Motion and forces Revision Notes

Momentum and Collision Types

Momentum (p) is calculated as p = m × v, where m is mass and v is velocity. In a closed system, momentum before and after a collision is always the same if no outside forces like friction affect the outcome. This principle applies to all collisions, making it central to understanding interactions between moving objects.

Examples and Key Differences

Billiard balls colliding on a table (elastic collision) and two cars sticking together after a crash (inelastic collision) illustrate how momentum is involved in real scenarios. In elastic collisions, both momentum and kinetic energy are conserved. In inelastic collisions, momentum is still conserved but kinetic energy is not, as some energy transforms into heat and sound.

Momentum and Its Change

Momentum (p) is calculated as p = m × v, giving units of kg m/s. If an object’s mass or velocity changes, its momentum changes by Δp = (mv - mu). Here, u is initial velocity and v is final velocity. The direction of momentum is the same as the velocity direction, making it a vector quantity.

Newton’s Second Law and Rate of Momentum Change

In terms of momentum, Newton’s Second Law can be expressed as F = (mv - mu) / t. This forms a direct link between force, change in momentum, and time interval. A greater force or longer duration increases the change in momentum, which is important in understanding topics such as vehicle safety and sports impacts.

Components of Stopping Distance

The total distance a vehicle travels from the moment a driver detects a hazard to the moment it comes to a complete stop is called the stopping distance. It is crucial to remember that this distance is composed of two distinct parts: the thinking distance and the braking distance. Stopping distance = Thinking distance + Braking distance.

Thinking Distance

Thinking distance is the distance travelled by the vehicle during the driver's reaction time. This is the period between the driver identifying a need to stop and the moment they actually apply the brakes. Factors like speed, tiredness, alcohol, drugs, and distractions can all increase thinking distance.

Braking Distance

Braking distance is the distance travelled by the vehicle from the moment the brakes are applied until the vehicle comes to a complete stop. Factors affecting braking distance include initial speed, condition of tyres and brakes, road conditions (e.g., wet or icy), and the vehicle's mass. Higher speeds significantly increase braking distance.

Components of Stopping Distance

Stopping distance is the sum of thinking distance (distance during the driver's reaction) and braking distance (distance after brakes are applied). Both parts are affected by different sets of factors and together they determine how far a vehicle will travel before stopping in response to a hazard.

How Various Factors Affect Stopping Distance

Greater vehicle mass gives less deceleration from a given braking force, so braking distance is longer. Higher speeds increase both the distance covered during a driver’s reaction and the braking distance, which rises with the speed squared. A slower reaction (from distraction, tiredness, or drugs) increases only thinking distance. Worn or faulty brakes, poor road conditions (like wet or icy roads), and less friction from smooth or worn tyres all increase braking distance by reducing how quickly the vehicle can slow down.