)

The belt prevents the passenger from hitting the steering wheel, dashboard, or other fixed objects due to inertia.

B. Increasing Impact Time and Spreading Force

• A seat belt is slightly elastic; it stretches a tiny bit during a severe collision. This stretching action increases the time (\(\Delta t\)) over which the body comes to a stop, significantly reducing the force.
• The belt spreads the stopping force over a wide, strong area of the body (the chest and pelvis), preventing the force from concentrating on small, vulnerable parts (like the neck).

Common Mistake to Avoid: A student might think the seatbelt stops you instantly. It doesn't! The slight stretching and yielding are crucial for reducing the force.

3. Air Bags

Air bags are designed to deploy immediately upon collision, acting as a soft cushion between the passenger and the rigid interior of the vehicle.

• How they work: An air bag provides a soft barrier which compresses when struck by the passenger's head or chest.
• Physics Link: Like crumple zones and seat belts, the process of compressing the air bag increases the time (\(\Delta t\)) taken for the head and chest to lose momentum. This results in a smaller average force and helps prevent serious injury.
• They also distribute the force over a wider area of the head and upper body.

Encouraging Note: You can see that all these features are applying the same core physics principle: If you can't change the momentum, change the time!

Summary Table: Safety Features and Physics Principles

Safety Feature	Physics Principle in Use	Result (Effect on Passenger)
Crumple Zones	Deformation absorbs energy	Increases \(\Delta t\), reduces force on passenger cell.
Seat Belts	Stretching/Elasticity	Increases \(\Delta t\), prevents inertia-driven movement, spreads force.
Air Bags	Cushioning and compression	Increases \(\Delta t\), spreads force over head/chest.

Section 4: Total Stopping Distance (Extended Focus)

While safety features help during a crash, avoiding the crash entirely is the best protection. The total distance a vehicle travels before coming to a complete stop is called the Stopping Distance.

Stopping Distance is made up of two parts:

\[\text{Stopping Distance} = \text{Thinking Distance} + \text{Braking Distance}\]

1. Thinking Distance (Reaction Time)

This is the distance the vehicle travels from the moment the driver sees a hazard until they apply the brakes.

• Physics/Force Link: During this time, the car is still moving at the initial speed, and no resultant braking force has been applied yet.
• Factors increasing Thinking Distance: Tiredness, distractions (e.g., mobile phones), alcohol/drugs, high initial speed.

2. Braking Distance (Deceleration)

This is the distance the vehicle travels after the brakes are applied until it stops completely. The brakes apply a frictional force to the wheels, causing deceleration.

• Physics/Force Link: The braking distance depends on the braking force and the initial speed.
• Factors increasing Braking Distance: Poor road conditions (ice, wet roads lead to less friction), worn tires, faulty brakes, high initial speed.

The Effect of Speed on Stopping Distance

This is a critical point! Both thinking and braking distance increase dramatically with speed:

• Thinking distance is directly proportional to speed (double the speed = double the thinking distance).
• Braking distance is proportional to the square of the speed (\(v^2\)). If you double your speed, the braking distance increases by a factor of four (2 x 2 = 4).

This explains why speed limits are vital for safety; reducing speed has a massive impact on the total stopping distance and the severity of any potential collision.

Key Takeaway (Stopping): A small increase in speed leads to a disproportionately large increase in the braking distance, making crashes at higher speeds much more dangerous because the vehicle covers far more distance before the necessary stopping force can halt it.

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That wraps up our look at safety! Remember that the physics you learn about forces, momentum, and time is directly applied every day to save lives in public transport.