MAP 3 The Physics of Road Traffic Collisions JB

Skidding and Newton's Laws

  • Newton’s 1st Law: Objects at a fixed velocity tend to maintain that velocity and resist changes to their state of motion.

  • Skidding: Occurs when a vehicle loses traction between tires and road surface, leading to a loss of control.

Friction

  • Definition: Friction is a force that opposes motion, always acting in the direction opposite to the movement of the object.

  • Frictional Force Equation:

    • F_f = -mg (where F_f is the frictional force, m is mass, and g is gravitational acceleration).

    • The coefficient of friction varies with different surfaces and states (static vs kinetic).

Types of Friction

Static Friction

  • Causes: At the molecular level, static friction is due to temporary bonds BEING FORMED between the contact surfaces.

Kinetic Friction

  • Characteristics: Weaker than static friction due to relative motion preventing strong attractive molecular bonds from forming.

Threshold of Motion

  • Static vs Kinetic Friction Relationship: Static friction is greater than kinetic friction.

  • The object remains static until the applied force exceeds the static friction, leading to motion (break the threshold).

Example of Static Friction

  • Case Study: A wooden block with a weight of 100 N on concrete shows the coefficient of static friction = 0.1.

    • Applied Force: 5 N, not sufficient to overcome static friction (10 N required).

Rolling and Kinetic Friction

Rolling Friction

  • Occurs when a wheel rolls without slipping, creating maximum traction (static situation). due to static contact (-mewmg) There is no relative horizontal movement providing traction & control.

Kinetic Friction During a Skid

  • Kinetic friction reduces traction because the contact remains static during a skid. reducing traction & control

Friction in Car Skidding

  • A car fails to navigate turns if it cannot generate sufficient frictional force directed towards the center of the turn.

  • This limits the maximum speed to a rate where the tires dont slip.

Effects of a Skid

  • Difficulty in Recovery: Once a skid begins, magnitude of frictional force decreases, making it harder to regain control.

  • Inertia causes the vehicle to continue moving straight, often towards obstacles.

Braking Distance Calculation

  • Formula: s = (u^2 - v^2)/(-2gµ)

    • where s = braking distance, g = gravity (9.81 m/s²), u = initial speed, v = final speed, µ = coefficient of friction.

    • Doubling initial speed increases stopping distance four times.

Law of Inertia

  • Defines that an object at rest stays at rest, and an object in motion continues in motion unless acted upon by an external force, observed in sudden starts/stops of vehicles.

Inertia and Road Traffic Trauma

  • Translational Inertia: Resistance to change in linear motion.

  • Rotational Inertia: Resistance to change in rotational motion.

Whiplash

  • Definition: Physical damage to the upper spinal column due to inertia during rear-end collisions.

  • Mechanism: Head inertia leads to backward motion then forward motion against restraints. and because the head weighs about 5 kg its prone to tearing muscles ligaments and other soft tissues

Active Head Restraints

  • Function: Oppose effects of whiplash by reducing head movement relative to the torso.

Blunt Trauma Considerations

  • Inertia of internal organs during impacts can cause injuries to the thoracic cavity (lungs, heart, liver).

Brain Trauma

  • Coup-contrecoup injuries occur due to the inertia of the brain against the skull during impacts.

Quiz and Revision

  • Question: Whiplash injuries occur in rear-end collisions due to inertia; justification through Newton’s 1st Law.

  • Additional Quiz Question: Push force vs friction force on a crate moving with constant velocity (options a, b, c).

  • Revision Note: Lectures 5 and 6 cover similar topics; study both together.