Q26 of 26 Page 18

A man is standing on top of a building 100 m high. He throws two balls vertically, one at t = 0 and other after a time interval (less than 2 seconds). The later ball is thrown at a velocity of half the first. The vertical gap between first and second ball is +15 m at t = 2 s. The gap is found to remain constant. Calculate the velocity with which the balls were thrown and the exact time interval between their throw.

The distance between the two balls can be constant if and only if they have the same velocity. For the balls to have the same velocity, the velocity of the first ball released should clearly be same as the second ball when it is released at time (t) otherwise this scenario won’t be possible.

Let the initial velocity of the first ball be (u).


Let the second ball be released at time t = t seconds.


Then the initial velocity of the second ball and the final velocity of the first ball at time (t) is u/2


For the first ball



(v -> final velocity, u -> initial velocity, t->time).



(g is acceleration due to gravity)



The first ball would have gone up by 15 m till now because then only they will maintain a constant distance (s) of 15 m thereafter.




Taking (g) as 10m/s2, we get u = 20 m/s. Putting the value of (u) to get (t) which is equal to u/2g, we get t = 1 s.


Thus, the first ball was thrown at 20 m/s.


The second ball was thrown at 10m/s.


The time interval was 1 second.


More from this chapter

All 26 →
22

The velocity-displacement graph of a particle is shown in Fig. 3.5.

(a) Write the relation between v and x.


(b) Obtain the relation between acceleration and displacement and plot it.



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. (g = 10m/s2)


(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 !!)


24

A motor car moving at a speed of 72km/h cannot come to a stop in less than 3.0 s while for a truck this time interval is 5.0 s. On a highway the car is behind the truck both moving at 72km/h. 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 0.5s.

(Comment : This is to illustrate why vehicles carry the message on the rear side. “Keep safe Distance”)


25

A monkey climbs up a slippery pole for 3 seconds and subsequently slips for 3 seconds. Its velocity at time t is given by v(t) = 2t (3-t); 0< t < 3 and v (t)=–(t–3)(6–t) for 3 < t < 6 s in m/s. 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?


(d) How many cycles (counting fractions) are required to reach the top?