Converting miles per hour (mph) to pound-force (lb-force) is not directly possible, as they measure different physical quantities: speed and force, respectively. To calculate force from a deceleration scenario, one can use the formula F = MA, where F is force, M is mass in pounds, and A is acceleration. The discussion highlights confusion about the conversion process, emphasizing the need for context when discussing units of measurement. Ultimately, speed cannot be converted into force without additional information regarding mass and acceleration. Understanding the distinction between these units is crucial for accurate calculations.