**Define terminal velocity?**

Difficulty: Hard

**Termina****l velocity:**

The maximum and constant velocity of an object falling vertically downward is called terminal velocity.

Terminal velocity = $V_{t}=2rg^{2}\rho/9 \: \eta$

Where g = acceleration due to gravity, r = radius, ρ = density, η = viscosity.

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**QUICK QUIZ**

**1. A ladder leaning against a wall as shown in the figure is in equilibrium. How?**

**Ans: **In this case three forces involved are:

- The weight of the ladder
- The reaction at the wall (R
_{1})-at right angles because the wall is smooth. - The reaction at the ground (R
_{2})-not at a right angle

As the ground is rough and all the forces pass through the same point. The vector diagram for the three forces will cancel the effect of each other therefore ladder leaning at a wall will be in equilibrium.

**1. The weight of the ladder in the figure produces an anticlockwise torque. The wall pushes the ladder at its top end thus producing a clockwise torque. Does the ladder satisfy the second condition for equilibrium?**

**Ans: **Yes, the ladder satisfies the second condition for equilibrium because the clockwise torque will cancel the effect of anticlockwise torque. So, the resultant torque acting in this situation is zero.

**1. Does the speed of a ceiling fan go on increasing all the time?**

**Ans: **No,** **the speed of a ceiling fan does not go on increasing all the time. The fan will move with constant speed.

**2. Does the fan satisfy the second condition for equilibrium when rotating with uniform speed?**

**Ans: **Yes, a rotating ceiling fan satisfies the second condition for equilibrium. Because ceiling fan rotating at constant speed is in equilibrium as net torque acting on it is zero

** ∑ τ = 0**

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