A block has velocity function $\vec{v}\left(t\right)$ that increases with time along the $+\text{x}$ direction. What must be true about the net force on the block?

• A. The net force must alternate direction since velocity is positive.
• B. The net force must be in the $+\text{x}$ direction because the acceleration is positive.
• C. The net force must be upward because the speed is changing.
• D. The net force must be zero because the object is already moving.

Answer: (B)