Question

A student pulls a block over a rough surface with a constant force FP that is at an angle θ above the horizontal, as shown above. If FP remains constant but the angle θ is increased, which of the following is true at some later time?
A.The force of friction between the block and surface will increase.

B.The force of friction between the block and surface will decrease.

C.The weight of the block will increase.

D.The weight of the block will decrease.

Answer 1

Answer:

B.The force of friction between the block and surface will decrease.

Explanation:

The force of friction is given by

$F_s = \mu N$

where $\mu$ is the coefficient of friction and $N$ is the normal force.

When the student pulls on the block with force $F_p$ at an angle $\theta$, the normal force on the block becomes

$N = Mg- F_psin(\theta)$

and hence the frictional force becomes

$F_s = \mu (Mg- F_psin(\theta))$.

Now, as we increase $\theta$, $sin(\theta)$ increases which as a result decreases the normal force $Mg- F_psin(\theta)$, which also means the frictional force decreases; Hence choice B stands true.

P.S: Choice D is tempting but incorrect since the weight $W=mg$ is independent of the external forces on the block.

Answer 2

Answer:

The force of friction between the block and surface will decrease.

Explanation: