Laws of Motion and Momentum

  • Created by: CPev3
  • Created on: 16-06-20 11:31

Newton's first law of motion

  • An object will remain at rest or continue to move with constant velocity unless acted upon by a resultant force


  • Velocity is a vector quantity, so an object's velocity changes if its speed and/ or direction changes
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Newton's third law of motion

  • When two objects interact, they exert equal and opposite forces on each other


  • The forces acting on the interacting objects are always of the same type
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The four fundamental forces

  • Gravitational
  • Electromagnetic
  • Strong nuclear
  • Weak nuclear
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Linear momentum equation

p (momentum) = m (mass) * v (velocity)

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Principle of conservation of momentum

For a system of interacting objects, the total momentum in a specified direction remains constant, as long as no external forces act on the system

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Example of zero momentum

A gun recoils when a bullet is fired


The momentum of the gun and the momentum of the bullet have the same magnitude but act in oppsite directions


The total momentum of the cloesed system remains the same and is equal to zero

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Perfectly elastic collision

  • Momentum conserved
  • Total energy conserved 
  • Total kinetic energy conserved
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Inelastic collision

  • Momentum conserved
  • Total energy conserved
  • Total kinetic energy not conserved
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Newton's third law of motion

  • The net force acting on an object is directly proportional to the rate of change of its momentum, and is in the same direction


  • F (force)Δp (change in momentum) / Δt (time)
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Force equation

F = Δp / Δt


F = (mv - mu) / t


F = m ((v-u) / t)


F = ma

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Why momentum is conserved in collisions

According to Newton's third law of motion, each object experiences an equal but opposite force


The net force acting on the objects in this closed system is zero


According to Newton's second law of motion, Δp / Δt = 0


The change in momentum of both objects must be zero


The total momentum of the objects does not change


Momentum is always conserved

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Impulse of a force

  • The product of force and the time for which this force acts on an object


  • Change in momentum


  • The area under a force-time graph
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