The net external torque on a system of particles about an axis is zero. Which of the following are compatible with it?
(a) | The forces may be acting radially from a point on the axis. |
(b) | The forces may be acting on the axis of rotation. |
(c) | The forces may be acting parallel to the axis of rotation. |
(d) | The torque caused by some forces may be equal and opposite to that caused by other forces. |
Choose the correct option.
1. (a, c, d)
2. (a, b, d)
3. (a, b, c, d)
4. (b, c, d)
We know that torque on a system of particles →Γ=→r×→F=Fr sin θ ˆn ....(i)
where θ is the angle between →r and →F. and →n is a unit vector perpendicular to both →r and →F.
(a) When forces act radially then θ=0. Therefore, ∣∣∣→Γ∣∣∣=0 [from Eq. (i)]
(b) When forces are acting on the axis of rotation, r=0, therefore, |→Γ∣∣∣=0 [from Eq. (i)]
(c) When forces acting parallel to the axis of rotation then, θ=0∘,∣∣∣→Γ∣∣∣=0 [from Eq. (i)]
(d) When torque by forces are equal and opposite. Therefore, →Γ net =→Γ1−→Γ2=0
Hence, option (3) is the correct answer.
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