Three large blocks each having flat surfaces are kept stationary over one another as shown in the figure. A horizontal force of 20N acts on the middle block B, then choose the incorrect statement: (Take g=10m/s2)
A
Acceleration of the lowest block is 7m/s2
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B
Friction between middle and lower blocks is 12N
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C
Acceleration of the upper most block is 2m/s2
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D
Friction between middle and upper blocks is 2N
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Solution
The correct option is B Friction between middle and lower blocks is 12N FBD for block A:
N=1g=10N f1max=μN=0.2×10=2N
FBD for Block B:
N=2g=20N f2max=μN=0.8×20=16N
FBD for block C:
N=3g=30N f3max=μN=0.3×30=9N
Since, the applied external force i.e 20N is greater than f1max and f2max, the blocks will slide on each other with different acceleration.
Hence, acceleration of the lowest block C is aC=f2max−f3maxmC=16−91=7m/s2 acceleration of upper most block A is aA=f1maxmA=21=2m/s2 and, acceleration of middle block B is aB=20−f1max−f2maxmB=20−2−161=2m/s2