Newton's Laws and Momentum

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  • Created by: lucysnell
  • Created on: 29-01-16 20:44

What is momentum?

  • Momentum is a physical quantity that a moving object has
  • The momentum of an object depends on its mass (m) and velocity (v)
  • Momentum is a VECTOR quantity - therefore has magnitude and direction
  • Momentum is always conserved
  • Hence, total momentum before a collision = total momentum after a collision
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Collisions - Elastic/inelastic?

Elastic Collision

  • Momentum is always conserved
  • Kinetic energy is conserved
  • No energy is dissapated to the surroundings

Inelastic Collision

  • Momentum is always conserved
  • Kinetic energy is not conserved
  • Energy is dissapated to the surroundings eg. heat, sound
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Impulse = change in momentum

Newton's Second Law says - the force is equal to the rate of change of momentum

  • F = (mv-mu)/t .... rearrange to get Ft = mv-mu
  • Impulse = average force x time
  • Therefore, the force of an impact can be reduced by increasing the time of the impact
  • This is why cars have crumple zones
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Newton's First Law

An object will continue in a state of rest of of uniform motion unless acted upon by a resultant force.

  • If forces are not balanced eg. there is a resultant force, then the overall resultant force will cause the object to accelerate
  • Acceleration of an object can mean a change in direction or speed, or both
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Newton's Second Law

The rate of change of momentum of an object is directly proportional to the resultant force on the object and in the same direction.

  • If mass is constant then F=ma applies
  • The more force you have on a mass, the more the mass accelerates
  • For a given force, the more mass you have, the smaller the acceleration
  • F=ma cannot be used when the mass of an object is changing
  • In this situation Newton's Second Law still applies but you must use Force=change in momentum/time
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Newton's Third Law

If object A exerts a force on object B, then object B exerts an equal and opposite force of the same type of object A.

  • The forces represent the same interaction but from different perspectives
  • Newton's Third Law is a consequence of the conservation of momentum
  • A resultant force acting upon an object means a change in mass or acceleration
  • Hence this means a change in momentum
  • Momentum is always conserved, so the forces must be equal so that the overall change in momentum is zero
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