Q33 of 42 Page 34

A person in an elevator accelerating upwards with an acceleration of 2 m s-2, tosses a coin vertically upwards with a speed of 20 m s1. After how much time will the coin fall back into his hand? (g = 10 m s-2)

Here, the initial velocity of the coin is, u = 20 m s1

And the acceleration of the elevator is, a = 2 m s-2


Here, let’s think this problem from the elevator reference frame. For the person inside the elevator going up with acceleration 2 m s-2 will experience a net acceleration of (g+a) which is 12 m s-2.


And as the coin return back to its original position (because we are in the elevator reference frame), the net displacement (s) is zero.


So we can use the laws of motion to solve the problem.



Here, a’=a+g = 12 m s-2




So the time of flight of the coin is 3.33 seconds.


More from this chapter

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31

A 100 kg gun fires a ball of 1kg horizontally from a cliff of height 500m. It falls on the ground at a distance of 400m from the bottom of the cliff. Find the recoil velocity of the gun. (acceleration due to gravity = 10 m s-2)

 32

Figure 5.8 shows (x, t), (y, t ) diagram of a particle moving in 2-dimensions.


If the particle has a mass of 500 g, find the force (direction and magnitude) acting on the particle.


 34

There are three forces F1, F2 and F3 acting on a body, all acting on a point P on the body. The body is found to move with uniform speed.

(a) Show that the forces are coplanar.


(b) Show that the torque acting on the body about any point due to these three forces is zero.


 35

When a body slides down from rest along a smooth inclined plane making an angle of 45° with the horizontal, it takes time T. When the same body slides down from rest along a rough inclined plane making the same angle and through the same distance, it is seen to take time pT, where p is some number greater than 1. Calculate the co-efficient of friction between the body and the rough plane.