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Easy

Question

A force of left parenthesis 2 i with hat on top minus 4 j with hat on top plus 2 k with hat on top right parenthesis  Newton acts at a point   left parenthesis 3 i with hat on top plus 2 j with hat on top minus 4 k with hat on top right parenthesis metre from the origin. The magnitude of torque is -

  1. zero
  2. 24.4 N-m         
  3. 0.244 N-m
  4. 2.444 N-m

The correct answer is: 24.4 N-m


    F with rightwards arrow on top equals left parenthesis 2 i with hat on top minus 4 j with hat on top plus 2 k with hat on top space right parenthesis
r with rightwards arrow on top equals left parenthesis 3 i with hat on top plus 2 j with hat on top minus 4 k with hat on top space right parenthesis
T o r q u e space equals space r with rightwards arrow on top cross times F with rightwards arrow on top
    
vertical line left parenthesis 3 i with not stretchy hat on top plus 2 j with not stretchy hat on top minus 4 k with not stretchy hat on top right parenthesis cross times left parenthesis 2 i with not stretchy hat on top minus 4 j with not stretchy hat on top plus 2 k with not stretchy hat on top right parenthesis vertical line
    open vertical bar table row i j k row 3 2 cell negative 4 end cell row 2 cell negative 4 end cell 2 end table close vertical bar
    
i with not stretchy hat on top left parenthesis 4 minus 16 right parenthesis minus j with not stretchy hat on top left parenthesis 6 plus 8 right parenthesis plus k with not stretchy hat on top left parenthesis negative 12 minus 4 right parenthesis
    
equals vertical line minus 12 i with not stretchy hat on top minus 14 j with not stretchy hat on top minus 16 k with not stretchy hat on top vertical line
    equals square root of left parenthesis 12 right parenthesis squared plus left parenthesis 14 right parenthesis squared plus left parenthesis negative 16 right parenthesis squared end root equals 24.4 N minus m

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