IB DP Physics: SL复习笔记2.4.3 Conservation of Linear Momentum

Conservation of Linear Momentum

  • The principle of conservation of linear momentum states:

The total momentum before a collision = the total momentum after a collision provided no external force acts

  • Linear momentum is the momentum of an object that only moves in one dimension
  • Momentum is a vector quantity
    • This means oppositely-directed vectors can cancel each other out resulting in a net momentum of zero
    • If after a collision an object starts to move in the opposite direction to which it was initially travelling, its velocity will now be negative
  • Momentum, just like energy, is always conserved

conversation-of-momentum

The conservation of momentum for two objects A and B colliding then moving apart

External & Internal Forces

  • External forces are forces that act on a system from outside of it e.g. friction and weight
  • Internal forces are forces exchanged by the particles in the system e.g. tension in a string
  • Forces that are internal or external will depend on the system itself
  • For example, in a mass-spring system:
    • The internal force is the spring force
    • The external force is the weight

3.2.1.1-External-and-internal-forces-on-a-mass-on-a-spring

Internal and external forces on a mass on a spring

  • Systems with no external forces may be described as ‘closed’ or ‘isolated’
    • These are keywords that refer to a system that is not affected by external forces
  • For example, a swimmer diving from a boat:
    • The diver will move forwards, and, to conserve momentum, the boat will move backwards
    • This is because the momentum beforehand was zero and no external forces were present to affect the motion of the diver or the boat

Worked Example

Trolley A of mass 0.80 kg collides head-on with stationary trolley B whilst travelling at3.0 m s–1. Trolley B has twice the mass of trolley A. On impact, the trolleys stick together.

Using the conversation of momentum, calculate the common velocity of both trolleys after the collision.

Worked-example-1D-momentum-quesions-solution

Worked Example

The diagram shows a car and a van, just before and just after the car collided with the van, which is initially at rest.5.9.2-WE-Conservation-of-Momentum-Question-image-

Use the idea of conservation of momentum to calculate the velocity of the van when it is pushed forward by the collision.

5.9.2-Conservation-of-Momentum-WE-15.9.2-Conservation-of-Momentum-WE-2-1

Exam Tip

If it is not given in the question already, drawing a diagram of before and after helps keep track of all the masses and velocities (and directions) in the conversation of momentum questions.

 

 

 

 

 

转载自savemyexams

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