Q12 of 28 Page 52

Figure 7.5 shows a lamina in x-y plane. Two axes z and z′ pass perpendicular to its plane. A force F acts in the plane of lamina at point P as shown. Which of the following are true? (The point P is closer to z′-axis than the z-axis.)


As we know that


And direction of z is perpendicular to plane of r and F by right hand thumb rule.


a) Torque caused by force F about z-axis is in +k direction by right hand thumb rule.


b) Similarly Torque caused by force F about z’-axis is in –k direction.


c) r>r’ and θ>θ’


So, rFsinθ > r’Fsinθ’


Hence


d) There is no sense in adding torques about 2 different axes.

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The net external torque on a system of particles about an axis is zero. Which of the following are compatible with it?

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With reference to Fig. 7.6 of a cube of edge an and mass m, state whether the following are true or false. (O is the centre of the cube.)


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