A stone is thrown horizontally with an initial speed 30 m/s from a bridge. Find the stone's total speed when it enters the water4 s later. (Ignore air resistance.)

Explanation

The stone is thrown with an initial horizontal velocity of 30 m/s, so the horizontal component of its velocity remains constant at 30 m/s because there is no horizontal acceleration. The vertical component of velocity can be calculated using the equation vf = vi + at, where vi is the initial vertical velocity (0 m/s), a is the acceleration due to gravity (10 m/s2), and t is the time (4 s). Therefore, the vertical velocity after 4 seconds is vf = 0 + 10(4) = 40 m/s. The total speed of the stone is the resultant velocity, which is the vector sum of the horizontal and vertical components: √(302 + 402) = √2500 = 50 m/s.Options A and B are incorrect because they represent only one component of the motion. Option D is incorrect because it overestimates the resultant speed.