A uniform cube of mass $$m$$ and side $$a$$ is placed on a frictionless horizontal surface. A vertical force $$F$$ is applied to the edge as shown in figure. Match the following (most appropriate choice)
| (a) | $$ m g / 4<F<m g / 2 $$ |
(i) | Cube will move up. |
|---|---|---|---|
| (b) | $$F > mg/2$$ | (ii) | Cube will not exhibit motion. |
| (c) | $$F > mg$$ | (iii) | Cube will begin to rotate and slip at A. |
| (d) | $$F=mg/4$$ | (iv) | Normal reaction effectively at a/3 from A, no motion. |
Consider the below diagram
Moment of the force $$F$$ about point $$A, \tau_1=F \times a$$ (anti-clockwise)
Moment of weight $$m g$$ of the cube about point $$A$$,
$$\tau_2=m g \times \frac{a}{2}(\text { clockwise })$$
Cube will not exhibit motion, if $$\tau_1=\tau_2$$
($$\because$$ In this case, both the torque will cancel the effect of each other)
$$\therefore \quad F \times a=m g \times \frac{a}{2} \Rightarrow F=\frac{m g}{2}$$
$$\begin{aligned} &\text { Cube will rotate only when, } \tau_1>\tau_2\\ &\Rightarrow \quad F \times a>m g \times \frac{a}{2} \Rightarrow F>\frac{m g}{2} \end{aligned}$$
$$\begin{aligned} & \text { Let normal reaction is acting at } \frac{a}{3} \text { from point } A \text {, then } \\ & \qquad m g \times \frac{a}{3}=F \times a \text { or } F=\frac{m g}{3} \quad \text{For no motion} \end{aligned}$$
$$\begin{aligned} &\text { When } F=\frac{m g}{4} \text { which is less than } \frac{m g}{3} \text {,. }\quad\left(F<\frac{m g}{3}\right) \end{aligned}$$
there will be no motion.
$$\therefore \quad$$ (a) $$\rightarrow$$ (ii) (b) $$\rightarrow$$ (iii) (c) $$\rightarrow$$ (i) (d) $$\rightarrow$$ (iv)
A uniform sphere of mass $$m$$ and radius $$R$$ is placed on a rough horizontal surface (figure). The sphere is struck horizontally at a height $$h$$ from the floor. Match the following
| (a) | $$h=R/2$$ | (i) | Sphere rolls without slipping with a constant veloci8ty and no loss of energy. |
|---|---|---|---|
| (b) | $$h > R$$ | (ii) | Sphere spins clockwise, loses energy by friction. |
| (c) | $$h > 3R/2$$ | (iii) | Sphere spins anti-clockwise, loses energy by friction. |
| (d) | $$h=7R/5$$ | (iv) | Sphere has only a translational motion, loses energy by friction. |
Consider the diagram where a sphere of $$m$$ and radius $$R$$, struck horizontally at height $$h$$ above the floor
The sphere will roll without slipping when $$\omega=\frac{v}{r}$$, where, $$v$$ is linear velocity and $$\omega$$ is angular velocity of the sphere.
Now, angular momentum of sphere, about centre of mass
[We are applying conservation of angular momentum just before and after struck]
$$\begin{aligned} m v(h-R) & =I \omega=\left(\frac{2}{5} m R^2\right)\left(\frac{v}{R}\right) \\ \Rightarrow \quad m v(h-R) & =\frac{2}{5} m v R \\ h-R & =\frac{2}{5} R \Rightarrow h=\frac{7}{5} R \end{aligned}$$
Therefore, the sphere will roll without slipping with a constant velocity and hence, no loss of energy, so (d) $$\rightarrow$$ (i)
Torque due to applied force, $$F$$ about centre of mass
$$\tau=F(h-R)\quad \text{(clockwise)}$$
For $$\tau=0, h=R$$, sphere will have only translational motion. It would lose energy by friction.
Hence, (b) $$\rightarrow$$ (iv)
The sphere will spin clockwise when $$\tau>0 \Rightarrow h>R$$
Therefore, (c) $$\rightarrow$$ (ii)
The sphere will spin anti-clockwise when $$\tau<0 \Rightarrow h< R$$, (a) $$\rightarrow$$ (iii)
The vector sum of a system of non-collinear forces acting on a rigid body is given to be non-zero. If the vector sum of all the torques due to the system of forces about a certain point is found to be zero, does this mean that it is necessarily zero about any arbitrary point?
No, not necessarily.
Given, $$\quad \sum_\limits i F_i \neq 0$$
$$\text { The sum of torques about a certain point } O, \sum_\limits i \mathbf{r}_i \times \mathbf{F}_i=0$$
$$\begin{aligned} &\text { The sum of torques about any other point } \mathrm{O}^{\prime}\\ &\sum_i\left(\mathbf{r}_i-\mathbf{a}\right) \times \mathbf{F}_i=\sum_i \mathrm{r}_i \times \mathbf{F}_i-\mathbf{a} \times \sum_i \mathrm{~F}_i \end{aligned}$$
Here, the second term need not vanish.
Therefore, sum of all the torques about any arbitrary point need not be zero necessarily.
A wheel in uniform motion about an axis passing through its centre and perpendicular to its plane is considered to be in mechanical (translational plus rotational) equilibrium because no net external force or torque is required to sustain its motion. However, the particles that constitute the wheel do experience a centripetal the acceleration directed towards the centre. How do you reconcile this fact with the wheel being in equilibrium?
How would you set a half wheel into uniform motion about an axis passing through the centre of mass of the wheel and perpendicular to its plane? Will you require external forces to sustain the motion?
Wheel is a rigid body. The particles that constitute the wheel do experience a centripetal acceleration directed towards the centre. This acceleration arises due to internal elastic forces, which cancel out in pairs. In a half wheel, the distribution of mass about its centre of mass (through which axis of rotation passes) is not symmetrical. Therefore, the direction of angular momentum of the wheel does not coincide with the direction of its angular velocity. Hence, an external torque is required to maintain the motion of the half wheel.
A door is hinged at one end and is free to rotate about a vertical axis (figure). Does its weight cause any torque about this axis? Give reason for your answer.
Consider the diagram, where weight of the door acts along negative y-axis.
A force can produce torque only along a direction normal to itself as $$\tau=\mathbf{r} \times \mathbf{F}$$. So, when the door is in the $$x y$$-plane, the torque produced by gravity can only be along $$\pm z$$ direction, never about an axis passing through $$y$$-direction. Hence, the weight will not produce any torque about $$y$$-axis.