Question

A uniform bar is lying on a flat horizontal table. The bar is acted upon by $F_1$ and $F_2$,besides the gravitational and normal forces (which cancel), $F_1$ and $F_2$ are parallel to the surface of the table. If the net force on the rod is zero, then which one of the following statement is true?

• A. The net torque on the bar must also be zero
• B. The bar can accelerate translationally if $F_1$ and $F_2$ are not applied at the same point
• C. The net torque will be zero if $F_1$ and $F_2$ are applied at the same point
• D. The bar cannot accelerate translationally or rotationally
• E. None of the above

Answer 1

Answer: (C)

The net torque will be zero if $F_1$ and $F_2$ are applied at the same point

The bar cannot accelerate translationally or rotationally because net force on it ,is zero. It is not compulsory that if net force is zero ,net torque will also be zero ,infact torque is produced by two equal and opposite forces on a body when their lines of action are not on the same point if these forces are acting on the same point then net torque would be zero because in this situation forces will cancel each other.