Question

An external force is applied to a box that is free to move on a rough surface. Which of the following statements could be correct?

• A. The frictional force is larger than the applied force
• B. The frictional force is smaller than the applied force
• C. The weight of the object is larger than the applied force
• D. The weight of the object is smaller than the applied force
• E. All of these statements could be correct

Answer 1

Answer: (E)

All of these statements could be correct

If the box is not moving it means the frictional force is larger than the applied force

If the box is moving it means the frictional force is smaller than the applied force

If the box is not moving it means the weight of the object is larger than the applied force

if $mg>P$ where $P$ is applied force

then $R>P$ where $R$ is normal reaction ($mg=R$)

so $F>P$ where $F$ is the frictional force ($F\propto R$)

If the box is moving it means the weight of the object is smaller than the applied force

if $mg<P$ where $P$ is applied force

then $R<P$ where $R$ is normal reaction ($mg=R$)

so $F<P$ where $F$ is the frictional force ($F\propto R$)

therefore all statements could be correct.