text: # What amount of force must be applied to 6kg block moving at 13m/s to slow down to 5m/s in a distance of 4.5m?The Friction between them is 75N?
  
  jeew-m | College Teacher | (Level 1) Educator Emeritus
  
  Posted on
  
  We can do this using Newtons second law of motion.
  
  To stop the moving block we have to apply a force against its motion. Both friction and the applied force result in decrease the velocity of the block.
  
  Using equations of motion
  
  `V^2 = U^2+2aS`
  
  `a = (V^2-U^2)/(2S)`
  
  `a = (5^2-13^2)/(2xx4.5)`
  
  `a = -16m/s^2`
  
  Acceleration is negative because we have a deceleration.
  
  Let us say the force is P at the opposite direction to the movement.
  
  Using F = ma to the direction of motion
  
  `-P-75 = 6xx(-16)`
  
  `P = 21N`
  
  So the required external force is 21N at the direction opposite to the movement of block.
  
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