# NCERT Physics Class 11 Exemplar Ch 3 Motion In A Straight Line Part 5

Q. 23 It is a common observation that rain clouds can be at about a kilometre altitude above the ground.

(a) If a rain drop falls from such a height freely under gravity, what will be its speed? Also calculate in km/h.

(b) A typical rain drop is about 4mm diameter. Momentum is mass x speed in magnitude. Estimate its momentum when it hits ground.

(c) Estimate the time required to flatten the drop.

(d) Rate of change of momentum is force. Estimate how much force such a drop would exert on you. (e) Estimate the order of magnitude force on umbrella. Typical lateral separation between two rain drops is 5 cm. (Assume that umbrella is circular and has a diameter of 1m and cloth is not pierced through !!)

(b)

.

(c) Diameter

(d)

(e) Area of cross section

With average separation of 5 cm, no. of drops that will fall almost simultaneously is

Net force(Practically drops are damped by air viscosity).

Q. 24 A motor car moving at a speed of can not come to a stop in less than 3.0 s while for a truck this time interval is On a higway the car is behind the truck both moving at The truck gives a signal that it is going to stop at emergency. At what distance the car should be from the truck so that it does not bump onto (collide with) the truck. Human response time is

Regardation of truck

Let the truck be at a distance x from the car when breaks are applied

Distance of truck from

Distance of car from A is

If the two meet

To find x,

Which gives

Therefore,

Therefore,

Q. 25 A monkey climbs up a slippery pole for 3 seconds and subsequently slips for 3 seconds. Its velocity at time t is given It repeats this cycle till it reaches the height of 20 m.

(a) At what time is its velocity maximum?

(b) At what time is its average velocity maximum?

(c) At what time is its acceleration maximum in magnitude?