Concept explainers
If the net force on a system is zero, is the net torque also zero? If the net torque or a system is zero, is the net force zero? Explain and give examples.
To Explain: The two situations mentioned in the question with examples.
Answer to Problem 5Q
Solution:
No, for both situations
Explanation of Solution
- Consider a rod of length l , which has its weight ( mg ) acting in the downward direction, while an upward force of equal magnitude F acts on one of its ends to keep it steady. The net force acting on the rod is
- Consider a rod of length l, which has its weight ( mg ) acting in the downward direction, while two upward forces of equal magnitude F act on it, one at each end. F is still equal to mg . One of these forces tries to rotate the rod in the clockwise direction (positive), while the other tries to rotate it in counter-clockwise direction (negative)
Therefore, even though the forces are balanced and the net force is zero on the rod, there is net upward torque acting on it.
Therefore, net force,
Taking torque about the center of the rod,
Thus, even though the net torque is zero, there is net upward force acting on the rod.
Conclusion
Therefore, if the net force on a system is zero, there can still be net torque acting on it, and even if the net torque is zero, there can be net force acting on it.
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