P10 - Forces in motion

• F = ma, where F is force (N), m is mass (kg), and a is acceleration (m/s^2).
• The acceleration of an object is directly proportionate to the resultant force, and inversely proportionate to the mass of the object.
• The inertia of an object is its tendency to stay at rest or continue in uniform motion.
• The inertial mass of an object is a measure of the difficulty of changing the objects velocity.
• Terminal velocity is where the velocity of an object (usually falling) becomes constant as the force of the weight and the force of air resistance equalise due to acceleration.
• The greater the resultant force of an object, the greater the object's acceleration.
• The greater the mass of an object, the smaller its acceleration for a given force.
• The weight of an object is the force acting on the object due to gravity.
• The mass of an object is the quantity of matter in the object.
• An object acted on only by gravity accelerates at about 10 m/s².
• When an object is moving at terminal velocity, the resultant force on it is 0.
• Friction and air resistance oppose the driving force of a vehicle, slowing it down.
• stopping distance = thinking distance + braking distance
• Braking distance can be affected by:

  • high speed
  • poor weather conditions
  • poor vehicle maintenance
• Thinking distance is affected by:

  • Poor reaction time (tiredness)
  • Alcohol
  • Drugs
  • Distractions
  • High speed
• p = mv, where p is momentum (kgm/s), m is mass (kg) and v is velocity (m/s)
• A closed system is a system in which the total momentum before an event is the same as the total momentum after the event (commonly called conservation of momentum).
• Momentum is a vector quantity.
• When two objects push each other apart, they move with different speeds if they have unequal masses, and with equal and opposite momentum, so their total momentum is 0.
• (mass of object 1) x (velocity of object 1) = (mass of object 2) x (velocity of object 2); when two objects recoil from each other
• impact force in a crash = change of momentum / impact time; the shorter the impact time, the greater the force.
• Cycle helmets and cushioned surfaces reduce impact forces by increasing the impact time.
• Seat belts and air bags spread the impact force across the chest and increase impact time.
• Side impact bars and crumple zones give way in an impact, and so increase the impact time.
• An object is called elastic if it returns to its original shape after removing the force deforming it.