Q. 433.7( 3 Votes )
Two tall buildings face each other and are at a distance of 180 m from each other. With what velocity must a ball be thrown horizontally from a window 55m above the ground in one building so that the ball enters a window 10.9 m above the ground in the second building?
Let, initial velocity = u
Now, this is projectile motion. We can resolve the motion along X and Y axis.
u = velocity of projection
Since the initial velocity is along X-axis (horizontally)
Initial velocity along x-axis
Initial velocity along y-axis
= Acceleration along x axis
= Acceleration along y axis
Let, time taken to travel 180 m horizontally and (55–10.9) m vertically = t
Motion along the Y axis,
Motion along X the axis
Hence, the ball must be thrown with the velocity
Rate this question :
A particle is projected in air at an angle β to a surface which itself is inclined at an angle α to the horizontal (Fig. 4.6).
(a) Find an expression of range on the plane surface (distance on the plane from the point of projection at which particle will hit the surface).
(b) Time of flight.
(c) β at which range will be maximum.
(Hint: This problem can be solved in two different ways:
(i) Point P at which particle hits the plane can be seen as intersection of its trajectory (parabola) and straight line. Remember particle is projected at an angle (α + β) w.r.t. horizontal.
(ii) We can take x-direction along the plane and y-direction perpendicular to the plane. In that case resolve g (acceleration due to gravity) in two different components, gx along the plane and gy perpendicular to the plane. Now the problem can be solved as two independent motions in x and y directions respectively with time as a common parameter.)
Physics - Exemplar
A particle falling vertically from a height hits a plane surface inclined to horizontal at an angle θ with speed vo and rebounds elastically (Fig 4.7). Find the distance along the plane where it will hit second time.
(Hint: (i) After rebound, particle still has speed Vo to start.
(ii) Work out angle particle speed has with horizontal after it rebounds.
(iii) Rest is similar to if particle is projected up the incline.)
Physics - Exemplar