Momentum

• Momentum

  Defined by the equation: p = mv, where p = momentum in kilogram metre per second (kg m/s), m = mass in kilograms (kg), v = velocity in metres per second (m/s)
• An object at rest (i.e v = 0) has no momentum
• Momentum

  • Keeps an object moving in the same direction, making it difficult to change the direction of an object with a large momentum
  • Since velocity is a vector, the momentum of an object also depends on its direction of travel
  • Momentum can be either positive or negative
• An object travelling to the right has positive momentum


  An object travelling in the opposite direction (to the left) will have negative momentum
• The tennis ball's momentum is negative when it moves in the opposite direction to which it initially was travelling in
• Momentum changes if

  1. The object accelerates (speeds up) or decelerates (slows down)

  2. Changes direction

  3. Its mass changes
• Conservation of Momentum

  The total momentum before a collision = The total momentum after a collision
• Closed system

  • The energy within the system is constant and there is an absence of external forces (e.g. friction)
• Since momentum is a vector quantity, a system of objects moving in opposite directions (e.g. towards each other) at the same speed will have an overall momentum of 0 since they will cancel out
• Momentum is always conserved over time
• Force and momentum equation

  Force = Rate of change in momentum = (Final momentum - Initial momentum) / Change in time
• Newton's Third Law

  Whenever two bodies interact, the forces they exert on each other are equal and opposite
• Newton's third law explains the forces that enable someone to walk
• When two objects collide, both objects will react, generally causing one object to speed up (gain momentum) and the other object to slow down (lose momentum)
• For objects of equal mass, they will have equal accelerations. For objects of unequal mass, they will have unequal accelerations
• Contact time

  The time in which the person is in contact with what they have collided with
• Safety features

  • Designed to absorb energy upon an impact by changing shape
  • The main vehicle safety features are crumple zones, seat belts and airbags
  • For a given force upon impact, these absorb the energy from the impact and increase the time over which the force takes place
• The usefulness of safety equipment depends on two main factors: mass and velocity
• Seat belts

  Designed to stop a passenger from colliding with the interior of a vehicle by keeping them fixed to their seat in an abrupt stop. They are designed to stretch slightly to increase the time for the passenger's momentum to reach zero and reduce the force on them in a collision
• Airbags

  Deployed at the front on the dashboard and steering wheel when a collision occurs. They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact
• Crumple zones

  Designed into the exterior of vehicles at the front and back. They are designed to crush or crumple in a controlled way in a collision to increase the time over which the vehicle comes to rest, lowering the impact force on the passengers
• Crash mats

  Thick and soft to offer shock absorption of the force created by the person landing on the mat. They increase the contact time over which the person's momentum is reduced, creating a smaller impact force and a lower chance of injury
• Thin crash mats are suitable for low-impact activities where a person/adult will fall with a low velocity such as falling from lower heights
• Safety features are intended to reduce the chance of serious injury but do not completely prevent it in all cases