A force of $20$ N is applied to a $3$ kg block on a rough surface where the frictional force is $5$ N. Find the acceleration of the block.
A$8.33$ m s$^{-2}$
B$1.67$ m s$^{-2}$
C$6.67$ m s$^{-2}$
D$5$ m s$^{-2}$CORRECT
JUPEBPhysicsKinematics and Dynamics
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Why the answer is D, and why the others tempt you.
The net force is $20 - 5 = 15$ N, so $a = \dfrac{F_{net}}{m} = \dfrac{15}{3} = 5$ m s$^{-2}$. Using the applied force alone gives $6.67$ m s$^{-2}$, and adding friction instead of subtracting gives $8.33$.
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