The same heavy wooden block is dragged by a force along a rough steel plate, as shown below for two possible situations.The magnitude of the acceleration is the same for the two situations; do not assume that the magnitude of is the same.The magnitude of the frictional force in (ii), as compared with that in (i)is:
A)the same
B)greater
C)less
D)less for some angles and greater for others
E)can be less or greater, depending on the magnitude of the applied force.

Question

The same heavy wooden block is dragged by a force  along a rough steel plate, as shown below for two possible situations.The magnitude of the acceleration is the same for the two situations; do not assume that the magnitude of  is the same.The magnitude of the frictional force in (ii), as compared with that in (i)is: 
A)the same 
B)greater 
C)less 
D)less for some angles and greater for others 
E)can be less or greater, depending on the magnitude of the applied force.

Quizplus · Accepted Answer

The frictional force is given by $f=\mu N$, where $\mu$ is the coefficient of friction and $N$ is the normal force. Since the same block is used in both cases, the normal force is the same, but the magnitude of the applied force is not necessarily the same. Therefore, the frictional force will be less in situation (ii) than in situation (i) because the force needed to achieve the same acceleration is less due to the decrease in friction. Thus, the correct answer is (C) less.

The Same Heavy Wooden Block Is Dragged by a Force